index.js

/***********************************************************************

  A JavaScript tokenizer / parser / beautifier / compressor.
  https://github.com/mishoo/UglifyJS2

  -------------------------------- (C) ---------------------------------

                           Author: Mihai Bazon
                         <mihai.bazon@gmail.com>
                       http://mihai.bazon.net/blog

  Distributed under the BSD license:

    Copyright 2012 (c) Mihai Bazon <mihai.bazon@gmail.com>

    Redistribution and use in source and binary forms, with or without
    modification, are permitted provided that the following conditions
    are met:

        * Redistributions of source code must retain the above
          copyright notice, this list of conditions and the following
          disclaimer.

        * Redistributions in binary form must reproduce the above
          copyright notice, this list of conditions and the following
          disclaimer in the documentation and/or other materials
          provided with the distribution.

    THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDER “AS IS” AND ANY
    EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE
    IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR
    PURPOSE ARE DISCLAIMED. IN NO EVENT SHALL THE COPYRIGHT HOLDER BE
    LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY,
    OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT LIMITED TO,
    PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS OF USE, DATA, OR
    PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND ON ANY
    THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY, OR
    TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF
    THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF
    SUCH DAMAGE.

 ***********************************************************************/

import {
    AST_Accessor,
    AST_Array,
    AST_Arrow,
    AST_Assign,
    AST_BigInt,
    AST_Binary,
    AST_Block,
    AST_BlockStatement,
    AST_Boolean,
    AST_Break,
    AST_Call,
    AST_Catch,
    AST_Chain,
    AST_Class,
    AST_ClassExpression,
    AST_ClassProperty,
    AST_ConciseMethod,
    AST_Conditional,
    AST_Const,
    AST_Constant,
    AST_Debugger,
    AST_Default,
    AST_DefaultAssign,
    AST_DefClass,
    AST_Definitions,
    AST_Defun,
    AST_Destructuring,
    AST_Directive,
    AST_Do,
    AST_Dot,
    AST_DWLoop,
    AST_EmptyStatement,
    AST_Exit,
    AST_Expansion,
    AST_Export,
    AST_False,
    AST_For,
    AST_ForIn,
    AST_Function,
    AST_Hole,
    AST_If,
    AST_Import,
    AST_Infinity,
    AST_IterationStatement,
    AST_LabeledStatement,
    AST_Lambda,
    AST_Let,
    AST_NaN,
    AST_New,
    AST_Node,
    AST_Null,
    AST_Number,
    AST_Object,
    AST_ObjectKeyVal,
    AST_ObjectProperty,
    AST_PrefixedTemplateString,
    AST_PropAccess,
    AST_RegExp,
    AST_Return,
    AST_Scope,
    AST_Sequence,
    AST_SimpleStatement,
    AST_Statement,
    AST_String,
    AST_Sub,
    AST_Switch,
    AST_SwitchBranch,
    AST_Symbol,
    AST_SymbolBlockDeclaration,
    AST_SymbolCatch,
    AST_SymbolClassProperty,
    AST_SymbolDeclaration,
    AST_SymbolDefun,
    AST_SymbolExport,
    AST_SymbolFunarg,
    AST_SymbolLambda,
    AST_SymbolLet,
    AST_SymbolMethod,
    AST_SymbolRef,
    AST_SymbolVar,
    AST_TemplateString,
    AST_This,
    AST_Toplevel,
    AST_True,
    AST_Try,
    AST_Unary,
    AST_UnaryPostfix,
    AST_UnaryPrefix,
    AST_Undefined,
    AST_Var,
    AST_VarDef,
    AST_While,
    AST_With,
    AST_Yield,

    TreeTransformer,
    TreeWalker,
    walk,
    walk_abort,
    walk_parent,

    _INLINE,
    _NOINLINE,
    _PURE
} from "../ast.js";
import {
    defaults,
    HOP,
    keep_name,
    make_node,
    makePredicate,
    map_add,
    MAP,
    member,
    remove,
    return_false,
    return_true,
    regexp_source_fix,
    has_annotation
} from "../utils/index.js";
import { first_in_statement } from "../utils/first_in_statement.js";
import { equivalent_to } from "../equivalent-to.js";
import {
    is_basic_identifier_string,
    JS_Parse_Error,
    parse,
    PRECEDENCE,
} from "../parse.js";
import { OutputStream } from "../output.js";
import {
    base54,
    SymbolDef,
} from "../scope.js";
import "../size.js";

import { nullish } from "./evaluate.js";
import "./drop-side-effect-free.js";
import "./reduce-vars.js";
import {
    is_undeclared_ref,
    lazy_op,
    is_nullish,
    is_undefined,
    is_lhs,
    aborts,
} from "./inference.js";
import {
    SQUEEZED,
    OPTIMIZED,
    INLINED,
    CLEAR_BETWEEN_PASSES,
    TOP,
    WRITE_ONLY,
    UNDEFINED,
    UNUSED,
    TRUTHY,
    FALSY,

    has_flag,
    set_flag,
    clear_flag,
} from "./compressor-flags.js";
import {
    make_sequence,
    best_of,
    best_of_expression,
    make_node_from_constant,
    merge_sequence,
    get_simple_key,
    has_break_or_continue,
    maintain_this_binding,
    identifier_atom,
    is_identifier_atom,
    is_func_expr,
    is_ref_of,
    can_be_evicted_from_block,
    as_statement_array,
    is_iife_call,
    is_recursive_ref
} from "./common.js";
import { tighten_body, trim_unreachable_code } from "./tighten-body.js";

class Compressor extends TreeWalker {
    constructor(options, { false_by_default = false, mangle_options = false }) {
        super();
        if (options.defaults !== undefined && !options.defaults) false_by_default = true;
        this.options = defaults(options, {
            arguments     : false,
            arrows        : !false_by_default,
            booleans      : !false_by_default,
            booleans_as_integers : false,
            collapse_vars : !false_by_default,
            comparisons   : !false_by_default,
            computed_props: !false_by_default,
            conditionals  : !false_by_default,
            dead_code     : !false_by_default,
            defaults      : true,
            directives    : !false_by_default,
            drop_console  : false,
            drop_debugger : !false_by_default,
            ecma          : 5,
            evaluate      : !false_by_default,
            expression    : false,
            global_defs   : false,
            hoist_funs    : false,
            hoist_props   : !false_by_default,
            hoist_vars    : false,
            ie8           : false,
            if_return     : !false_by_default,
            inline        : !false_by_default,
            join_vars     : !false_by_default,
            keep_classnames: false,
            keep_fargs    : true,
            keep_fnames   : false,
            keep_infinity : false,
            loops         : !false_by_default,
            module        : false,
            negate_iife   : !false_by_default,
            passes        : 1,
            properties    : !false_by_default,
            pure_getters  : !false_by_default && "strict",
            pure_funcs    : null,
            reduce_funcs  : !false_by_default,
            reduce_vars   : !false_by_default,
            sequences     : !false_by_default,
            side_effects  : !false_by_default,
            switches      : !false_by_default,
            top_retain    : null,
            toplevel      : !!(options && options["top_retain"]),
            typeofs       : !false_by_default,
            unsafe        : false,
            unsafe_arrows : false,
            unsafe_comps  : false,
            unsafe_Function: false,
            unsafe_math   : false,
            unsafe_symbols: false,
            unsafe_methods: false,
            unsafe_proto  : false,
            unsafe_regexp : false,
            unsafe_undefined: false,
            unused        : !false_by_default,
            warnings      : false  // legacy
        }, true);
        var global_defs = this.options["global_defs"];
        if (typeof global_defs == "object") for (var key in global_defs) {
            if (key[0] === "@" && HOP(global_defs, key)) {
                global_defs[key.slice(1)] = parse(global_defs[key], {
                    expression: true
                });
            }
        }
        if (this.options["inline"] === true) this.options["inline"] = 3;
        var pure_funcs = this.options["pure_funcs"];
        if (typeof pure_funcs == "function") {
            this.pure_funcs = pure_funcs;
        } else {
            this.pure_funcs = pure_funcs ? function(node) {
                return !pure_funcs.includes(node.expression.print_to_string());
            } : return_true;
        }
        var top_retain = this.options["top_retain"];
        if (top_retain instanceof RegExp) {
            this.top_retain = function(def) {
                return top_retain.test(def.name);
            };
        } else if (typeof top_retain == "function") {
            this.top_retain = top_retain;
        } else if (top_retain) {
            if (typeof top_retain == "string") {
                top_retain = top_retain.split(/,/);
            }
            this.top_retain = function(def) {
                return top_retain.includes(def.name);
            };
        }
        if (this.options["module"]) {
            this.directives["use strict"] = true;
            this.options["toplevel"] = true;
        }
        var toplevel = this.options["toplevel"];
        this.toplevel = typeof toplevel == "string" ? {
            funcs: /funcs/.test(toplevel),
            vars: /vars/.test(toplevel)
        } : {
            funcs: toplevel,
            vars: toplevel
        };
        var sequences = this.options["sequences"];
        this.sequences_limit = sequences == 1 ? 800 : sequences | 0;
        this.evaluated_regexps = new Map();
        this._toplevel = undefined;
        this.mangle_options = mangle_options;
    }

    option(key) {
        return this.options[key];
    }

    exposed(def) {
        if (def.export) return true;
        if (def.global) for (var i = 0, len = def.orig.length; i < len; i++)
            if (!this.toplevel[def.orig[i] instanceof AST_SymbolDefun ? "funcs" : "vars"])
                return true;
        return false;
    }

    in_boolean_context() {
        if (!this.option("booleans")) return false;
        var self = this.self();
        for (var i = 0, p; p = this.parent(i); i++) {
            if (p instanceof AST_SimpleStatement
                || p instanceof AST_Conditional && p.condition === self
                || p instanceof AST_DWLoop && p.condition === self
                || p instanceof AST_For && p.condition === self
                || p instanceof AST_If && p.condition === self
                || p instanceof AST_UnaryPrefix && p.operator == "!" && p.expression === self) {
                return true;
            }
            if (
                p instanceof AST_Binary
                    && (
                        p.operator == "&&"
                        || p.operator == "||"
                        || p.operator == "??"
                    )
                || p instanceof AST_Conditional
                || p.tail_node() === self
            ) {
                self = p;
            } else {
                return false;
            }
        }
    }

    get_toplevel() {
        return this._toplevel;
    }

    compress(toplevel) {
        toplevel = toplevel.resolve_defines(this);
        this._toplevel = toplevel;
        if (this.option("expression")) {
            this._toplevel.process_expression(true);
        }
        var passes = +this.options.passes || 1;
        var min_count = 1 / 0;
        var stopping = false;
        var nth_identifier = this.mangle_options && this.mangle_options.nth_identifier || base54;
        var mangle = { ie8: this.option("ie8"), nth_identifier: nth_identifier };
        for (var pass = 0; pass < passes; pass++) {
            this._toplevel.figure_out_scope(mangle);
            if (pass === 0 && this.option("drop_console")) {
                // must be run before reduce_vars and compress pass
                this._toplevel = this._toplevel.drop_console();
            }
            if (pass > 0 || this.option("reduce_vars")) {
                this._toplevel.reset_opt_flags(this);
            }
            this._toplevel = this._toplevel.transform(this);
            if (passes > 1) {
                let count = 0;
                walk(this._toplevel, () => { count++; });
                if (count < min_count) {
                    min_count = count;
                    stopping = false;
                } else if (stopping) {
                    break;
                } else {
                    stopping = true;
                }
            }
        }
        if (this.option("expression")) {
            this._toplevel.process_expression(false);
        }
        toplevel = this._toplevel;
        this._toplevel = undefined;
        return toplevel;
    }

    before(node, descend) {
        if (has_flag(node, SQUEEZED)) return node;
        var was_scope = false;
        if (node instanceof AST_Scope) {
            node = node.hoist_properties(this);
            node = node.hoist_declarations(this);
            was_scope = true;
        }
        // Before https://github.com/mishoo/UglifyJS2/pull/1602 AST_Node.optimize()
        // would call AST_Node.transform() if a different instance of AST_Node is
        // produced after def_optimize().
        // This corrupts TreeWalker.stack, which cause AST look-ups to malfunction.
        // Migrate and defer all children's AST_Node.transform() to below, which
        // will now happen after this parent AST_Node has been properly substituted
        // thus gives a consistent AST snapshot.
        descend(node, this);
        // Existing code relies on how AST_Node.optimize() worked, and omitting the
        // following replacement call would result in degraded efficiency of both
        // output and performance.
        descend(node, this);
        var opt = node.optimize(this);
        if (was_scope && opt instanceof AST_Scope) {
            opt.drop_unused(this);
            descend(opt, this);
        }
        if (opt === node) set_flag(opt, SQUEEZED);
        return opt;
    }
}

function def_optimize(node, optimizer) {
    node.DEFMETHOD("optimize", function(compressor) {
        var self = this;
        if (has_flag(self, OPTIMIZED)) return self;
        if (compressor.has_directive("use asm")) return self;
        var opt = optimizer(self, compressor);
        set_flag(opt, OPTIMIZED);
        return opt;
    });
}

def_optimize(AST_Node, function(self) {
    return self;
});

AST_Toplevel.DEFMETHOD("drop_console", function() {
    return this.transform(new TreeTransformer(function(self) {
        if (self.TYPE == "Call") {
            var exp = self.expression;
            if (exp instanceof AST_PropAccess) {
                var name = exp.expression;
                while (name.expression) {
                    name = name.expression;
                }
                if (is_undeclared_ref(name) && name.name == "console") {
                    return make_node(AST_Undefined, self);
                }
            }
        }
    }));
});

AST_Node.DEFMETHOD("equivalent_to", function(node) {
    return equivalent_to(this, node);
});

AST_Scope.DEFMETHOD("process_expression", function(insert, compressor) {
    var self = this;
    var tt = new TreeTransformer(function(node) {
        if (insert && node instanceof AST_SimpleStatement) {
            return make_node(AST_Return, node, {
                value: node.body
            });
        }
        if (!insert && node instanceof AST_Return) {
            if (compressor) {
                var value = node.value && node.value.drop_side_effect_free(compressor, true);
                return value
                    ? make_node(AST_SimpleStatement, node, { body: value })
                    : make_node(AST_EmptyStatement, node);
            }
            return make_node(AST_SimpleStatement, node, {
                body: node.value || make_node(AST_UnaryPrefix, node, {
                    operator: "void",
                    expression: make_node(AST_Number, node, {
                        value: 0
                    })
                })
            });
        }
        if (node instanceof AST_Class || node instanceof AST_Lambda && node !== self) {
            return node;
        }
        if (node instanceof AST_Block) {
            var index = node.body.length - 1;
            if (index >= 0) {
                node.body[index] = node.body[index].transform(tt);
            }
        } else if (node instanceof AST_If) {
            node.body = node.body.transform(tt);
            if (node.alternative) {
                node.alternative = node.alternative.transform(tt);
            }
        } else if (node instanceof AST_With) {
            node.body = node.body.transform(tt);
        }
        return node;
    });
    self.transform(tt);
});

AST_Toplevel.DEFMETHOD("reset_opt_flags", function(compressor) {
    const self = this;
    const reduce_vars = compressor.option("reduce_vars");

    const preparation = new TreeWalker(function(node, descend) {
        clear_flag(node, CLEAR_BETWEEN_PASSES);
        if (reduce_vars) {
            if (compressor.top_retain
                && node instanceof AST_Defun  // Only functions are retained
                && preparation.parent() === self
            ) {
                set_flag(node, TOP);
            }
            return node.reduce_vars(preparation, descend, compressor);
        }
    });
    // Stack of look-up tables to keep track of whether a `SymbolDef` has been
    // properly assigned before use:
    // - `push()` & `pop()` when visiting conditional branches
    preparation.safe_ids = Object.create(null);
    preparation.in_loop = null;
    preparation.loop_ids = new Map();
    preparation.defs_to_safe_ids = new Map();
    self.walk(preparation);
});

AST_Symbol.DEFMETHOD("fixed_value", function() {
    var fixed = this.thedef.fixed;
    if (!fixed || fixed instanceof AST_Node) return fixed;
    return fixed();
});

AST_SymbolRef.DEFMETHOD("is_immutable", function() {
    var orig = this.definition().orig;
    return orig.length == 1 && orig[0] instanceof AST_SymbolLambda;
});

function find_scope(tw) {
    for (let i = 0;;i++) {
        const p = tw.parent(i);
        if (p instanceof AST_Toplevel) return p;
        if (p instanceof AST_Lambda) return p;
        if (p.block_scope) return p.block_scope;
    }
}

function find_variable(compressor, name) {
    var scope, i = 0;
    while (scope = compressor.parent(i++)) {
        if (scope instanceof AST_Scope) break;
        if (scope instanceof AST_Catch && scope.argname) {
            scope = scope.argname.definition().scope;
            break;
        }
    }
    return scope.find_variable(name);
}

function is_empty(thing) {
    if (thing === null) return true;
    if (thing instanceof AST_EmptyStatement) return true;
    if (thing instanceof AST_BlockStatement) return thing.body.length == 0;
    return false;
}

var global_names = makePredicate("Array Boolean clearInterval clearTimeout console Date decodeURI decodeURIComponent encodeURI encodeURIComponent Error escape eval EvalError Function isFinite isNaN JSON Math Number parseFloat parseInt RangeError ReferenceError RegExp Object setInterval setTimeout String SyntaxError TypeError unescape URIError");
AST_SymbolRef.DEFMETHOD("is_declared", function(compressor) {
    return !this.definition().undeclared
        || compressor.option("unsafe") && global_names.has(this.name);
});

/* -----[ optimizers ]----- */

var directives = new Set(["use asm", "use strict"]);
def_optimize(AST_Directive, function(self, compressor) {
    if (compressor.option("directives")
        && (!directives.has(self.value) || compressor.has_directive(self.value) !== self)) {
        return make_node(AST_EmptyStatement, self);
    }
    return self;
});

def_optimize(AST_Debugger, function(self, compressor) {
    if (compressor.option("drop_debugger"))
        return make_node(AST_EmptyStatement, self);
    return self;
});

def_optimize(AST_LabeledStatement, function(self, compressor) {
    if (self.body instanceof AST_Break
        && compressor.loopcontrol_target(self.body) === self.body) {
        return make_node(AST_EmptyStatement, self);
    }
    return self.label.references.length == 0 ? self.body : self;
});

def_optimize(AST_Block, function(self, compressor) {
    tighten_body(self.body, compressor);
    return self;
});

function can_be_extracted_from_if_block(node) {
    return !(
        node instanceof AST_Const
        || node instanceof AST_Let
        || node instanceof AST_Class
    );
}

def_optimize(AST_BlockStatement, function(self, compressor) {
    tighten_body(self.body, compressor);
    switch (self.body.length) {
      case 1:
        if (!compressor.has_directive("use strict")
            && compressor.parent() instanceof AST_If
            && can_be_extracted_from_if_block(self.body[0])
            || can_be_evicted_from_block(self.body[0])) {
            return self.body[0];
        }
        break;
      case 0: return make_node(AST_EmptyStatement, self);
    }
    return self;
});

function opt_AST_Lambda(self, compressor) {
    tighten_body(self.body, compressor);
    if (compressor.option("side_effects")
        && self.body.length == 1
        && self.body[0] === compressor.has_directive("use strict")) {
        self.body.length = 0;
    }
    return self;
}
def_optimize(AST_Lambda, opt_AST_Lambda);

const r_keep_assign = /keep_assign/;
AST_Scope.DEFMETHOD("drop_unused", function(compressor) {
    if (!compressor.option("unused")) return;
    if (compressor.has_directive("use asm")) return;
    var self = this;
    if (self.pinned()) return;
    var drop_funcs = !(self instanceof AST_Toplevel) || compressor.toplevel.funcs;
    var drop_vars = !(self instanceof AST_Toplevel) || compressor.toplevel.vars;
    const assign_as_unused = r_keep_assign.test(compressor.option("unused")) ? return_false : function(node) {
        if (node instanceof AST_Assign
            && !node.logical
            && (has_flag(node, WRITE_ONLY) || node.operator == "=")
        ) {
            return node.left;
        }
        if (node instanceof AST_Unary && has_flag(node, WRITE_ONLY)) {
            return node.expression;
        }
    };
    var in_use_ids = new Map();
    var fixed_ids = new Map();
    if (self instanceof AST_Toplevel && compressor.top_retain) {
        self.variables.forEach(function(def) {
            if (compressor.top_retain(def) && !in_use_ids.has(def.id)) {
                in_use_ids.set(def.id, def);
            }
        });
    }
    var var_defs_by_id = new Map();
    var initializations = new Map();
    // pass 1: find out which symbols are directly used in
    // this scope (not in nested scopes).
    var scope = this;
    var tw = new TreeWalker(function(node, descend) {
        if (node instanceof AST_Lambda && node.uses_arguments && !tw.has_directive("use strict")) {
            node.argnames.forEach(function(argname) {
                if (!(argname instanceof AST_SymbolDeclaration)) return;
                var def = argname.definition();
                if (!in_use_ids.has(def.id)) {
                    in_use_ids.set(def.id, def);
                }
            });
        }
        if (node === self) return;
        if (node instanceof AST_Defun || node instanceof AST_DefClass) {
            var node_def = node.name.definition();
            const in_export = tw.parent() instanceof AST_Export;
            if (in_export || !drop_funcs && scope === self) {
                if (node_def.global && !in_use_ids.has(node_def.id)) {
                    in_use_ids.set(node_def.id, node_def);
                }
            }
            if (node instanceof AST_DefClass) {
                if (
                    node.extends
                    && (node.extends.has_side_effects(compressor)
                    || node.extends.may_throw(compressor))
                ) {
                    node.extends.walk(tw);
                }
                for (const prop of node.properties) {
                    if (
                        prop.has_side_effects(compressor) ||
                        prop.may_throw(compressor)
                    ) {
                        prop.walk(tw);
                    }
                }
            }
            map_add(initializations, node_def.id, node);
            return true; // don't go in nested scopes
        }
        if (node instanceof AST_SymbolFunarg && scope === self) {
            map_add(var_defs_by_id, node.definition().id, node);
        }
        if (node instanceof AST_Definitions && scope === self) {
            const in_export = tw.parent() instanceof AST_Export;
            node.definitions.forEach(function(def) {
                if (def.name instanceof AST_SymbolVar) {
                    map_add(var_defs_by_id, def.name.definition().id, def);
                }
                if (in_export || !drop_vars) {
                    walk(def.name, node => {
                        if (node instanceof AST_SymbolDeclaration) {
                            const def = node.definition();
                            if (
                                (in_export || def.global)
                                && !in_use_ids.has(def.id)
                            ) {
                                in_use_ids.set(def.id, def);
                            }
                        }
                    });
                }
                if (def.value) {
                    if (def.name instanceof AST_Destructuring) {
                        def.walk(tw);
                    } else {
                        var node_def = def.name.definition();
                        map_add(initializations, node_def.id, def.value);
                        if (!node_def.chained && def.name.fixed_value() === def.value) {
                            fixed_ids.set(node_def.id, def);
                        }
                    }
                    if (def.value.has_side_effects(compressor)) {
                        def.value.walk(tw);
                    }
                }
            });
            return true;
        }
        return scan_ref_scoped(node, descend);
    });
    self.walk(tw);
    // pass 2: for every used symbol we need to walk its
    // initialization code to figure out if it uses other
    // symbols (that may not be in_use).
    tw = new TreeWalker(scan_ref_scoped);
    in_use_ids.forEach(function (def) {
        var init = initializations.get(def.id);
        if (init) init.forEach(function(init) {
            init.walk(tw);
        });
    });
    // pass 3: we should drop declarations not in_use
    var tt = new TreeTransformer(
        function before(node, descend, in_list) {
            var parent = tt.parent();
            if (drop_vars) {
                const sym = assign_as_unused(node);
                if (sym instanceof AST_SymbolRef) {
                    var def = sym.definition();
                    var in_use = in_use_ids.has(def.id);
                    if (node instanceof AST_Assign) {
                        if (!in_use || fixed_ids.has(def.id) && fixed_ids.get(def.id) !== node) {
                            return maintain_this_binding(parent, node, node.right.transform(tt));
                        }
                    } else if (!in_use) return in_list ? MAP.skip : make_node(AST_Number, node, {
                        value: 0
                    });
                }
            }
            if (scope !== self) return;
            var def;
            if (node.name
                && (node instanceof AST_ClassExpression
                    && !keep_name(compressor.option("keep_classnames"), (def = node.name.definition()).name)
                || node instanceof AST_Function
                    && !keep_name(compressor.option("keep_fnames"), (def = node.name.definition()).name))) {
                // any declarations with same name will overshadow
                // name of this anonymous function and can therefore
                // never be used anywhere
                if (!in_use_ids.has(def.id) || def.orig.length > 1) node.name = null;
            }
            if (node instanceof AST_Lambda && !(node instanceof AST_Accessor)) {
                var trim = !compressor.option("keep_fargs");
                for (var a = node.argnames, i = a.length; --i >= 0;) {
                    var sym = a[i];
                    if (sym instanceof AST_Expansion) {
                        sym = sym.expression;
                    }
                    if (sym instanceof AST_DefaultAssign) {
                        sym = sym.left;
                    }
                    // Do not drop destructuring arguments.
                    // They constitute a type assertion, so dropping
                    // them would stop that TypeError which would happen
                    // if someone called it with an incorrectly formatted
                    // parameter.
                    if (!(sym instanceof AST_Destructuring) && !in_use_ids.has(sym.definition().id)) {
                        set_flag(sym, UNUSED);
                        if (trim) {
                            a.pop();
                        }
                    } else {
                        trim = false;
                    }
                }
            }
            if ((node instanceof AST_Defun || node instanceof AST_DefClass) && node !== self) {
                const def = node.name.definition();
                let keep = def.global && !drop_funcs || in_use_ids.has(def.id);
                if (!keep) {
                    def.eliminated++;
                    if (node instanceof AST_DefClass) {
                        // Classes might have extends with side effects
                        const side_effects = node.drop_side_effect_free(compressor);
                        if (side_effects) {
                            return make_node(AST_SimpleStatement, node, {
                                body: side_effects
                            });
                        }
                    }
                    return in_list ? MAP.skip : make_node(AST_EmptyStatement, node);
                }
            }
            if (node instanceof AST_Definitions && !(parent instanceof AST_ForIn && parent.init === node)) {
                var drop_block = !(parent instanceof AST_Toplevel) && !(node instanceof AST_Var);
                // place uninitialized names at the start
                var body = [], head = [], tail = [];
                // for unused names whose initialization has
                // side effects, we can cascade the init. code
                // into the next one, or next statement.
                var side_effects = [];
                node.definitions.forEach(function(def) {
                    if (def.value) def.value = def.value.transform(tt);
                    var is_destructure = def.name instanceof AST_Destructuring;
                    var sym = is_destructure
                        ? new SymbolDef(null, { name: "<destructure>" }) /* fake SymbolDef */
                        : def.name.definition();
                    if (drop_block && sym.global) return tail.push(def);
                    if (!(drop_vars || drop_block)
                        || is_destructure
                            && (def.name.names.length
                                || def.name.is_array
                                || compressor.option("pure_getters") != true)
                        || in_use_ids.has(sym.id)
                    ) {
                        if (def.value && fixed_ids.has(sym.id) && fixed_ids.get(sym.id) !== def) {
                            def.value = def.value.drop_side_effect_free(compressor);
                        }
                        if (def.name instanceof AST_SymbolVar) {
                            var var_defs = var_defs_by_id.get(sym.id);
                            if (var_defs.length > 1 && (!def.value || sym.orig.indexOf(def.name) > sym.eliminated)) {
                                if (def.value) {
                                    var ref = make_node(AST_SymbolRef, def.name, def.name);
                                    sym.references.push(ref);
                                    var assign = make_node(AST_Assign, def, {
                                        operator: "=",
                                        logical: false,
                                        left: ref,
                                        right: def.value
                                    });
                                    if (fixed_ids.get(sym.id) === def) {
                                        fixed_ids.set(sym.id, assign);
                                    }
                                    side_effects.push(assign.transform(tt));
                                }
                                remove(var_defs, def);
                                sym.eliminated++;
                                return;
                            }
                        }
                        if (def.value) {
                            if (side_effects.length > 0) {
                                if (tail.length > 0) {
                                    side_effects.push(def.value);
                                    def.value = make_sequence(def.value, side_effects);
                                } else {
                                    body.push(make_node(AST_SimpleStatement, node, {
                                        body: make_sequence(node, side_effects)
                                    }));
                                }
                                side_effects = [];
                            }
                            tail.push(def);
                        } else {
                            head.push(def);
                        }
                    } else if (sym.orig[0] instanceof AST_SymbolCatch) {
                        var value = def.value && def.value.drop_side_effect_free(compressor);
                        if (value) side_effects.push(value);
                        def.value = null;
                        head.push(def);
                    } else {
                        var value = def.value && def.value.drop_side_effect_free(compressor);
                        if (value) {
                            side_effects.push(value);
                        }
                        sym.eliminated++;
                    }
                });
                if (head.length > 0 || tail.length > 0) {
                    node.definitions = head.concat(tail);
                    body.push(node);
                }
                if (side_effects.length > 0) {
                    body.push(make_node(AST_SimpleStatement, node, {
                        body: make_sequence(node, side_effects)
                    }));
                }
                switch (body.length) {
                  case 0:
                    return in_list ? MAP.skip : make_node(AST_EmptyStatement, node);
                  case 1:
                    return body[0];
                  default:
                    return in_list ? MAP.splice(body) : make_node(AST_BlockStatement, node, {
                        body: body
                    });
                }
            }
            // certain combination of unused name + side effect leads to:
            //    https://github.com/mishoo/UglifyJS2/issues/44
            //    https://github.com/mishoo/UglifyJS2/issues/1830
            //    https://github.com/mishoo/UglifyJS2/issues/1838
            // that's an invalid AST.
            // We fix it at this stage by moving the `var` outside the `for`.
            if (node instanceof AST_For) {
                descend(node, this);
                var block;
                if (node.init instanceof AST_BlockStatement) {
                    block = node.init;
                    node.init = block.body.pop();
                    block.body.push(node);
                }
                if (node.init instanceof AST_SimpleStatement) {
                    node.init = node.init.body;
                } else if (is_empty(node.init)) {
                    node.init = null;
                }
                return !block ? node : in_list ? MAP.splice(block.body) : block;
            }
            if (node instanceof AST_LabeledStatement
                && node.body instanceof AST_For
            ) {
                descend(node, this);
                if (node.body instanceof AST_BlockStatement) {
                    var block = node.body;
                    node.body = block.body.pop();
                    block.body.push(node);
                    return in_list ? MAP.splice(block.body) : block;
                }
                return node;
            }
            if (node instanceof AST_BlockStatement) {
                descend(node, this);
                if (in_list && node.body.every(can_be_evicted_from_block)) {
                    return MAP.splice(node.body);
                }
                return node;
            }
            if (node instanceof AST_Scope) {
                const save_scope = scope;
                scope = node;
                descend(node, this);
                scope = save_scope;
                return node;
            }
        }
    );

    self.transform(tt);

    function scan_ref_scoped(node, descend) {
        var node_def;
        const sym = assign_as_unused(node);
        if (sym instanceof AST_SymbolRef
            && !is_ref_of(node.left, AST_SymbolBlockDeclaration)
            && self.variables.get(sym.name) === (node_def = sym.definition())
        ) {
            if (node instanceof AST_Assign) {
                node.right.walk(tw);
                if (!node_def.chained && node.left.fixed_value() === node.right) {
                    fixed_ids.set(node_def.id, node);
                }
            }
            return true;
        }
        if (node instanceof AST_SymbolRef) {
            node_def = node.definition();
            if (!in_use_ids.has(node_def.id)) {
                in_use_ids.set(node_def.id, node_def);
                if (node_def.orig[0] instanceof AST_SymbolCatch) {
                    const redef = node_def.scope.is_block_scope()
                        && node_def.scope.get_defun_scope().variables.get(node_def.name);
                    if (redef) in_use_ids.set(redef.id, redef);
                }
            }
            return true;
        }
        if (node instanceof AST_Scope) {
            var save_scope = scope;
            scope = node;
            descend();
            scope = save_scope;
            return true;
        }
    }
});

AST_Scope.DEFMETHOD("hoist_declarations", function(compressor) {
    var self = this;
    if (compressor.has_directive("use asm")) return self;
    // Hoisting makes no sense in an arrow func
    if (!Array.isArray(self.body)) return self;

    var hoist_funs = compressor.option("hoist_funs");
    var hoist_vars = compressor.option("hoist_vars");

    if (hoist_funs || hoist_vars) {
        var dirs = [];
        var hoisted = [];
        var vars = new Map(), vars_found = 0, var_decl = 0;
        // let's count var_decl first, we seem to waste a lot of
        // space if we hoist `var` when there's only one.
        walk(self, node => {
            if (node instanceof AST_Scope && node !== self)
                return true;
            if (node instanceof AST_Var) {
                ++var_decl;
                return true;
            }
        });
        hoist_vars = hoist_vars && var_decl > 1;
        var tt = new TreeTransformer(
            function before(node) {
                if (node !== self) {
                    if (node instanceof AST_Directive) {
                        dirs.push(node);
                        return make_node(AST_EmptyStatement, node);
                    }
                    if (hoist_funs && node instanceof AST_Defun
                        && !(tt.parent() instanceof AST_Export)
                        && tt.parent() === self) {
                        hoisted.push(node);
                        return make_node(AST_EmptyStatement, node);
                    }
                    if (
                        hoist_vars
                        && node instanceof AST_Var
                        && !node.definitions.some(def => def.name instanceof AST_Destructuring)
                    ) {
                        node.definitions.forEach(function(def) {
                            vars.set(def.name.name, def);
                            ++vars_found;
                        });
                        var seq = node.to_assignments(compressor);
                        var p = tt.parent();
                        if (p instanceof AST_ForIn && p.init === node) {
                            if (seq == null) {
                                var def = node.definitions[0].name;
                                return make_node(AST_SymbolRef, def, def);
                            }
                            return seq;
                        }
                        if (p instanceof AST_For && p.init === node) {
                            return seq;
                        }
                        if (!seq) return make_node(AST_EmptyStatement, node);
                        return make_node(AST_SimpleStatement, node, {
                            body: seq
                        });
                    }
                    if (node instanceof AST_Scope)
                        return node; // to avoid descending in nested scopes
                }
            }
        );
        self = self.transform(tt);
        if (vars_found > 0) {
            // collect only vars which don't show up in self's arguments list
            var defs = [];
            const is_lambda = self instanceof AST_Lambda;
            const args_as_names = is_lambda ? self.args_as_names() : null;
            vars.forEach((def, name) => {
                if (is_lambda && args_as_names.some((x) => x.name === def.name.name)) {
                    vars.delete(name);
                } else {
                    def = def.clone();
                    def.value = null;
                    defs.push(def);
                    vars.set(name, def);
                }
            });
            if (defs.length > 0) {
                // try to merge in assignments
                for (var i = 0; i < self.body.length;) {
                    if (self.body[i] instanceof AST_SimpleStatement) {
                        var expr = self.body[i].body, sym, assign;
                        if (expr instanceof AST_Assign
                            && expr.operator == "="
                            && (sym = expr.left) instanceof AST_Symbol
                            && vars.has(sym.name)
                        ) {
                            var def = vars.get(sym.name);
                            if (def.value) break;
                            def.value = expr.right;
                            remove(defs, def);
                            defs.push(def);
                            self.body.splice(i, 1);
                            continue;
                        }
                        if (expr instanceof AST_Sequence
                            && (assign = expr.expressions[0]) instanceof AST_Assign
                            && assign.operator == "="
                            && (sym = assign.left) instanceof AST_Symbol
                            && vars.has(sym.name)
                        ) {
                            var def = vars.get(sym.name);
                            if (def.value) break;
                            def.value = assign.right;
                            remove(defs, def);
                            defs.push(def);
                            self.body[i].body = make_sequence(expr, expr.expressions.slice(1));
                            continue;
                        }
                    }
                    if (self.body[i] instanceof AST_EmptyStatement) {
                        self.body.splice(i, 1);
                        continue;
                    }
                    if (self.body[i] instanceof AST_BlockStatement) {
                        self.body.splice(i, 1, ...self.body[i].body);
                        continue;
                    }
                    break;
                }
                defs = make_node(AST_Var, self, {
                    definitions: defs
                });
                hoisted.push(defs);
            }
        }
        self.body = dirs.concat(hoisted, self.body);
    }
    return self;
});

AST_Scope.DEFMETHOD("hoist_properties", function(compressor) {
    var self = this;
    if (!compressor.option("hoist_props") || compressor.has_directive("use asm")) return self;
    var top_retain = self instanceof AST_Toplevel && compressor.top_retain || return_false;
    var defs_by_id = new Map();
    var hoister = new TreeTransformer(function(node, descend) {
        if (node instanceof AST_Definitions
            && hoister.parent() instanceof AST_Export) return node;
        if (node instanceof AST_VarDef) {
            const sym = node.name;
            let def;
            let value;
            if (sym.scope === self
                && (def = sym.definition()).escaped != 1
                && !def.assignments
                && !def.direct_access
                && !def.single_use
                && !compressor.exposed(def)
                && !top_retain(def)
                && (value = sym.fixed_value()) === node.value
                && value instanceof AST_Object
                && !value.properties.some(prop =>
                    prop instanceof AST_Expansion || prop.computed_key()
                )
            ) {
                descend(node, this);
                const defs = new Map();
                const assignments = [];
                value.properties.forEach(({ key, value }) => {
                    const scope = find_scope(hoister);
                    const symbol = self.create_symbol(sym.CTOR, {
                        source: sym,
                        scope,
                        conflict_scopes: new Set([
                            scope,
                            ...sym.definition().references.map(ref => ref.scope)
                        ]),
                        tentative_name: sym.name + "_" + key
                    });

                    defs.set(String(key), symbol.definition());

                    assignments.push(make_node(AST_VarDef, node, {
                        name: symbol,
                        value
                    }));
                });
                defs_by_id.set(def.id, defs);
                return MAP.splice(assignments);
            }
        } else if (node instanceof AST_PropAccess
            && node.expression instanceof AST_SymbolRef
        ) {
            const defs = defs_by_id.get(node.expression.definition().id);
            if (defs) {
                const def = defs.get(String(get_simple_key(node.property)));
                const sym = make_node(AST_SymbolRef, node, {
                    name: def.name,
                    scope: node.expression.scope,
                    thedef: def
                });
                sym.reference({});
                return sym;
            }
        }
    });
    return self.transform(hoister);
});

def_optimize(AST_SimpleStatement, function(self, compressor) {
    if (compressor.option("side_effects")) {
        var body = self.body;
        var node = body.drop_side_effect_free(compressor, true);
        if (!node) {
            return make_node(AST_EmptyStatement, self);
        }
        if (node !== body) {
            return make_node(AST_SimpleStatement, self, { body: node });
        }
    }
    return self;
});

def_optimize(AST_While, function(self, compressor) {
    return compressor.option("loops") ? make_node(AST_For, self, self).optimize(compressor) : self;
});

def_optimize(AST_Do, function(self, compressor) {
    if (!compressor.option("loops")) return self;
    var cond = self.condition.tail_node().evaluate(compressor);
    if (!(cond instanceof AST_Node)) {
        if (cond) return make_node(AST_For, self, {
            body: make_node(AST_BlockStatement, self.body, {
                body: [
                    self.body,
                    make_node(AST_SimpleStatement, self.condition, {
                        body: self.condition
                    })
                ]
            })
        }).optimize(compressor);
        if (!has_break_or_continue(self, compressor.parent())) {
            return make_node(AST_BlockStatement, self.body, {
                body: [
                    self.body,
                    make_node(AST_SimpleStatement, self.condition, {
                        body: self.condition
                    })
                ]
            }).optimize(compressor);
        }
    }
    return self;
});

function if_break_in_loop(self, compressor) {
    var first = self.body instanceof AST_BlockStatement ? self.body.body[0] : self.body;
    if (compressor.option("dead_code") && is_break(first)) {
        var body = [];
        if (self.init instanceof AST_Statement) {
            body.push(self.init);
        } else if (self.init) {
            body.push(make_node(AST_SimpleStatement, self.init, {
                body: self.init
            }));
        }
        if (self.condition) {
            body.push(make_node(AST_SimpleStatement, self.condition, {
                body: self.condition
            }));
        }
        trim_unreachable_code(compressor, self.body, body);
        return make_node(AST_BlockStatement, self, {
            body: body
        });
    }
    if (first instanceof AST_If) {
        if (is_break(first.body)) {
            if (self.condition) {
                self.condition = make_node(AST_Binary, self.condition, {
                    left: self.condition,
                    operator: "&&",
                    right: first.condition.negate(compressor),
                });
            } else {
                self.condition = first.condition.negate(compressor);
            }
            drop_it(first.alternative);
        } else if (is_break(first.alternative)) {
            if (self.condition) {
                self.condition = make_node(AST_Binary, self.condition, {
                    left: self.condition,
                    operator: "&&",
                    right: first.condition,
                });
            } else {
                self.condition = first.condition;
            }
            drop_it(first.body);
        }
    }
    return self;

    function is_break(node) {
        return node instanceof AST_Break
            && compressor.loopcontrol_target(node) === compressor.self();
    }

    function drop_it(rest) {
        rest = as_statement_array(rest);
        if (self.body instanceof AST_BlockStatement) {
            self.body = self.body.clone();
            self.body.body = rest.concat(self.body.body.slice(1));
            self.body = self.body.transform(compressor);
        } else {
            self.body = make_node(AST_BlockStatement, self.body, {
                body: rest
            }).transform(compressor);
        }
        self = if_break_in_loop(self, compressor);
    }
}

def_optimize(AST_For, function(self, compressor) {
    if (!compressor.option("loops")) return self;
    if (compressor.option("side_effects") && self.init) {
        self.init = self.init.drop_side_effect_free(compressor);
    }
    if (self.condition) {
        var cond = self.condition.evaluate(compressor);
        if (!(cond instanceof AST_Node)) {
            if (cond) self.condition = null;
            else if (!compressor.option("dead_code")) {
                var orig = self.condition;
                self.condition = make_node_from_constant(cond, self.condition);
                self.condition = best_of_expression(self.condition.transform(compressor), orig);
            }
        }
        if (compressor.option("dead_code")) {
            if (cond instanceof AST_Node) cond = self.condition.tail_node().evaluate(compressor);
            if (!cond) {
                var body = [];
                trim_unreachable_code(compressor, self.body, body);
                if (self.init instanceof AST_Statement) {
                    body.push(self.init);
                } else if (self.init) {
                    body.push(make_node(AST_SimpleStatement, self.init, {
                        body: self.init
                    }));
                }
                body.push(make_node(AST_SimpleStatement, self.condition, {
                    body: self.condition
                }));
                return make_node(AST_BlockStatement, self, { body: body }).optimize(compressor);
            }
        }
    }
    return if_break_in_loop(self, compressor);
});

def_optimize(AST_If, function(self, compressor) {
    if (is_empty(self.alternative)) self.alternative = null;

    if (!compressor.option("conditionals")) return self;
    // if condition can be statically determined, drop
    // one of the blocks.  note, statically determined implies
    // “has no side effects”; also it doesn't work for cases like
    // `x && true`, though it probably should.
    var cond = self.condition.evaluate(compressor);
    if (!compressor.option("dead_code") && !(cond instanceof AST_Node)) {
        var orig = self.condition;
        self.condition = make_node_from_constant(cond, orig);
        self.condition = best_of_expression(self.condition.transform(compressor), orig);
    }
    if (compressor.option("dead_code")) {
        if (cond instanceof AST_Node) cond = self.condition.tail_node().evaluate(compressor);
        if (!cond) {
            var body = [];
            trim_unreachable_code(compressor, self.body, body);
            body.push(make_node(AST_SimpleStatement, self.condition, {
                body: self.condition
            }));
            if (self.alternative) body.push(self.alternative);
            return make_node(AST_BlockStatement, self, { body: body }).optimize(compressor);
        } else if (!(cond instanceof AST_Node)) {
            var body = [];
            body.push(make_node(AST_SimpleStatement, self.condition, {
                body: self.condition
            }));
            body.push(self.body);
            if (self.alternative) {
                trim_unreachable_code(compressor, self.alternative, body);
            }
            return make_node(AST_BlockStatement, self, { body: body }).optimize(compressor);
        }
    }
    var negated = self.condition.negate(compressor);
    var self_condition_length = self.condition.size();
    var negated_length = negated.size();
    var negated_is_best = negated_length < self_condition_length;
    if (self.alternative && negated_is_best) {
        negated_is_best = false; // because we already do the switch here.
        // no need to swap values of self_condition_length and negated_length
        // here because they are only used in an equality comparison later on.
        self.condition = negated;
        var tmp = self.body;
        self.body = self.alternative || make_node(AST_EmptyStatement, self);
        self.alternative = tmp;
    }
    if (is_empty(self.body) && is_empty(self.alternative)) {
        return make_node(AST_SimpleStatement, self.condition, {
            body: self.condition.clone()
        }).optimize(compressor);
    }
    if (self.body instanceof AST_SimpleStatement
        && self.alternative instanceof AST_SimpleStatement) {
        return make_node(AST_SimpleStatement, self, {
            body: make_node(AST_Conditional, self, {
                condition   : self.condition,
                consequent  : self.body.body,
                alternative : self.alternative.body
            })
        }).optimize(compressor);
    }
    if (is_empty(self.alternative) && self.body instanceof AST_SimpleStatement) {
        if (self_condition_length === negated_length && !negated_is_best
            && self.condition instanceof AST_Binary && self.condition.operator == "||") {
            // although the code length of self.condition and negated are the same,
            // negated does not require additional surrounding parentheses.
            // see https://github.com/mishoo/UglifyJS2/issues/979
            negated_is_best = true;
        }
        if (negated_is_best) return make_node(AST_SimpleStatement, self, {
            body: make_node(AST_Binary, self, {
                operator : "||",
                left     : negated,
                right    : self.body.body
            })
        }).optimize(compressor);
        return make_node(AST_SimpleStatement, self, {
            body: make_node(AST_Binary, self, {
                operator : "&&",
                left     : self.condition,
                right    : self.body.body
            })
        }).optimize(compressor);
    }
    if (self.body instanceof AST_EmptyStatement
        && self.alternative instanceof AST_SimpleStatement) {
        return make_node(AST_SimpleStatement, self, {
            body: make_node(AST_Binary, self, {
                operator : "||",
                left     : self.condition,
                right    : self.alternative.body
            })
        }).optimize(compressor);
    }
    if (self.body instanceof AST_Exit
        && self.alternative instanceof AST_Exit
        && self.body.TYPE == self.alternative.TYPE) {
        return make_node(self.body.CTOR, self, {
            value: make_node(AST_Conditional, self, {
                condition   : self.condition,
                consequent  : self.body.value || make_node(AST_Undefined, self.body),
                alternative : self.alternative.value || make_node(AST_Undefined, self.alternative)
            }).transform(compressor)
        }).optimize(compressor);
    }
    if (self.body instanceof AST_If
        && !self.body.alternative
        && !self.alternative) {
        self = make_node(AST_If, self, {
            condition: make_node(AST_Binary, self.condition, {
                operator: "&&",
                left: self.condition,
                right: self.body.condition
            }),
            body: self.body.body,
            alternative: null
        });
    }
    if (aborts(self.body)) {
        if (self.alternative) {
            var alt = self.alternative;
            self.alternative = null;
            return make_node(AST_BlockStatement, self, {
                body: [ self, alt ]
            }).optimize(compressor);
        }
    }
    if (aborts(self.alternative)) {
        var body = self.body;
        self.body = self.alternative;
        self.condition = negated_is_best ? negated : self.condition.negate(compressor);
        self.alternative = null;
        return make_node(AST_BlockStatement, self, {
            body: [ self, body ]
        }).optimize(compressor);
    }
    return self;
});

def_optimize(AST_Switch, function(self, compressor) {
    if (!compressor.option("switches")) return self;
    var branch;
    var value = self.expression.evaluate(compressor);
    if (!(value instanceof AST_Node)) {
        var orig = self.expression;
        self.expression = make_node_from_constant(value, orig);
        self.expression = best_of_expression(self.expression.transform(compressor), orig);
    }
    if (!compressor.option("dead_code")) return self;
    if (value instanceof AST_Node) {
        value = self.expression.tail_node().evaluate(compressor);
    }
    var decl = [];
    var body = [];
    var default_branch;
    var exact_match;
    for (var i = 0, len = self.body.length; i < len && !exact_match; i++) {
        branch = self.body[i];
        if (branch instanceof AST_Default) {
            if (!default_branch) {
                default_branch = branch;
            } else {
                eliminate_branch(branch, body[body.length - 1]);
            }
        } else if (!(value instanceof AST_Node)) {
            var exp = branch.expression.evaluate(compressor);
            if (!(exp instanceof AST_Node) && exp !== value) {
                eliminate_branch(branch, body[body.length - 1]);
                continue;
            }
            if (exp instanceof AST_Node) exp = branch.expression.tail_node().evaluate(compressor);
            if (exp === value) {
                exact_match = branch;
                if (default_branch) {
                    var default_index = body.indexOf(default_branch);
                    body.splice(default_index, 1);
                    eliminate_branch(default_branch, body[default_index - 1]);
                    default_branch = null;
                }
            }
        }
        body.push(branch);
    }
    while (i < len) eliminate_branch(self.body[i++], body[body.length - 1]);
    self.body = body;

    for (let i = 0; i < body.length; i++) {
        let branch = body[i];
        if (branch.body.length === 0) continue;
        if (!aborts(branch)) continue;

        for (let j = i + 1; j < body.length; i++, j++) {
            let next = body[j];
            if (next.body.length === 0) continue;
            if (
                branches_equivalent(next, branch, false)
                || (j === body.length - 1 && branches_equivalent(next, branch, true))
            ) {
                branch.body = [];
                branch = next;
                continue;
            }
            break;
        }
    }

    let default_or_exact = default_branch || exact_match;
    default_branch = null;
    exact_match = null;

    // Prune any empty branches at the end of the switch statement.
    {
        let i = body.length - 1;
        for (; i >= 0; i--) {
            let bbody = body[i].body;
            if (is_break(bbody[bbody.length - 1], compressor)) bbody.pop();
            if (!is_inert_body(body[i])) break;
        }
        // i now points to the index of a branch that contains a body. By incrementing, it's
        // pointing to the first branch that's empty.
        i++;
        if (!default_or_exact || body.indexOf(default_or_exact) >= i) {
            // The default behavior is to do nothing. We can take advantage of that to
            // remove all case expressions that are side-effect free that also do
            // nothing, since they'll default to doing nothing. But we can't remove any
            // case expressions before one that would side-effect, since they may cause
            // the side-effect to be skipped.
            for (let j = body.length - 1; j >= i; j--) {
                let branch = body[j];
                if (branch === default_or_exact) {
                    default_or_exact = null;
                    body.pop();
                } else if (!branch.expression.has_side_effects(compressor)) {
                    body.pop();
                } else {
                    break;
                }
            }
        }
    }


    // Prune side-effect free branches that fall into default.
    if (default_or_exact) {
        let default_index = body.indexOf(default_or_exact);
        let default_body_index = default_index;
        for (; default_body_index < body.length - 1; default_body_index++) {
            if (!is_inert_body(body[default_body_index])) break;
        }
        let side_effect_index = body.length - 1;
        for (; side_effect_index >= 0; side_effect_index--) {
            let branch = body[side_effect_index];
            if (branch === default_or_exact) continue;
            if (branch.expression.has_side_effects(compressor)) break;
        }
        // If the default behavior comes after any side-effect case expressions,
        // then we can fold all side-effect free cases into the default branch.
        // If the side-effect case is after the default, then any side-effect
        // free cases could prevent the side-effect from occurring.
        if (default_body_index > side_effect_index) {
            let prev_body_index = default_index - 1;
            for (; prev_body_index >= 0; prev_body_index--) {
                if (!is_inert_body(body[prev_body_index])) break;
            }
            let before = Math.max(side_effect_index, prev_body_index) + 1;
            let after = default_index;
            if (side_effect_index > default_index) {
                // If the default falls into the same body as a side-effect
                // case, then we need preserve that case and only prune the
                // cases after it.
                after = side_effect_index;
                body[side_effect_index].body = body[default_body_index].body;
            } else {
                // The default will be the last branch.
                default_or_exact.body = body[default_body_index].body;
            }

            // Prune everything after the default (or last side-effect case)
            // until the next case with a body.
            body.splice(after + 1, default_body_index - after);
            // Prune everything before the default that falls into it.
            body.splice(before, default_index - before);
        }
    }

    // See if we can remove the switch entirely if all cases (the default) fall into the same case body.
    DEFAULT: if (default_or_exact) {
        let i = body.findIndex(branch => !is_inert_body(branch));
        let caseBody;
        // `i` is equal to one of the following:
        // - `-1`, there is no body in the switch statement.
        // - `body.length - 1`, all cases fall into the same body.
        // - anything else, there are multiple bodies in the switch.
        if (i === body.length - 1) {
            // All cases fall into the case body.
            let branch = body[i];
            if (has_nested_break(self)) break DEFAULT;

            // This is the last case body, and we've already pruned any breaks, so it's
            // safe to hoist.
            caseBody = make_node(AST_BlockStatement, branch, {
                body: branch.body
            });
            branch.body = [];
        } else if (i !== -1) {
            // If there are multiple bodies, then we cannot optimize anything.
            break DEFAULT;
        }

        let sideEffect = body.find(branch => {
            return (
                branch !== default_or_exact
                && branch.expression.has_side_effects(compressor)
            );
        });
        // If no cases cause a side-effect, we can eliminate the switch entirely.
        if (!sideEffect) {
            return make_node(AST_BlockStatement, self, {
                body: decl.concat(
                    statement(self.expression),
                    default_or_exact.expression ? statement(default_or_exact.expression) : [],
                    caseBody || []
                )
            }).optimize(compressor);
        }

        // If we're this far, either there was no body or all cases fell into the same body.
        // If there was no body, then we don't need a default branch (because the default is
        // do nothing). If there was a body, we'll extract it to after the switch, so the
        // switch's new default is to do nothing and we can still prune it.
        const default_index = body.indexOf(default_or_exact);
        body.splice(default_index, 1);
        default_or_exact = null;

        if (caseBody) {
            // Recurse into switch statement one more time so that we can append the case body
            // outside of the switch. This recursion will only happen once since we've pruned
            // the default case.
            return make_node(AST_BlockStatement, self, {
                body: decl.concat(self, caseBody)
            }).optimize(compressor);
        }
        // If we fall here, there is a default branch somewhere, there are no case bodies,
        // and there's a side-effect somewhere. Just let the below paths take care of it.
    }

    if (body.length > 0) {
        body[0].body = decl.concat(body[0].body);
    }

    if (body.length == 0) {
        return make_node(AST_BlockStatement, self, {
            body: decl.concat(statement(self.expression))
        }).optimize(compressor);
    }
    if (body.length == 1 && !has_nested_break(self)) {
        // This is the last case body, and we've already pruned any breaks, so it's
        // safe to hoist.
        let branch = body[0];
        return make_node(AST_If, self, {
            condition: make_node(AST_Binary, self, {
                operator: "===",
                left: self.expression,
                right: branch.expression,
            }),
            body: make_node(AST_BlockStatement, branch, {
                body: branch.body
            }),
            alternative: null
        }).optimize(compressor);
    }
    if (body.length === 2 && default_or_exact && !has_nested_break(self)) {
        let branch = body[0] === default_or_exact ? body[1] : body[0];
        let exact_exp = default_or_exact.expression && statement(default_or_exact.expression);
        if (aborts(body[0])) {
            // Only the first branch body could have a break (at the last statement)
            let first = body[0];
            if (is_break(first.body[first.body.length - 1], compressor)) {
                first.body.pop();
            }
            return make_node(AST_If, self, {
                condition: make_node(AST_Binary, self, {
                    operator: "===",
                    left: self.expression,
                    right: branch.expression,
                }),
                body: make_node(AST_BlockStatement, branch, {
                    body: branch.body
                }),
                alternative: make_node(AST_BlockStatement, default_or_exact, {
                    body: [].concat(
                        exact_exp || [],
                        default_or_exact.body
                    )
                })
            }).optimize(compressor);
        }
        let operator = "===";
        let consequent = make_node(AST_BlockStatement, branch, {
            body: branch.body,
        });
        let always = make_node(AST_BlockStatement, default_or_exact, {
            body: [].concat(
                exact_exp || [],
                default_or_exact.body
            )
        });
        if (body[0] === default_or_exact) {
            operator = "!==";
            let tmp = always;
            always = consequent;
            consequent = tmp;
        }
        return make_node(AST_BlockStatement, self, {
            body: [
                make_node(AST_If, self, {
                    condition: make_node(AST_Binary, self, {
                        operator: operator,
                        left: self.expression,
                        right: branch.expression,
                    }),
                    body: consequent,
                    alternative: null
                })
            ].concat(always)
        }).optimize(compressor);
    }
    return self;

    function eliminate_branch(branch, prev) {
        if (prev && !aborts(prev)) {
            prev.body = prev.body.concat(branch.body);
        } else {
            trim_unreachable_code(compressor, branch, decl);
        }
    }
    function branches_equivalent(branch, prev, insertBreak) {
        let bbody = branch.body;
        let pbody = prev.body;
        if (insertBreak) {
            bbody = bbody.concat(make_node(AST_Break));
        }
        if (bbody.length !== pbody.length) return false;
        let bblock = make_node(AST_BlockStatement, branch, { body: bbody });
        let pblock = make_node(AST_BlockStatement, prev, { body: pbody });
        return bblock.equivalent_to(pblock);
    }
    function statement(expression) {
        return make_node(AST_SimpleStatement, expression, {
            body: expression
        });
    }
    function has_nested_break(root) {
        let has_break = false;
        let tw = new TreeWalker(node => {
            if (has_break) return true;
            if (node instanceof AST_Lambda) return true;
            if (node instanceof AST_SimpleStatement) return true;
            if (!is_break(node, tw)) return;
            let parent = tw.parent();
            if (
                parent instanceof AST_SwitchBranch
                && parent.body[parent.body.length - 1] === node
            ) {
                return;
            }
            has_break = true;
        });
        root.walk(tw);
        return has_break;
    }
    function is_break(node, stack) {
        return node instanceof AST_Break
            && stack.loopcontrol_target(node) === self;
    }
    function is_inert_body(branch) {
        return !aborts(branch) && !make_node(AST_BlockStatement, branch, {
            body: branch.body
        }).has_side_effects(compressor);
    }
});

def_optimize(AST_Try, function(self, compressor) {
    tighten_body(self.body, compressor);
    if (self.bcatch && self.bfinally && self.bfinally.body.every(is_empty)) self.bfinally = null;
    if (compressor.option("dead_code") && self.body.every(is_empty)) {
        var body = [];
        if (self.bcatch) {
            trim_unreachable_code(compressor, self.bcatch, body);
        }
        if (self.bfinally) body.push(...self.bfinally.body);
        return make_node(AST_BlockStatement, self, {
            body: body
        }).optimize(compressor);
    }
    return self;
});

AST_Definitions.DEFMETHOD("remove_initializers", function() {
    var decls = [];
    this.definitions.forEach(function(def) {
        if (def.name instanceof AST_SymbolDeclaration) {
            def.value = null;
            decls.push(def);
        } else {
            walk(def.name, node => {
                if (node instanceof AST_SymbolDeclaration) {
                    decls.push(make_node(AST_VarDef, def, {
                        name: node,
                        value: null
                    }));
                }
            });
        }
    });
    this.definitions = decls;
});

AST_Definitions.DEFMETHOD("to_assignments", function(compressor) {
    var reduce_vars = compressor.option("reduce_vars");
    var assignments = [];

    for (const def of this.definitions) {
        if (def.value) {
            var name = make_node(AST_SymbolRef, def.name, def.name);
            assignments.push(make_node(AST_Assign, def, {
                operator : "=",
                logical: false,
                left     : name,
                right    : def.value
            }));
            if (reduce_vars) name.definition().fixed = false;
        } else if (def.value) {
            // Because it's a destructuring, do not turn into an assignment.
            var varDef = make_node(AST_VarDef, def, {
                name: def.name,
                value: def.value
            });
            var var_ = make_node(AST_Var, def, {
                definitions: [ varDef ]
            });
            assignments.push(var_);
        }
        const thedef = def.name.definition();
        thedef.eliminated++;
        thedef.replaced--;
    }

    if (assignments.length == 0) return null;
    return make_sequence(this, assignments);
});

def_optimize(AST_Definitions, function(self) {
    if (self.definitions.length == 0)
        return make_node(AST_EmptyStatement, self);
    return self;
});

def_optimize(AST_VarDef, function(self, compressor) {
    if (
        self.name instanceof AST_SymbolLet
        && self.value != null
        && is_undefined(self.value, compressor)
    ) {
        self.value = null;
    }
    return self;
});

def_optimize(AST_Import, function(self) {
    return self;
});

// TODO this only works with AST_Defun, shouldn't it work for other ways of defining functions?
function retain_top_func(fn, compressor) {
    return compressor.top_retain
        && fn instanceof AST_Defun
        && has_flag(fn, TOP)
        && fn.name
        && compressor.top_retain(fn.name);
}

def_optimize(AST_Call, function(self, compressor) {
    var exp = self.expression;
    var fn = exp;
    inline_array_like_spread(self.args);
    var simple_args = self.args.every((arg) =>
        !(arg instanceof AST_Expansion)
    );

    if (compressor.option("reduce_vars")
        && fn instanceof AST_SymbolRef
        && !has_annotation(self, _NOINLINE)
    ) {
        const fixed = fn.fixed_value();
        if (!retain_top_func(fixed, compressor)) {
            fn = fixed;
        }
    }

    var is_func = fn instanceof AST_Lambda;

    if (is_func && fn.pinned()) return self;

    if (compressor.option("unused")
        && simple_args
        && is_func
        && !fn.uses_arguments) {
        var pos = 0, last = 0;
        for (var i = 0, len = self.args.length; i < len; i++) {
            if (fn.argnames[i] instanceof AST_Expansion) {
                if (has_flag(fn.argnames[i].expression, UNUSED)) while (i < len) {
                    var node = self.args[i++].drop_side_effect_free(compressor);
                    if (node) {
                        self.args[pos++] = node;
                    }
                } else while (i < len) {
                    self.args[pos++] = self.args[i++];
                }
                last = pos;
                break;
            }
            var trim = i >= fn.argnames.length;
            if (trim || has_flag(fn.argnames[i], UNUSED)) {
                var node = self.args[i].drop_side_effect_free(compressor);
                if (node) {
                    self.args[pos++] = node;
                } else if (!trim) {
                    self.args[pos++] = make_node(AST_Number, self.args[i], {
                        value: 0
                    });
                    continue;
                }
            } else {
                self.args[pos++] = self.args[i];
            }
            last = pos;
        }
        self.args.length = last;
    }

    if (compressor.option("unsafe")) {
        if (exp instanceof AST_Dot && exp.start.value === "Array" && exp.property === "from" && self.args.length === 1) {
            const [argument] = self.args;
            if (argument instanceof AST_Array) {
                return make_node(AST_Array, argument, {
                    elements: argument.elements
                }).optimize(compressor);
            }
        }
        if (is_undeclared_ref(exp)) switch (exp.name) {
          case "Array":
            if (self.args.length != 1) {
                return make_node(AST_Array, self, {
                    elements: self.args
                }).optimize(compressor);
            } else if (self.args[0] instanceof AST_Number && self.args[0].value <= 11) {
                const elements = [];
                for (let i = 0; i < self.args[0].value; i++) elements.push(new AST_Hole);
                return new AST_Array({ elements });
            }
            break;
          case "Object":
            if (self.args.length == 0) {
                return make_node(AST_Object, self, {
                    properties: []
                });
            }
            break;
          case "String":
            if (self.args.length == 0) return make_node(AST_String, self, {
                value: ""
            });
            if (self.args.length <= 1) return make_node(AST_Binary, self, {
                left: self.args[0],
                operator: "+",
                right: make_node(AST_String, self, { value: "" })
            }).optimize(compressor);
            break;
          case "Number":
            if (self.args.length == 0) return make_node(AST_Number, self, {
                value: 0
            });
            if (self.args.length == 1 && compressor.option("unsafe_math")) {
                return make_node(AST_UnaryPrefix, self, {
                    expression: self.args[0],
                    operator: "+"
                }).optimize(compressor);
            }
            break;
          case "Symbol":
            if (self.args.length == 1 && self.args[0] instanceof AST_String && compressor.option("unsafe_symbols"))
                self.args.length = 0;
                break;
          case "Boolean":
            if (self.args.length == 0) return make_node(AST_False, self);
            if (self.args.length == 1) return make_node(AST_UnaryPrefix, self, {
                expression: make_node(AST_UnaryPrefix, self, {
                    expression: self.args[0],
                    operator: "!"
                }),
                operator: "!"
            }).optimize(compressor);
            break;
          case "RegExp":
            var params = [];
            if (self.args.length >= 1
                && self.args.length <= 2
                && self.args.every((arg) => {
                    var value = arg.evaluate(compressor);
                    params.push(value);
                    return arg !== value;
                })
            ) {
                let [ source, flags ] = params;
                source = regexp_source_fix(new RegExp(source).source);
                const rx = make_node(AST_RegExp, self, {
                    value: { source, flags }
                });
                if (rx._eval(compressor) !== rx) {
                    return rx;
                }
            }
            break;
        } else if (exp instanceof AST_Dot) switch(exp.property) {
          case "toString":
            if (self.args.length == 0 && !exp.expression.may_throw_on_access(compressor)) {
                return make_node(AST_Binary, self, {
                    left: make_node(AST_String, self, { value: "" }),
                    operator: "+",
                    right: exp.expression
                }).optimize(compressor);
            }
            break;
          case "join":
            if (exp.expression instanceof AST_Array) EXIT: {
                var separator;
                if (self.args.length > 0) {
                    separator = self.args[0].evaluate(compressor);
                    if (separator === self.args[0]) break EXIT; // not a constant
                }
                var elements = [];
                var consts = [];
                for (var i = 0, len = exp.expression.elements.length; i < len; i++) {
                    var el = exp.expression.elements[i];
                    if (el instanceof AST_Expansion) break EXIT;
                    var value = el.evaluate(compressor);
                    if (value !== el) {
                        consts.push(value);
                    } else {
                        if (consts.length > 0) {
                            elements.push(make_node(AST_String, self, {
                                value: consts.join(separator)
                            }));
                            consts.length = 0;
                        }
                        elements.push(el);
                    }
                }
                if (consts.length > 0) {
                    elements.push(make_node(AST_String, self, {
                        value: consts.join(separator)
                    }));
                }
                if (elements.length == 0) return make_node(AST_String, self, { value: "" });
                if (elements.length == 1) {
                    if (elements[0].is_string(compressor)) {
                        return elements[0];
                    }
                    return make_node(AST_Binary, elements[0], {
                        operator : "+",
                        left     : make_node(AST_String, self, { value: "" }),
                        right    : elements[0]
                    });
                }
                if (separator == "") {
                    var first;
                    if (elements[0].is_string(compressor)
                        || elements[1].is_string(compressor)) {
                        first = elements.shift();
                    } else {
                        first = make_node(AST_String, self, { value: "" });
                    }
                    return elements.reduce(function(prev, el) {
                        return make_node(AST_Binary, el, {
                            operator : "+",
                            left     : prev,
                            right    : el
                        });
                    }, first).optimize(compressor);
                }
                // need this awkward cloning to not affect original element
                // best_of will decide which one to get through.
                var node = self.clone();
                node.expression = node.expression.clone();
                node.expression.expression = node.expression.expression.clone();
                node.expression.expression.elements = elements;
                return best_of(compressor, self, node);
            }
            break;
          case "charAt":
            if (exp.expression.is_string(compressor)) {
                var arg = self.args[0];
                var index = arg ? arg.evaluate(compressor) : 0;
                if (index !== arg) {
                    return make_node(AST_Sub, exp, {
                        expression: exp.expression,
                        property: make_node_from_constant(index | 0, arg || exp)
                    }).optimize(compressor);
                }
            }
            break;
          case "apply":
            if (self.args.length == 2 && self.args[1] instanceof AST_Array) {
                var args = self.args[1].elements.slice();
                args.unshift(self.args[0]);
                return make_node(AST_Call, self, {
                    expression: make_node(AST_Dot, exp, {
                        expression: exp.expression,
                        optional: false,
                        property: "call"
                    }),
                    args: args
                }).optimize(compressor);
            }
            break;
          case "call":
            var func = exp.expression;
            if (func instanceof AST_SymbolRef) {
                func = func.fixed_value();
            }
            if (func instanceof AST_Lambda && !func.contains_this()) {
                return (self.args.length ? make_sequence(this, [
                    self.args[0],
                    make_node(AST_Call, self, {
                        expression: exp.expression,
                        args: self.args.slice(1)
                    })
                ]) : make_node(AST_Call, self, {
                    expression: exp.expression,
                    args: []
                })).optimize(compressor);
            }
            break;
        }
    }

    if (compressor.option("unsafe_Function")
        && is_undeclared_ref(exp)
        && exp.name == "Function") {
        // new Function() => function(){}
        if (self.args.length == 0) return make_node(AST_Function, self, {
            argnames: [],
            body: []
        }).optimize(compressor);
        var nth_identifier = compressor.mangle_options && compressor.mangle_options.nth_identifier || base54;
        if (self.args.every((x) => x instanceof AST_String)) {
            // quite a corner-case, but we can handle it:
            //   https://github.com/mishoo/UglifyJS2/issues/203
            // if the code argument is a constant, then we can minify it.
            try {
                var code = "n(function(" + self.args.slice(0, -1).map(function(arg) {
                    return arg.value;
                }).join(",") + "){" + self.args[self.args.length - 1].value + "})";
                var ast = parse(code);
                var mangle = { ie8: compressor.option("ie8"), nth_identifier: nth_identifier };
                ast.figure_out_scope(mangle);
                var comp = new Compressor(compressor.options, {
                    mangle_options: compressor.mangle_options
                });
                ast = ast.transform(comp);
                ast.figure_out_scope(mangle);
                ast.compute_char_frequency(mangle);
                ast.mangle_names(mangle);
                var fun;
                walk(ast, node => {
                    if (is_func_expr(node)) {
                        fun = node;
                        return walk_abort;
                    }
                });
                var code = OutputStream();
                AST_BlockStatement.prototype._codegen.call(fun, fun, code);
                self.args = [
                    make_node(AST_String, self, {
                        value: fun.argnames.map(function(arg) {
                            return arg.print_to_string();
                        }).join(",")
                    }),
                    make_node(AST_String, self.args[self.args.length - 1], {
                        value: code.get().replace(/^{|}$/g, "")
                    })
                ];
                return self;
            } catch (ex) {
                if (!(ex instanceof JS_Parse_Error)) {
                    throw ex;
                }

                // Otherwise, it crashes at runtime. Or maybe it's nonstandard syntax.
            }
        }
    }

    var stat = is_func && fn.body[0];
    var is_regular_func = is_func && !fn.is_generator && !fn.async;
    var can_inline = is_regular_func && compressor.option("inline") && !self.is_callee_pure(compressor);
    if (can_inline && stat instanceof AST_Return) {
        let returned = stat.value;
        if (!returned || returned.is_constant_expression()) {
            if (returned) {
                returned = returned.clone(true);
            } else {
                returned = make_node(AST_Undefined, self);
            }
            const args = self.args.concat(returned);
            return make_sequence(self, args).optimize(compressor);
        }

        // optimize identity function
        if (
            fn.argnames.length === 1
            && (fn.argnames[0] instanceof AST_SymbolFunarg)
            && self.args.length < 2
            && returned instanceof AST_SymbolRef
            && returned.name === fn.argnames[0].name
        ) {
            const replacement =
                (self.args[0] || make_node(AST_Undefined)).optimize(compressor);

            let parent;
            if (
                replacement instanceof AST_PropAccess
                && (parent = compressor.parent()) instanceof AST_Call
                && parent.expression === self
            ) {
                // identity function was being used to remove `this`, like in
                //
                // id(bag.no_this)(...)
                //
                // Replace with a larger but more effish (0, bag.no_this) wrapper.

                return make_sequence(self, [
                    make_node(AST_Number, self, { value: 0 }),
                    replacement
                ]);
            }
            // replace call with first argument or undefined if none passed
            return replacement;
        }
    }

    if (can_inline) {
        var scope, in_loop, level = -1;
        let def;
        let returned_value;
        let nearest_scope;
        if (simple_args
            && !fn.uses_arguments
            && !(compressor.parent() instanceof AST_Class)
            && !(fn.name && fn instanceof AST_Function)
            && (returned_value = can_flatten_body(stat))
            && (exp === fn
                || has_annotation(self, _INLINE)
                || compressor.option("unused")
                    && (def = exp.definition()).references.length == 1
                    && !is_recursive_ref(compressor, def)
                    && fn.is_constant_expression(exp.scope))
            && !has_annotation(self, _PURE | _NOINLINE)
            && !fn.contains_this()
            && can_inject_symbols()
            && (nearest_scope = find_scope(compressor))
            && !scope_encloses_variables_in_this_scope(nearest_scope, fn)
            && !(function in_default_assign() {
                    // Due to the fact function parameters have their own scope
                    // which can't use `var something` in the function body within,
                    // we simply don't inline into DefaultAssign.
                    let i = 0;
                    let p;
                    while ((p = compressor.parent(i++))) {
                        if (p instanceof AST_DefaultAssign) return true;
                        if (p instanceof AST_Block) break;
                    }
                    return false;
                })()
            && !(scope instanceof AST_Class)
        ) {
            set_flag(fn, SQUEEZED);
            nearest_scope.add_child_scope(fn);
            return make_sequence(self, flatten_fn(returned_value)).optimize(compressor);
        }
    }

    if (can_inline && has_annotation(self, _INLINE)) {
        set_flag(fn, SQUEEZED);
        fn = make_node(fn.CTOR === AST_Defun ? AST_Function : fn.CTOR, fn, fn);
        fn.figure_out_scope({}, {
            parent_scope: find_scope(compressor),
            toplevel: compressor.get_toplevel()
        });

        return make_node(AST_Call, self, {
            expression: fn,
            args: self.args,
        }).optimize(compressor);
    }

    const can_drop_this_call = is_regular_func && compressor.option("side_effects") && fn.body.every(is_empty);
    if (can_drop_this_call) {
        var args = self.args.concat(make_node(AST_Undefined, self));
        return make_sequence(self, args).optimize(compressor);
    }

    if (compressor.option("negate_iife")
        && compressor.parent() instanceof AST_SimpleStatement
        && is_iife_call(self)) {
        return self.negate(compressor, true);
    }

    var ev = self.evaluate(compressor);
    if (ev !== self && ev !== nullish) {
        ev = make_node_from_constant(ev, self).optimize(compressor);
        return best_of(compressor, ev, self);
    }

    return self;

    function return_value(stat) {
        if (!stat) return make_node(AST_Undefined, self);
        if (stat instanceof AST_Return) {
            if (!stat.value) return make_node(AST_Undefined, self);
            return stat.value.clone(true);
        }
        if (stat instanceof AST_SimpleStatement) {
            return make_node(AST_UnaryPrefix, stat, {
                operator: "void",
                expression: stat.body.clone(true)
            });
        }
    }

    function can_flatten_body(stat) {
        var body = fn.body;
        var len = body.length;
        if (compressor.option("inline") < 3) {
            return len == 1 && return_value(stat);
        }
        stat = null;
        for (var i = 0; i < len; i++) {
            var line = body[i];
            if (line instanceof AST_Var) {
                if (stat && !line.definitions.every((var_def) =>
                    !var_def.value
                )) {
                    return false;
                }
            } else if (stat) {
                return false;
            } else if (!(line instanceof AST_EmptyStatement)) {
                stat = line;
            }
        }
        return return_value(stat);
    }

    function can_inject_args(block_scoped, safe_to_inject) {
        for (var i = 0, len = fn.argnames.length; i < len; i++) {
            var arg = fn.argnames[i];
            if (arg instanceof AST_DefaultAssign) {
                if (has_flag(arg.left, UNUSED)) continue;
                return false;
            }
            if (arg instanceof AST_Destructuring) return false;
            if (arg instanceof AST_Expansion) {
                if (has_flag(arg.expression, UNUSED)) continue;
                return false;
            }
            if (has_flag(arg, UNUSED)) continue;
            if (!safe_to_inject
                || block_scoped.has(arg.name)
                || identifier_atom.has(arg.name)
                || scope.conflicting_def(arg.name)) {
                return false;
            }
            if (in_loop) in_loop.push(arg.definition());
        }
        return true;
    }

    function can_inject_vars(block_scoped, safe_to_inject) {
        var len = fn.body.length;
        for (var i = 0; i < len; i++) {
            var stat = fn.body[i];
            if (!(stat instanceof AST_Var)) continue;
            if (!safe_to_inject) return false;
            for (var j = stat.definitions.length; --j >= 0;) {
                var name = stat.definitions[j].name;
                if (name instanceof AST_Destructuring
                    || block_scoped.has(name.name)
                    || identifier_atom.has(name.name)
                    || scope.conflicting_def(name.name)) {
                    return false;
                }
                if (in_loop) in_loop.push(name.definition());
            }
        }
        return true;
    }

    function can_inject_symbols() {
        var block_scoped = new Set();
        do {
            scope = compressor.parent(++level);
            if (scope.is_block_scope() && scope.block_scope) {
                // TODO this is sometimes undefined during compression.
                // But it should always have a value!
                scope.block_scope.variables.forEach(function (variable) {
                    block_scoped.add(variable.name);
                });
            }
            if (scope instanceof AST_Catch) {
                // TODO can we delete? AST_Catch is a block scope.
                if (scope.argname) {
                    block_scoped.add(scope.argname.name);
                }
            } else if (scope instanceof AST_IterationStatement) {
                in_loop = [];
            } else if (scope instanceof AST_SymbolRef) {
                if (scope.fixed_value() instanceof AST_Scope) return false;
            }
        } while (!(scope instanceof AST_Scope));

        var safe_to_inject = !(scope instanceof AST_Toplevel) || compressor.toplevel.vars;
        var inline = compressor.option("inline");
        if (!can_inject_vars(block_scoped, inline >= 3 && safe_to_inject)) return false;
        if (!can_inject_args(block_scoped, inline >= 2 && safe_to_inject)) return false;
        return !in_loop || in_loop.length == 0 || !is_reachable(fn, in_loop);
    }

    function append_var(decls, expressions, name, value) {
        var def = name.definition();

        // Name already exists, only when a function argument had the same name
        const already_appended = scope.variables.has(name.name);
        if (!already_appended) {
            scope.variables.set(name.name, def);
            scope.enclosed.push(def);
            decls.push(make_node(AST_VarDef, name, {
                name: name,
                value: null
            }));
        }

        var sym = make_node(AST_SymbolRef, name, name);
        def.references.push(sym);
        if (value) expressions.push(make_node(AST_Assign, self, {
            operator: "=",
            logical: false,
            left: sym,
            right: value.clone()
        }));
    }

    function flatten_args(decls, expressions) {
        var len = fn.argnames.length;
        for (var i = self.args.length; --i >= len;) {
            expressions.push(self.args[i]);
        }
        for (i = len; --i >= 0;) {
            var name = fn.argnames[i];
            var value = self.args[i];
            if (has_flag(name, UNUSED) || !name.name || scope.conflicting_def(name.name)) {
                if (value) expressions.push(value);
            } else {
                var symbol = make_node(AST_SymbolVar, name, name);
                name.definition().orig.push(symbol);
                if (!value && in_loop) value = make_node(AST_Undefined, self);
                append_var(decls, expressions, symbol, value);
            }
        }
        decls.reverse();
        expressions.reverse();
    }

    function flatten_vars(decls, expressions) {
        var pos = expressions.length;
        for (var i = 0, lines = fn.body.length; i < lines; i++) {
            var stat = fn.body[i];
            if (!(stat instanceof AST_Var)) continue;
            for (var j = 0, defs = stat.definitions.length; j < defs; j++) {
                var var_def = stat.definitions[j];
                var name = var_def.name;
                append_var(decls, expressions, name, var_def.value);
                if (in_loop && fn.argnames.every((argname) =>
                    argname.name != name.name
                )) {
                    var def = fn.variables.get(name.name);
                    var sym = make_node(AST_SymbolRef, name, name);
                    def.references.push(sym);
                    expressions.splice(pos++, 0, make_node(AST_Assign, var_def, {
                        operator: "=",
                        logical: false,
                        left: sym,
                        right: make_node(AST_Undefined, name)
                    }));
                }
            }
        }
    }

    function flatten_fn(returned_value) {
        var decls = [];
        var expressions = [];
        flatten_args(decls, expressions);
        flatten_vars(decls, expressions);
        expressions.push(returned_value);

        if (decls.length) {
            const i = scope.body.indexOf(compressor.parent(level - 1)) + 1;
            scope.body.splice(i, 0, make_node(AST_Var, fn, {
                definitions: decls
            }));
        }

        return expressions.map(exp => exp.clone(true));
    }
});

def_optimize(AST_New, function(self, compressor) {
    if (
        compressor.option("unsafe") &&
        is_undeclared_ref(self.expression) &&
        ["Object", "RegExp", "Function", "Error", "Array"].includes(self.expression.name)
    ) return make_node(AST_Call, self, self).transform(compressor);
    return self;
});

def_optimize(AST_Sequence, function(self, compressor) {
    if (!compressor.option("side_effects")) return self;
    var expressions = [];
    filter_for_side_effects();
    var end = expressions.length - 1;
    trim_right_for_undefined();
    if (end == 0) {
        self = maintain_this_binding(compressor.parent(), compressor.self(), expressions[0]);
        if (!(self instanceof AST_Sequence)) self = self.optimize(compressor);
        return self;
    }
    self.expressions = expressions;
    return self;

    function filter_for_side_effects() {
        var first = first_in_statement(compressor);
        var last = self.expressions.length - 1;
        self.expressions.forEach(function(expr, index) {
            if (index < last) expr = expr.drop_side_effect_free(compressor, first);
            if (expr) {
                merge_sequence(expressions, expr);
                first = false;
            }
        });
    }

    function trim_right_for_undefined() {
        while (end > 0 && is_undefined(expressions[end], compressor)) end--;
        if (end < expressions.length - 1) {
            expressions[end] = make_node(AST_UnaryPrefix, self, {
                operator   : "void",
                expression : expressions[end]
            });
            expressions.length = end + 1;
        }
    }
});

AST_Unary.DEFMETHOD("lift_sequences", function(compressor) {
    if (compressor.option("sequences")) {
        if (this.expression instanceof AST_Sequence) {
            var x = this.expression.expressions.slice();
            var e = this.clone();
            e.expression = x.pop();
            x.push(e);
            return make_sequence(this, x).optimize(compressor);
        }
    }
    return this;
});

def_optimize(AST_UnaryPostfix, function(self, compressor) {
    return self.lift_sequences(compressor);
});

def_optimize(AST_UnaryPrefix, function(self, compressor) {
    var e = self.expression;
    if (self.operator == "delete"
        && !(e instanceof AST_SymbolRef
            || e instanceof AST_PropAccess
            || is_identifier_atom(e))) {
        if (e instanceof AST_Sequence) {
            const exprs = e.expressions.slice();
            exprs.push(make_node(AST_True, self));
            return make_sequence(self, exprs).optimize(compressor);
        }
        return make_sequence(self, [ e, make_node(AST_True, self) ]).optimize(compressor);
    }
    var seq = self.lift_sequences(compressor);
    if (seq !== self) {
        return seq;
    }
    if (compressor.option("side_effects") && self.operator == "void") {
        e = e.drop_side_effect_free(compressor);
        if (e) {
            self.expression = e;
            return self;
        } else {
            return make_node(AST_Undefined, self).optimize(compressor);
        }
    }
    if (compressor.in_boolean_context()) {
        switch (self.operator) {
          case "!":
            if (e instanceof AST_UnaryPrefix && e.operator == "!") {
                // !!foo ==> foo, if we're in boolean context
                return e.expression;
            }
            if (e instanceof AST_Binary) {
                self = best_of(compressor, self, e.negate(compressor, first_in_statement(compressor)));
            }
            break;
          case "typeof":
            // typeof always returns a non-empty string, thus it's
            // always true in booleans
            // And we don't need to check if it's undeclared, because in typeof, that's OK
            return (e instanceof AST_SymbolRef ? make_node(AST_True, self) : make_sequence(self, [
                e,
                make_node(AST_True, self)
            ])).optimize(compressor);
        }
    }
    if (self.operator == "-" && e instanceof AST_Infinity) {
        e = e.transform(compressor);
    }
    if (e instanceof AST_Binary
        && (self.operator == "+" || self.operator == "-")
        && (e.operator == "*" || e.operator == "/" || e.operator == "%")) {
        return make_node(AST_Binary, self, {
            operator: e.operator,
            left: make_node(AST_UnaryPrefix, e.left, {
                operator: self.operator,
                expression: e.left
            }),
            right: e.right
        });
    }
    // avoids infinite recursion of numerals
    if (self.operator != "-"
        || !(e instanceof AST_Number || e instanceof AST_Infinity || e instanceof AST_BigInt)) {
        var ev = self.evaluate(compressor);
        if (ev !== self) {
            ev = make_node_from_constant(ev, self).optimize(compressor);
            return best_of(compressor, ev, self);
        }
    }
    return self;
});

AST_Binary.DEFMETHOD("lift_sequences", function(compressor) {
    if (compressor.option("sequences")) {
        if (this.left instanceof AST_Sequence) {
            var x = this.left.expressions.slice();
            var e = this.clone();
            e.left = x.pop();
            x.push(e);
            return make_sequence(this, x).optimize(compressor);
        }
        if (this.right instanceof AST_Sequence && !this.left.has_side_effects(compressor)) {
            var assign = this.operator == "=" && this.left instanceof AST_SymbolRef;
            var x = this.right.expressions;
            var last = x.length - 1;
            for (var i = 0; i < last; i++) {
                if (!assign && x[i].has_side_effects(compressor)) break;
            }
            if (i == last) {
                x = x.slice();
                var e = this.clone();
                e.right = x.pop();
                x.push(e);
                return make_sequence(this, x).optimize(compressor);
            } else if (i > 0) {
                var e = this.clone();
                e.right = make_sequence(this.right, x.slice(i));
                x = x.slice(0, i);
                x.push(e);
                return make_sequence(this, x).optimize(compressor);
            }
        }
    }
    return this;
});

var commutativeOperators = makePredicate("== === != !== * & | ^");
function is_object(node) {
    return node instanceof AST_Array
        || node instanceof AST_Lambda
        || node instanceof AST_Object
        || node instanceof AST_Class;
}

def_optimize(AST_Binary, function(self, compressor) {
    function reversible() {
        return self.left.is_constant()
            || self.right.is_constant()
            || !self.left.has_side_effects(compressor)
                && !self.right.has_side_effects(compressor);
    }
    function reverse(op) {
        if (reversible()) {
            if (op) self.operator = op;
            var tmp = self.left;
            self.left = self.right;
            self.right = tmp;
        }
    }
    if (commutativeOperators.has(self.operator)) {
        if (self.right.is_constant()
            && !self.left.is_constant()) {
            // if right is a constant, whatever side effects the
            // left side might have could not influence the
            // result.  hence, force switch.

            if (!(self.left instanceof AST_Binary
                  && PRECEDENCE[self.left.operator] >= PRECEDENCE[self.operator])) {
                reverse();
            }
        }
    }
    self = self.lift_sequences(compressor);
    if (compressor.option("comparisons")) switch (self.operator) {
      case "===":
      case "!==":
        var is_strict_comparison = true;
        if ((self.left.is_string(compressor) && self.right.is_string(compressor)) ||
            (self.left.is_number(compressor) && self.right.is_number(compressor)) ||
            (self.left.is_boolean() && self.right.is_boolean()) ||
            self.left.equivalent_to(self.right)) {
            self.operator = self.operator.substr(0, 2);
        }
        // XXX: intentionally falling down to the next case
      case "==":
      case "!=":
        // void 0 == x => null == x
        if (!is_strict_comparison && is_undefined(self.left, compressor)) {
            self.left = make_node(AST_Null, self.left);
        } else if (compressor.option("typeofs")
            // "undefined" == typeof x => undefined === x
            && self.left instanceof AST_String
            && self.left.value == "undefined"
            && self.right instanceof AST_UnaryPrefix
            && self.right.operator == "typeof") {
            var expr = self.right.expression;
            if (expr instanceof AST_SymbolRef ? expr.is_declared(compressor)
                : !(expr instanceof AST_PropAccess && compressor.option("ie8"))) {
                self.right = expr;
                self.left = make_node(AST_Undefined, self.left).optimize(compressor);
                if (self.operator.length == 2) self.operator += "=";
            }
        } else if (self.left instanceof AST_SymbolRef
            // obj !== obj => false
            && self.right instanceof AST_SymbolRef
            && self.left.definition() === self.right.definition()
            && is_object(self.left.fixed_value())) {
            return make_node(self.operator[0] == "=" ? AST_True : AST_False, self);
        }
        break;
      case "&&":
      case "||":
        var lhs = self.left;
        if (lhs.operator == self.operator) {
            lhs = lhs.right;
        }
        if (lhs instanceof AST_Binary
            && lhs.operator == (self.operator == "&&" ? "!==" : "===")
            && self.right instanceof AST_Binary
            && lhs.operator == self.right.operator
            && (is_undefined(lhs.left, compressor) && self.right.left instanceof AST_Null
                || lhs.left instanceof AST_Null && is_undefined(self.right.left, compressor))
            && !lhs.right.has_side_effects(compressor)
            && lhs.right.equivalent_to(self.right.right)) {
            var combined = make_node(AST_Binary, self, {
                operator: lhs.operator.slice(0, -1),
                left: make_node(AST_Null, self),
                right: lhs.right
            });
            if (lhs !== self.left) {
                combined = make_node(AST_Binary, self, {
                    operator: self.operator,
                    left: self.left.left,
                    right: combined
                });
            }
            return combined;
        }
        break;
    }
    if (self.operator == "+" && compressor.in_boolean_context()) {
        var ll = self.left.evaluate(compressor);
        var rr = self.right.evaluate(compressor);
        if (ll && typeof ll == "string") {
            return make_sequence(self, [
                self.right,
                make_node(AST_True, self)
            ]).optimize(compressor);
        }
        if (rr && typeof rr == "string") {
            return make_sequence(self, [
                self.left,
                make_node(AST_True, self)
            ]).optimize(compressor);
        }
    }
    if (compressor.option("comparisons") && self.is_boolean()) {
        if (!(compressor.parent() instanceof AST_Binary)
            || compressor.parent() instanceof AST_Assign) {
            var negated = make_node(AST_UnaryPrefix, self, {
                operator: "!",
                expression: self.negate(compressor, first_in_statement(compressor))
            });
            self = best_of(compressor, self, negated);
        }
        if (compressor.option("unsafe_comps")) {
            switch (self.operator) {
              case "<": reverse(">"); break;
              case "<=": reverse(">="); break;
            }
        }
    }
    if (self.operator == "+") {
        if (self.right instanceof AST_String
            && self.right.getValue() == ""
            && self.left.is_string(compressor)) {
            return self.left;
        }
        if (self.left instanceof AST_String
            && self.left.getValue() == ""
            && self.right.is_string(compressor)) {
            return self.right;
        }
        if (self.left instanceof AST_Binary
            && self.left.operator == "+"
            && self.left.left instanceof AST_String
            && self.left.left.getValue() == ""
            && self.right.is_string(compressor)) {
            self.left = self.left.right;
            return self;
        }
    }
    if (compressor.option("evaluate")) {
        switch (self.operator) {
          case "&&":
            var ll = has_flag(self.left, TRUTHY)
                ? true
                : has_flag(self.left, FALSY)
                    ? false
                    : self.left.evaluate(compressor);
            if (!ll) {
                return maintain_this_binding(compressor.parent(), compressor.self(), self.left).optimize(compressor);
            } else if (!(ll instanceof AST_Node)) {
                return make_sequence(self, [ self.left, self.right ]).optimize(compressor);
            }
            var rr = self.right.evaluate(compressor);
            if (!rr) {
                if (compressor.in_boolean_context()) {
                    return make_sequence(self, [
                        self.left,
                        make_node(AST_False, self)
                    ]).optimize(compressor);
                } else {
                    set_flag(self, FALSY);
                }
            } else if (!(rr instanceof AST_Node)) {
                var parent = compressor.parent();
                if (parent.operator == "&&" && parent.left === compressor.self() || compressor.in_boolean_context()) {
                    return self.left.optimize(compressor);
                }
            }
            // x || false && y ---> x ? y : false
            if (self.left.operator == "||") {
                var lr = self.left.right.evaluate(compressor);
                if (!lr) return make_node(AST_Conditional, self, {
                    condition: self.left.left,
                    consequent: self.right,
                    alternative: self.left.right
                }).optimize(compressor);
            }
            break;
          case "||":
            var ll = has_flag(self.left, TRUTHY)
              ? true
              : has_flag(self.left, FALSY)
                ? false
                : self.left.evaluate(compressor);
            if (!ll) {
                return make_sequence(self, [ self.left, self.right ]).optimize(compressor);
            } else if (!(ll instanceof AST_Node)) {
                return maintain_this_binding(compressor.parent(), compressor.self(), self.left).optimize(compressor);
            }
            var rr = self.right.evaluate(compressor);
            if (!rr) {
                var parent = compressor.parent();
                if (parent.operator == "||" && parent.left === compressor.self() || compressor.in_boolean_context()) {
                    return self.left.optimize(compressor);
                }
            } else if (!(rr instanceof AST_Node)) {
                if (compressor.in_boolean_context()) {
                    return make_sequence(self, [
                        self.left,
                        make_node(AST_True, self)
                    ]).optimize(compressor);
                } else {
                    set_flag(self, TRUTHY);
                }
            }
            if (self.left.operator == "&&") {
                var lr = self.left.right.evaluate(compressor);
                if (lr && !(lr instanceof AST_Node)) return make_node(AST_Conditional, self, {
                    condition: self.left.left,
                    consequent: self.left.right,
                    alternative: self.right
                }).optimize(compressor);
            }
            break;
          case "??":
            if (is_nullish(self.left, compressor)) {
                return self.right;
            }

            var ll = self.left.evaluate(compressor);
            if (!(ll instanceof AST_Node)) {
                // if we know the value for sure we can simply compute right away.
                return ll == null ? self.right : self.left;
            }

            if (compressor.in_boolean_context()) {
                const rr = self.right.evaluate(compressor);
                if (!(rr instanceof AST_Node) && !rr) {
                    return self.left;
                }
            }
        }
        var associative = true;
        switch (self.operator) {
          case "+":
            // (x + "foo") + "bar" => x + "foobar"
            if (self.right instanceof AST_Constant
                && self.left instanceof AST_Binary
                && self.left.operator == "+"
                && self.left.is_string(compressor)) {
                var binary = make_node(AST_Binary, self, {
                    operator: "+",
                    left: self.left.right,
                    right: self.right,
                });
                var r = binary.optimize(compressor);
                if (binary !== r) {
                    self = make_node(AST_Binary, self, {
                        operator: "+",
                        left: self.left.left,
                        right: r
                    });
                }
            }
            // (x + "foo") + ("bar" + y) => (x + "foobar") + y
            if (self.left instanceof AST_Binary
                && self.left.operator == "+"
                && self.left.is_string(compressor)
                && self.right instanceof AST_Binary
                && self.right.operator == "+"
                && self.right.is_string(compressor)) {
                var binary = make_node(AST_Binary, self, {
                    operator: "+",
                    left: self.left.right,
                    right: self.right.left,
                });
                var m = binary.optimize(compressor);
                if (binary !== m) {
                    self = make_node(AST_Binary, self, {
                        operator: "+",
                        left: make_node(AST_Binary, self.left, {
                            operator: "+",
                            left: self.left.left,
                            right: m
                        }),
                        right: self.right.right
                    });
                }
            }
            // a + -b => a - b
            if (self.right instanceof AST_UnaryPrefix
                && self.right.operator == "-"
                && self.left.is_number(compressor)) {
                self = make_node(AST_Binary, self, {
                    operator: "-",
                    left: self.left,
                    right: self.right.expression
                });
                break;
            }
            // -a + b => b - a
            if (self.left instanceof AST_UnaryPrefix
                && self.left.operator == "-"
                && reversible()
                && self.right.is_number(compressor)) {
                self = make_node(AST_Binary, self, {
                    operator: "-",
                    left: self.right,
                    right: self.left.expression
                });
                break;
            }
            // `foo${bar}baz` + 1 => `foo${bar}baz1`
            if (self.left instanceof AST_TemplateString) {
                var l = self.left;
                var r = self.right.evaluate(compressor);
                if (r != self.right) {
                    l.segments[l.segments.length - 1].value += String(r);
                    return l;
                }
            }
            // 1 + `foo${bar}baz` => `1foo${bar}baz`
            if (self.right instanceof AST_TemplateString) {
                var r = self.right;
                var l = self.left.evaluate(compressor);
                if (l != self.left) {
                    r.segments[0].value = String(l) + r.segments[0].value;
                    return r;
                }
            }
            // `1${bar}2` + `foo${bar}baz` => `1${bar}2foo${bar}baz`
            if (self.left instanceof AST_TemplateString
                && self.right instanceof AST_TemplateString) {
                var l = self.left;
                var segments = l.segments;
                var r = self.right;
                segments[segments.length - 1].value += r.segments[0].value;
                for (var i = 1; i < r.segments.length; i++) {
                    segments.push(r.segments[i]);
                }
                return l;
            }
          case "*":
            associative = compressor.option("unsafe_math");
          case "&":
          case "|":
          case "^":
            // a + +b => +b + a
            if (self.left.is_number(compressor)
                && self.right.is_number(compressor)
                && reversible()
                && !(self.left instanceof AST_Binary
                    && self.left.operator != self.operator
                    && PRECEDENCE[self.left.operator] >= PRECEDENCE[self.operator])) {
                var reversed = make_node(AST_Binary, self, {
                    operator: self.operator,
                    left: self.right,
                    right: self.left
                });
                if (self.right instanceof AST_Constant
                    && !(self.left instanceof AST_Constant)) {
                    self = best_of(compressor, reversed, self);
                } else {
                    self = best_of(compressor, self, reversed);
                }
            }
            if (associative && self.is_number(compressor)) {
                // a + (b + c) => (a + b) + c
                if (self.right instanceof AST_Binary
                    && self.right.operator == self.operator) {
                    self = make_node(AST_Binary, self, {
                        operator: self.operator,
                        left: make_node(AST_Binary, self.left, {
                            operator: self.operator,
                            left: self.left,
                            right: self.right.left,
                            start: self.left.start,
                            end: self.right.left.end
                        }),
                        right: self.right.right
                    });
                }
                // (n + 2) + 3 => 5 + n
                // (2 * n) * 3 => 6 + n
                if (self.right instanceof AST_Constant
                    && self.left instanceof AST_Binary
                    && self.left.operator == self.operator) {
                    if (self.left.left instanceof AST_Constant) {
                        self = make_node(AST_Binary, self, {
                            operator: self.operator,
                            left: make_node(AST_Binary, self.left, {
                                operator: self.operator,
                                left: self.left.left,
                                right: self.right,
                                start: self.left.left.start,
                                end: self.right.end
                            }),
                            right: self.left.right
                        });
                    } else if (self.left.right instanceof AST_Constant) {
                        self = make_node(AST_Binary, self, {
                            operator: self.operator,
                            left: make_node(AST_Binary, self.left, {
                                operator: self.operator,
                                left: self.left.right,
                                right: self.right,
                                start: self.left.right.start,
                                end: self.right.end
                            }),
                            right: self.left.left
                        });
                    }
                }
                // (a | 1) | (2 | d) => (3 | a) | b
                if (self.left instanceof AST_Binary
                    && self.left.operator == self.operator
                    && self.left.right instanceof AST_Constant
                    && self.right instanceof AST_Binary
                    && self.right.operator == self.operator
                    && self.right.left instanceof AST_Constant) {
                    self = make_node(AST_Binary, self, {
                        operator: self.operator,
                        left: make_node(AST_Binary, self.left, {
                            operator: self.operator,
                            left: make_node(AST_Binary, self.left.left, {
                                operator: self.operator,
                                left: self.left.right,
                                right: self.right.left,
                                start: self.left.right.start,
                                end: self.right.left.end
                            }),
                            right: self.left.left
                        }),
                        right: self.right.right
                    });
                }
            }
        }
    }
    // x && (y && z)  ==>  x && y && z
    // x || (y || z)  ==>  x || y || z
    // x + ("y" + z)  ==>  x + "y" + z
    // "x" + (y + "z")==>  "x" + y + "z"
    if (self.right instanceof AST_Binary
        && self.right.operator == self.operator
        && (lazy_op.has(self.operator)
            || (self.operator == "+"
                && (self.right.left.is_string(compressor)
                    || (self.left.is_string(compressor)
                        && self.right.right.is_string(compressor)))))
    ) {
        self.left = make_node(AST_Binary, self.left, {
            operator : self.operator,
            left     : self.left.transform(compressor),
            right    : self.right.left.transform(compressor)
        });
        self.right = self.right.right.transform(compressor);
        return self.transform(compressor);
    }
    var ev = self.evaluate(compressor);
    if (ev !== self) {
        ev = make_node_from_constant(ev, self).optimize(compressor);
        return best_of(compressor, ev, self);
    }
    return self;
});

def_optimize(AST_SymbolExport, function(self) {
    return self;
});

function within_array_or_object_literal(compressor) {
    var node, level = 0;
    while (node = compressor.parent(level++)) {
        if (node instanceof AST_Statement) return false;
        if (node instanceof AST_Array
            || node instanceof AST_ObjectKeyVal
            || node instanceof AST_Object) {
            return true;
        }
    }
    return false;
}

def_optimize(AST_SymbolRef, function(self, compressor) {
    if (
        !compressor.option("ie8")
        && is_undeclared_ref(self)
        && !compressor.find_parent(AST_With)
    ) {
        switch (self.name) {
          case "undefined":
            return make_node(AST_Undefined, self).optimize(compressor);
          case "NaN":
            return make_node(AST_NaN, self).optimize(compressor);
          case "Infinity":
            return make_node(AST_Infinity, self).optimize(compressor);
        }
    }

    const parent = compressor.parent();
    if (compressor.option("reduce_vars") && is_lhs(self, parent) !== self) {
        const def = self.definition();
        const nearest_scope = find_scope(compressor);
        if (compressor.top_retain && def.global && compressor.top_retain(def)) {
            def.fixed = false;
            def.single_use = false;
            return self;
        }

        let fixed = self.fixed_value();
        let single_use = def.single_use
            && !(parent instanceof AST_Call
                && (parent.is_callee_pure(compressor))
                    || has_annotation(parent, _NOINLINE))
            && !(parent instanceof AST_Export
                && fixed instanceof AST_Lambda
                && fixed.name);

        if (single_use && fixed instanceof AST_Node) {
            single_use =
                !fixed.has_side_effects(compressor)
                && !fixed.may_throw(compressor);
        }

        if (single_use && (fixed instanceof AST_Lambda || fixed instanceof AST_Class)) {
            if (retain_top_func(fixed, compressor)) {
                single_use = false;
            } else if (def.scope !== self.scope
                && (def.escaped == 1
                    || has_flag(fixed, INLINED)
                    || within_array_or_object_literal(compressor)
                    || !compressor.option("reduce_funcs"))) {
                single_use = false;
            } else if (is_recursive_ref(compressor, def)) {
                single_use = false;
            } else if (def.scope !== self.scope || def.orig[0] instanceof AST_SymbolFunarg) {
                single_use = fixed.is_constant_expression(self.scope);
                if (single_use == "f") {
                    var scope = self.scope;
                    do {
                        if (scope instanceof AST_Defun || is_func_expr(scope)) {
                            set_flag(scope, INLINED);
                        }
                    } while (scope = scope.parent_scope);
                }
            }
        }

        if (single_use && fixed instanceof AST_Lambda) {
            single_use =
                def.scope === self.scope
                    && !scope_encloses_variables_in_this_scope(nearest_scope, fixed)
                || parent instanceof AST_Call
                    && parent.expression === self
                    && !scope_encloses_variables_in_this_scope(nearest_scope, fixed)
                    && !(fixed.name && fixed.name.definition().recursive_refs > 0);
        }

        if (single_use && fixed) {
            if (fixed instanceof AST_DefClass) {
                set_flag(fixed, SQUEEZED);
                fixed = make_node(AST_ClassExpression, fixed, fixed);
            }
            if (fixed instanceof AST_Defun) {
                set_flag(fixed, SQUEEZED);
                fixed = make_node(AST_Function, fixed, fixed);
            }
            if (def.recursive_refs > 0 && fixed.name instanceof AST_SymbolDefun) {
                const defun_def = fixed.name.definition();
                let lambda_def = fixed.variables.get(fixed.name.name);
                let name = lambda_def && lambda_def.orig[0];
                if (!(name instanceof AST_SymbolLambda)) {
                    name = make_node(AST_SymbolLambda, fixed.name, fixed.name);
                    name.scope = fixed;
                    fixed.name = name;
                    lambda_def = fixed.def_function(name);
                }
                walk(fixed, node => {
                    if (node instanceof AST_SymbolRef && node.definition() === defun_def) {
                        node.thedef = lambda_def;
                        lambda_def.references.push(node);
                    }
                });
            }
            if (
                (fixed instanceof AST_Lambda || fixed instanceof AST_Class)
                && fixed.parent_scope !== nearest_scope
            ) {
                fixed = fixed.clone(true, compressor.get_toplevel());

                nearest_scope.add_child_scope(fixed);
            }
            return fixed.optimize(compressor);
        }

        // multiple uses
        if (fixed) {
            let replace;

            if (fixed instanceof AST_This) {
                if (!(def.orig[0] instanceof AST_SymbolFunarg)
                    && def.references.every((ref) =>
                        def.scope === ref.scope
                    )) {
                    replace = fixed;
                }
            } else {
                var ev = fixed.evaluate(compressor);
                if (
                    ev !== fixed
                    && (compressor.option("unsafe_regexp") || !(ev instanceof RegExp))
                ) {
                    replace = make_node_from_constant(ev, fixed);
                }
            }

            if (replace) {
                const name_length = self.size(compressor);
                const replace_size = replace.size(compressor);

                let overhead = 0;
                if (compressor.option("unused") && !compressor.exposed(def)) {
                    overhead =
                        (name_length + 2 + replace_size) /
                        (def.references.length - def.assignments);
                }

                if (replace_size <= name_length + overhead) {
                    return replace;
                }
            }
        }
    }

    return self;
});

function scope_encloses_variables_in_this_scope(scope, pulled_scope) {
    for (const enclosed of pulled_scope.enclosed) {
        if (pulled_scope.variables.has(enclosed.name)) {
            continue;
        }
        const looked_up = scope.find_variable(enclosed.name);
        if (looked_up) {
            if (looked_up === enclosed) continue;
            return true;
        }
    }
    return false;
}

function is_atomic(lhs, self) {
    return lhs instanceof AST_SymbolRef || lhs.TYPE === self.TYPE;
}

def_optimize(AST_Undefined, function(self, compressor) {
    if (compressor.option("unsafe_undefined")) {
        var undef = find_variable(compressor, "undefined");
        if (undef) {
            var ref = make_node(AST_SymbolRef, self, {
                name   : "undefined",
                scope  : undef.scope,
                thedef : undef
            });
            set_flag(ref, UNDEFINED);
            return ref;
        }
    }
    var lhs = is_lhs(compressor.self(), compressor.parent());
    if (lhs && is_atomic(lhs, self)) return self;
    return make_node(AST_UnaryPrefix, self, {
        operator: "void",
        expression: make_node(AST_Number, self, {
            value: 0
        })
    });
});

def_optimize(AST_Infinity, function(self, compressor) {
    var lhs = is_lhs(compressor.self(), compressor.parent());
    if (lhs && is_atomic(lhs, self)) return self;
    if (
        compressor.option("keep_infinity")
        && !(lhs && !is_atomic(lhs, self))
        && !find_variable(compressor, "Infinity")
    ) {
        return self;
    }
    return make_node(AST_Binary, self, {
        operator: "/",
        left: make_node(AST_Number, self, {
            value: 1
        }),
        right: make_node(AST_Number, self, {
            value: 0
        })
    });
});

def_optimize(AST_NaN, function(self, compressor) {
    var lhs = is_lhs(compressor.self(), compressor.parent());
    if (lhs && !is_atomic(lhs, self)
        || find_variable(compressor, "NaN")) {
        return make_node(AST_Binary, self, {
            operator: "/",
            left: make_node(AST_Number, self, {
                value: 0
            }),
            right: make_node(AST_Number, self, {
                value: 0
            })
        });
    }
    return self;
});

function is_reachable(self, defs) {
    const find_ref = node => {
        if (node instanceof AST_SymbolRef && member(node.definition(), defs)) {
            return walk_abort;
        }
    };

    return walk_parent(self, (node, info) => {
        if (node instanceof AST_Scope && node !== self) {
            var parent = info.parent();

            if (parent instanceof AST_Call && parent.expression === node) return;

            if (walk(node, find_ref)) return walk_abort;

            return true;
        }
    });
}

const ASSIGN_OPS = makePredicate("+ - / * % >> << >>> | ^ &");
const ASSIGN_OPS_COMMUTATIVE = makePredicate("* | ^ &");
def_optimize(AST_Assign, function(self, compressor) {
    if (self.logical) {
        return self.lift_sequences(compressor);
    }

    var def;
    if (compressor.option("dead_code")
        && self.left instanceof AST_SymbolRef
        && (def = self.left.definition()).scope === compressor.find_parent(AST_Lambda)) {
        var level = 0, node, parent = self;
        do {
            node = parent;
            parent = compressor.parent(level++);
            if (parent instanceof AST_Exit) {
                if (in_try(level, parent)) break;
                if (is_reachable(def.scope, [ def ])) break;
                if (self.operator == "=") return self.right;
                def.fixed = false;
                return make_node(AST_Binary, self, {
                    operator: self.operator.slice(0, -1),
                    left: self.left,
                    right: self.right
                }).optimize(compressor);
            }
        } while (parent instanceof AST_Binary && parent.right === node
            || parent instanceof AST_Sequence && parent.tail_node() === node);
    }
    self = self.lift_sequences(compressor);
    if (self.operator == "=" && self.left instanceof AST_SymbolRef && self.right instanceof AST_Binary) {
        // x = expr1 OP expr2
        if (self.right.left instanceof AST_SymbolRef
            && self.right.left.name == self.left.name
            && ASSIGN_OPS.has(self.right.operator)) {
            // x = x - 2  --->  x -= 2
            self.operator = self.right.operator + "=";
            self.right = self.right.right;
        } else if (self.right.right instanceof AST_SymbolRef
            && self.right.right.name == self.left.name
            && ASSIGN_OPS_COMMUTATIVE.has(self.right.operator)
            && !self.right.left.has_side_effects(compressor)) {
            // x = 2 & x  --->  x &= 2
            self.operator = self.right.operator + "=";
            self.right = self.right.left;
        }
    }
    return self;

    function in_try(level, node) {
        var right = self.right;
        self.right = make_node(AST_Null, right);
        var may_throw = node.may_throw(compressor);
        self.right = right;
        var scope = self.left.definition().scope;
        var parent;
        while ((parent = compressor.parent(level++)) !== scope) {
            if (parent instanceof AST_Try) {
                if (parent.bfinally) return true;
                if (may_throw && parent.bcatch) return true;
            }
        }
    }
});

def_optimize(AST_DefaultAssign, function(self, compressor) {
    if (!compressor.option("evaluate")) {
        return self;
    }
    var evaluateRight = self.right.evaluate(compressor);

    // `[x = undefined] = foo` ---> `[x] = foo`
    if (evaluateRight === undefined) {
        self = self.left;
    } else if (evaluateRight !== self.right) {
        evaluateRight = make_node_from_constant(evaluateRight, self.right);
        self.right = best_of_expression(evaluateRight, self.right);
    }

    return self;
});

function is_nullish_check(check, check_subject, compressor) {
    if (check_subject.may_throw(compressor)) return false;

    let nullish_side;

    // foo == null
    if (
        check instanceof AST_Binary
        && check.operator === "=="
        // which side is nullish?
        && (
            (nullish_side = is_nullish(check.left, compressor) && check.left)
            || (nullish_side = is_nullish(check.right, compressor) && check.right)
        )
        // is the other side the same as the check_subject
        && (
            nullish_side === check.left
                ? check.right
                : check.left
        ).equivalent_to(check_subject)
    ) {
        return true;
    }

    // foo === null || foo === undefined
    if (check instanceof AST_Binary && check.operator === "||") {
        let null_cmp;
        let undefined_cmp;

        const find_comparison = cmp => {
            if (!(
                cmp instanceof AST_Binary
                && (cmp.operator === "===" || cmp.operator === "==")
            )) {
                return false;
            }

            let found = 0;
            let defined_side;

            if (cmp.left instanceof AST_Null) {
                found++;
                null_cmp = cmp;
                defined_side = cmp.right;
            }
            if (cmp.right instanceof AST_Null) {
                found++;
                null_cmp = cmp;
                defined_side = cmp.left;
            }
            if (is_undefined(cmp.left, compressor)) {
                found++;
                undefined_cmp = cmp;
                defined_side = cmp.right;
            }
            if (is_undefined(cmp.right, compressor)) {
                found++;
                undefined_cmp = cmp;
                defined_side = cmp.left;
            }

            if (found !== 1) {
                return false;
            }

            if (!defined_side.equivalent_to(check_subject)) {
                return false;
            }

            return true;
        };

        if (!find_comparison(check.left)) return false;
        if (!find_comparison(check.right)) return false;

        if (null_cmp && undefined_cmp && null_cmp !== undefined_cmp) {
            return true;
        }
    }

    return false;
}

def_optimize(AST_Conditional, function(self, compressor) {
    if (!compressor.option("conditionals")) return self;
    // This looks like lift_sequences(), should probably be under "sequences"
    if (self.condition instanceof AST_Sequence) {
        var expressions = self.condition.expressions.slice();
        self.condition = expressions.pop();
        expressions.push(self);
        return make_sequence(self, expressions);
    }
    var cond = self.condition.evaluate(compressor);
    if (cond !== self.condition) {
        if (cond) {
            return maintain_this_binding(compressor.parent(), compressor.self(), self.consequent);
        } else {
            return maintain_this_binding(compressor.parent(), compressor.self(), self.alternative);
        }
    }
    var negated = cond.negate(compressor, first_in_statement(compressor));
    if (best_of(compressor, cond, negated) === negated) {
        self = make_node(AST_Conditional, self, {
            condition: negated,
            consequent: self.alternative,
            alternative: self.consequent
        });
    }
    var condition = self.condition;
    var consequent = self.consequent;
    var alternative = self.alternative;
    // x?x:y --> x||y
    if (condition instanceof AST_SymbolRef
        && consequent instanceof AST_SymbolRef
        && condition.definition() === consequent.definition()) {
        return make_node(AST_Binary, self, {
            operator: "||",
            left: condition,
            right: alternative
        });
    }
    // if (foo) exp = something; else exp = something_else;
    //                   |
    //                   v
    // exp = foo ? something : something_else;
    if (
        consequent instanceof AST_Assign
        && alternative instanceof AST_Assign
        && consequent.operator === alternative.operator
        && consequent.logical === alternative.logical
        && consequent.left.equivalent_to(alternative.left)
        && (!self.condition.has_side_effects(compressor)
            || consequent.operator == "="
                && !consequent.left.has_side_effects(compressor))
    ) {
        return make_node(AST_Assign, self, {
            operator: consequent.operator,
            left: consequent.left,
            logical: consequent.logical,
            right: make_node(AST_Conditional, self, {
                condition: self.condition,
                consequent: consequent.right,
                alternative: alternative.right
            })
        });
    }
    // x ? y(a) : y(b) --> y(x ? a : b)
    var arg_index;
    if (consequent instanceof AST_Call
        && alternative.TYPE === consequent.TYPE
        && consequent.args.length > 0
        && consequent.args.length == alternative.args.length
        && consequent.expression.equivalent_to(alternative.expression)
        && !self.condition.has_side_effects(compressor)
        && !consequent.expression.has_side_effects(compressor)
        && typeof (arg_index = single_arg_diff()) == "number") {
        var node = consequent.clone();
        node.args[arg_index] = make_node(AST_Conditional, self, {
            condition: self.condition,
            consequent: consequent.args[arg_index],
            alternative: alternative.args[arg_index]
        });
        return node;
    }
    // a ? b : c ? b : d --> (a || c) ? b : d
    if (alternative instanceof AST_Conditional
        && consequent.equivalent_to(alternative.consequent)) {
        return make_node(AST_Conditional, self, {
            condition: make_node(AST_Binary, self, {
                operator: "||",
                left: condition,
                right: alternative.condition
            }),
            consequent: consequent,
            alternative: alternative.alternative
        }).optimize(compressor);
    }

    // a == null ? b : a -> a ?? b
    if (
        compressor.option("ecma") >= 2020 &&
        is_nullish_check(condition, alternative, compressor)
    ) {
        return make_node(AST_Binary, self, {
            operator: "??",
            left: alternative,
            right: consequent
        }).optimize(compressor);
    }

    // a ? b : (c, b) --> (a || c), b
    if (alternative instanceof AST_Sequence
        && consequent.equivalent_to(alternative.expressions[alternative.expressions.length - 1])) {
        return make_sequence(self, [
            make_node(AST_Binary, self, {
                operator: "||",
                left: condition,
                right: make_sequence(self, alternative.expressions.slice(0, -1))
            }),
            consequent
        ]).optimize(compressor);
    }
    // a ? b : (c && b) --> (a || c) && b
    if (alternative instanceof AST_Binary
        && alternative.operator == "&&"
        && consequent.equivalent_to(alternative.right)) {
        return make_node(AST_Binary, self, {
            operator: "&&",
            left: make_node(AST_Binary, self, {
                operator: "||",
                left: condition,
                right: alternative.left
            }),
            right: consequent
        }).optimize(compressor);
    }
    // x?y?z:a:a --> x&&y?z:a
    if (consequent instanceof AST_Conditional
        && consequent.alternative.equivalent_to(alternative)) {
        return make_node(AST_Conditional, self, {
            condition: make_node(AST_Binary, self, {
                left: self.condition,
                operator: "&&",
                right: consequent.condition
            }),
            consequent: consequent.consequent,
            alternative: alternative
        });
    }
    // x ? y : y --> x, y
    if (consequent.equivalent_to(alternative)) {
        return make_sequence(self, [
            self.condition,
            consequent
        ]).optimize(compressor);
    }
    // x ? y || z : z --> x && y || z
    if (consequent instanceof AST_Binary
        && consequent.operator == "||"
        && consequent.right.equivalent_to(alternative)) {
        return make_node(AST_Binary, self, {
            operator: "||",
            left: make_node(AST_Binary, self, {
                operator: "&&",
                left: self.condition,
                right: consequent.left
            }),
            right: alternative
        }).optimize(compressor);
    }

    const in_bool = compressor.in_boolean_context();
    if (is_true(self.consequent)) {
        if (is_false(self.alternative)) {
            // c ? true : false ---> !!c
            return booleanize(self.condition);
        }
        // c ? true : x ---> !!c || x
        return make_node(AST_Binary, self, {
            operator: "||",
            left: booleanize(self.condition),
            right: self.alternative
        });
    }
    if (is_false(self.consequent)) {
        if (is_true(self.alternative)) {
            // c ? false : true ---> !c
            return booleanize(self.condition.negate(compressor));
        }
        // c ? false : x ---> !c && x
        return make_node(AST_Binary, self, {
            operator: "&&",
            left: booleanize(self.condition.negate(compressor)),
            right: self.alternative
        });
    }
    if (is_true(self.alternative)) {
        // c ? x : true ---> !c || x
        return make_node(AST_Binary, self, {
            operator: "||",
            left: booleanize(self.condition.negate(compressor)),
            right: self.consequent
        });
    }
    if (is_false(self.alternative)) {
        // c ? x : false ---> !!c && x
        return make_node(AST_Binary, self, {
            operator: "&&",
            left: booleanize(self.condition),
            right: self.consequent
        });
    }

    return self;

    function booleanize(node) {
        if (node.is_boolean()) return node;
        // !!expression
        return make_node(AST_UnaryPrefix, node, {
            operator: "!",
            expression: node.negate(compressor)
        });
    }

    // AST_True or !0
    function is_true(node) {
        return node instanceof AST_True
            || in_bool
                && node instanceof AST_Constant
                && node.getValue()
            || (node instanceof AST_UnaryPrefix
                && node.operator == "!"
                && node.expression instanceof AST_Constant
                && !node.expression.getValue());
    }
    // AST_False or !1
    function is_false(node) {
        return node instanceof AST_False
            || in_bool
                && node instanceof AST_Constant
                && !node.getValue()
            || (node instanceof AST_UnaryPrefix
                && node.operator == "!"
                && node.expression instanceof AST_Constant
                && node.expression.getValue());
    }

    function single_arg_diff() {
        var a = consequent.args;
        var b = alternative.args;
        for (var i = 0, len = a.length; i < len; i++) {
            if (a[i] instanceof AST_Expansion) return;
            if (!a[i].equivalent_to(b[i])) {
                if (b[i] instanceof AST_Expansion) return;
                for (var j = i + 1; j < len; j++) {
                    if (a[j] instanceof AST_Expansion) return;
                    if (!a[j].equivalent_to(b[j])) return;
                }
                return i;
            }
        }
    }
});

def_optimize(AST_Boolean, function(self, compressor) {
    if (compressor.in_boolean_context()) return make_node(AST_Number, self, {
        value: +self.value
    });
    var p = compressor.parent();
    if (compressor.option("booleans_as_integers")) {
        if (p instanceof AST_Binary && (p.operator == "===" || p.operator == "!==")) {
            p.operator = p.operator.replace(/=$/, "");
        }
        return make_node(AST_Number, self, {
            value: +self.value
        });
    }
    if (compressor.option("booleans")) {
        if (p instanceof AST_Binary && (p.operator == "=="
                                        || p.operator == "!=")) {
            return make_node(AST_Number, self, {
                value: +self.value
            });
        }
        return make_node(AST_UnaryPrefix, self, {
            operator: "!",
            expression: make_node(AST_Number, self, {
                value: 1 - self.value
            })
        });
    }
    return self;
});

function safe_to_flatten(value, compressor) {
    if (value instanceof AST_SymbolRef) {
        value = value.fixed_value();
    }
    if (!value) return false;
    if (!(value instanceof AST_Lambda || value instanceof AST_Class)) return true;
    if (!(value instanceof AST_Lambda && value.contains_this())) return true;
    return compressor.parent() instanceof AST_New;
}

AST_PropAccess.DEFMETHOD("flatten_object", function(key, compressor) {
    if (!compressor.option("properties")) return;
    if (key === "__proto__") return;

    var arrows = compressor.option("unsafe_arrows") && compressor.option("ecma") >= 2015;
    var expr = this.expression;
    if (expr instanceof AST_Object) {
        var props = expr.properties;

        for (var i = props.length; --i >= 0;) {
            var prop = props[i];

            if ("" + (prop instanceof AST_ConciseMethod ? prop.key.name : prop.key) == key) {
                const all_props_flattenable = props.every((p) =>
                    (p instanceof AST_ObjectKeyVal
                        || arrows && p instanceof AST_ConciseMethod && !p.is_generator
                    )
                    && !p.computed_key()
                );

                if (!all_props_flattenable) return;
                if (!safe_to_flatten(prop.value, compressor)) return;

                return make_node(AST_Sub, this, {
                    expression: make_node(AST_Array, expr, {
                        elements: props.map(function(prop) {
                            var v = prop.value;
                            if (v instanceof AST_Accessor) {
                                v = make_node(AST_Function, v, v);
                            }

                            var k = prop.key;
                            if (k instanceof AST_Node && !(k instanceof AST_SymbolMethod)) {
                                return make_sequence(prop, [ k, v ]);
                            }

                            return v;
                        })
                    }),
                    property: make_node(AST_Number, this, {
                        value: i
                    })
                });
            }
        }
    }
});

def_optimize(AST_Sub, function(self, compressor) {
    var expr = self.expression;
    var prop = self.property;
    if (compressor.option("properties")) {
        var key = prop.evaluate(compressor);
        if (key !== prop) {
            if (typeof key == "string") {
                if (key == "undefined") {
                    key = undefined;
                } else {
                    var value = parseFloat(key);
                    if (value.toString() == key) {
                        key = value;
                    }
                }
            }
            prop = self.property = best_of_expression(prop, make_node_from_constant(key, prop).transform(compressor));
            var property = "" + key;
            if (is_basic_identifier_string(property)
                && property.length <= prop.size() + 1) {
                return make_node(AST_Dot, self, {
                    expression: expr,
                    optional: self.optional,
                    property: property,
                    quote: prop.quote,
                }).optimize(compressor);
            }
        }
    }
    var fn;
    OPT_ARGUMENTS: if (compressor.option("arguments")
        && expr instanceof AST_SymbolRef
        && expr.name == "arguments"
        && expr.definition().orig.length == 1
        && (fn = expr.scope) instanceof AST_Lambda
        && fn.uses_arguments
        && !(fn instanceof AST_Arrow)
        && prop instanceof AST_Number) {
        var index = prop.getValue();
        var params = new Set();
        var argnames = fn.argnames;
        for (var n = 0; n < argnames.length; n++) {
            if (!(argnames[n] instanceof AST_SymbolFunarg)) {
                break OPT_ARGUMENTS; // destructuring parameter - bail
            }
            var param = argnames[n].name;
            if (params.has(param)) {
                break OPT_ARGUMENTS; // duplicate parameter - bail
            }
            params.add(param);
        }
        var argname = fn.argnames[index];
        if (argname && compressor.has_directive("use strict")) {
            var def = argname.definition();
            if (!compressor.option("reduce_vars") || def.assignments || def.orig.length > 1) {
                argname = null;
            }
        } else if (!argname && !compressor.option("keep_fargs") && index < fn.argnames.length + 5) {
            while (index >= fn.argnames.length) {
                argname = fn.create_symbol(AST_SymbolFunarg, {
                    source: fn,
                    scope: fn,
                    tentative_name: "argument_" + fn.argnames.length,
                });
                fn.argnames.push(argname);
            }
        }
        if (argname) {
            var sym = make_node(AST_SymbolRef, self, argname);
            sym.reference({});
            clear_flag(argname, UNUSED);
            return sym;
        }
    }
    if (is_lhs(self, compressor.parent())) return self;
    if (key !== prop) {
        var sub = self.flatten_object(property, compressor);
        if (sub) {
            expr = self.expression = sub.expression;
            prop = self.property = sub.property;
        }
    }
    if (compressor.option("properties") && compressor.option("side_effects")
        && prop instanceof AST_Number && expr instanceof AST_Array) {
        var index = prop.getValue();
        var elements = expr.elements;
        var retValue = elements[index];
        FLATTEN: if (safe_to_flatten(retValue, compressor)) {
            var flatten = true;
            var values = [];
            for (var i = elements.length; --i > index;) {
                var value = elements[i].drop_side_effect_free(compressor);
                if (value) {
                    values.unshift(value);
                    if (flatten && value.has_side_effects(compressor)) flatten = false;
                }
            }
            if (retValue instanceof AST_Expansion) break FLATTEN;
            retValue = retValue instanceof AST_Hole ? make_node(AST_Undefined, retValue) : retValue;
            if (!flatten) values.unshift(retValue);
            while (--i >= 0) {
                var value = elements[i];
                if (value instanceof AST_Expansion) break FLATTEN;
                value = value.drop_side_effect_free(compressor);
                if (value) values.unshift(value);
                else index--;
            }
            if (flatten) {
                values.push(retValue);
                return make_sequence(self, values).optimize(compressor);
            } else return make_node(AST_Sub, self, {
                expression: make_node(AST_Array, expr, {
                    elements: values
                }),
                property: make_node(AST_Number, prop, {
                    value: index
                })
            });
        }
    }
    var ev = self.evaluate(compressor);
    if (ev !== self && ev !== nullish) {
        ev = make_node_from_constant(ev, self).optimize(compressor);
        return best_of(compressor, ev, self);
    }
    return self;
});

def_optimize(AST_Chain, function (self, compressor) {
    if (is_nullish(self.expression, compressor)) return make_node(AST_Undefined, self);
    return self;
});

AST_Lambda.DEFMETHOD("contains_this", function() {
    return walk(this, node => {
        if (node instanceof AST_This) return walk_abort;
        if (
            node !== this
            && node instanceof AST_Scope
            && !(node instanceof AST_Arrow)
        ) {
            return true;
        }
    });
});

def_optimize(AST_Dot, function(self, compressor) {
    const parent = compressor.parent();
    if (is_lhs(self, parent)) return self;
    if (compressor.option("unsafe_proto")
        && self.expression instanceof AST_Dot
        && self.expression.property == "prototype") {
        var exp = self.expression.expression;
        if (is_undeclared_ref(exp)) switch (exp.name) {
          case "Array":
            self.expression = make_node(AST_Array, self.expression, {
                elements: []
            });
            break;
          case "Function":
            self.expression = make_node(AST_Function, self.expression, {
                argnames: [],
                body: []
            });
            break;
          case "Number":
            self.expression = make_node(AST_Number, self.expression, {
                value: 0
            });
            break;
          case "Object":
            self.expression = make_node(AST_Object, self.expression, {
                properties: []
            });
            break;
          case "RegExp":
            self.expression = make_node(AST_RegExp, self.expression, {
                value: { source: "t", flags: "" }
            });
            break;
          case "String":
            self.expression = make_node(AST_String, self.expression, {
                value: ""
            });
            break;
        }
    }
    if (!(parent instanceof AST_Call) || !has_annotation(parent, _NOINLINE)) {
        const sub = self.flatten_object(self.property, compressor);
        if (sub) return sub.optimize(compressor);
    }
    let ev = self.evaluate(compressor);
    if (ev !== self && ev !== nullish) {
        ev = make_node_from_constant(ev, self).optimize(compressor);
        return best_of(compressor, ev, self);
    }
    return self;
});

function literals_in_boolean_context(self, compressor) {
    if (compressor.in_boolean_context()) {
        return best_of(compressor, self, make_sequence(self, [
            self,
            make_node(AST_True, self)
        ]).optimize(compressor));
    }
    return self;
}

function inline_array_like_spread(elements) {
    for (var i = 0; i < elements.length; i++) {
        var el = elements[i];
        if (el instanceof AST_Expansion) {
            var expr = el.expression;
            if (
                expr instanceof AST_Array
                && !expr.elements.some(elm => elm instanceof AST_Hole)
            ) {
                elements.splice(i, 1, ...expr.elements);
                // Step back one, as the element at i is now new.
                i--;
            }
            // In array-like spread, spreading a non-iterable value is TypeError.
            // We therefore can’t optimize anything else, unlike with object spread.
        }
    }
}

def_optimize(AST_Array, function(self, compressor) {
    var optimized = literals_in_boolean_context(self, compressor);
    if (optimized !== self) {
        return optimized;
    }
    inline_array_like_spread(self.elements);
    return self;
});

function inline_object_prop_spread(props, compressor) {
    for (var i = 0; i < props.length; i++) {
        var prop = props[i];
        if (prop instanceof AST_Expansion) {
            const expr = prop.expression;
            if (
                expr instanceof AST_Object
                && expr.properties.every(prop => prop instanceof AST_ObjectKeyVal)
            ) {
                props.splice(i, 1, ...expr.properties);
                // Step back one, as the property at i is now new.
                i--;
            } else if (expr instanceof AST_Constant
                && !(expr instanceof AST_String)) {
                // Unlike array-like spread, in object spread, spreading a
                // non-iterable value silently does nothing; it is thus safe
                // to remove. AST_String is the only iterable AST_Constant.
                props.splice(i, 1);
            } else if (is_nullish(expr, compressor)) {
                // Likewise, null and undefined can be silently removed.
                props.splice(i, 1);
            }
        }
    }
}

def_optimize(AST_Object, function(self, compressor) {
    var optimized = literals_in_boolean_context(self, compressor);
    if (optimized !== self) {
        return optimized;
    }
    inline_object_prop_spread(self.properties, compressor);
    return self;
});

def_optimize(AST_RegExp, literals_in_boolean_context);

def_optimize(AST_Return, function(self, compressor) {
    if (self.value && is_undefined(self.value, compressor)) {
        self.value = null;
    }
    return self;
});

def_optimize(AST_Arrow, opt_AST_Lambda);

def_optimize(AST_Function, function(self, compressor) {
    self = opt_AST_Lambda(self, compressor);
    if (compressor.option("unsafe_arrows")
        && compressor.option("ecma") >= 2015
        && !self.name
        && !self.is_generator
        && !self.uses_arguments
        && !self.pinned()) {
        const uses_this = walk(self, node => {
            if (node instanceof AST_This) return walk_abort;
        });
        if (!uses_this) return make_node(AST_Arrow, self, self).optimize(compressor);
    }
    return self;
});

def_optimize(AST_Class, function(self) {
    // HACK to avoid compress failure.
    // AST_Class is not really an AST_Scope/AST_Block as it lacks a body.
    return self;
});

def_optimize(AST_Yield, function(self, compressor) {
    if (self.expression && !self.is_star && is_undefined(self.expression, compressor)) {
        self.expression = null;
    }
    return self;
});

def_optimize(AST_TemplateString, function(self, compressor) {
    if (
        !compressor.option("evaluate")
        || compressor.parent() instanceof AST_PrefixedTemplateString
    ) {
        return self;
    }

    var segments = [];
    for (var i = 0; i < self.segments.length; i++) {
        var segment = self.segments[i];
        if (segment instanceof AST_Node) {
            var result = segment.evaluate(compressor);
            // Evaluate to constant value
            // Constant value shorter than ${segment}
            if (result !== segment && (result + "").length <= segment.size() + "${}".length) {
                // There should always be a previous and next segment if segment is a node
                segments[segments.length - 1].value = segments[segments.length - 1].value + result + self.segments[++i].value;
                continue;
            }
            // `before ${`innerBefore ${any} innerAfter`} after` => `before innerBefore ${any} innerAfter after`
            // TODO:
            // `before ${'test' + foo} after` => `before innerBefore ${any} innerAfter after`
            // `before ${foo + 'test} after` => `before innerBefore ${any} innerAfter after`
            if (segment instanceof AST_TemplateString) {
                var inners = segment.segments;
                segments[segments.length - 1].value += inners[0].value;
                for (var j = 1; j < inners.length; j++) {
                    segment = inners[j];
                    segments.push(segment);
                }
                continue;
            }
        }
        segments.push(segment);
    }
    self.segments = segments;

    // `foo` => "foo"
    if (segments.length == 1) {
        return make_node(AST_String, self, segments[0]);
    }

    if (
        segments.length === 3
        && segments[1] instanceof AST_Node
        && (
            segments[1].is_string(compressor)
            || segments[1].is_number(compressor)
            || is_nullish(segments[1], compressor)
            || compressor.option("unsafe")
        )
    ) {
        // `foo${bar}` => "foo" + bar
        if (segments[2].value === "") {
            return make_node(AST_Binary, self, {
                operator: "+",
                left: make_node(AST_String, self, {
                    value: segments[0].value,
                }),
                right: segments[1],
            });
        }
        // `${bar}baz` => bar + "baz"
        if (segments[0].value === "") {
            return make_node(AST_Binary, self, {
                operator: "+",
                left: segments[1],
                right: make_node(AST_String, self, {
                    value: segments[2].value,
                }),
            });
        }
    }
    return self;
});

def_optimize(AST_PrefixedTemplateString, function(self) {
    return self;
});

// ["p"]:1 ---> p:1
// [42]:1 ---> 42:1
function lift_key(self, compressor) {
    if (!compressor.option("computed_props")) return self;
    // save a comparison in the typical case
    if (!(self.key instanceof AST_Constant)) return self;
    // allow certain acceptable props as not all AST_Constants are true constants
    if (self.key instanceof AST_String || self.key instanceof AST_Number) {
        if (self.key.value === "__proto__") return self;
        if (self.key.value == "constructor"
            && compressor.parent() instanceof AST_Class) return self;
        if (self instanceof AST_ObjectKeyVal) {
            self.quote = self.key.quote;
            self.key = self.key.value;
        } else if (self instanceof AST_ClassProperty) {
            self.quote = self.key.quote;
            self.key = make_node(AST_SymbolClassProperty, self.key, {
                name: self.key.value
            });
        } else {
            self.quote = self.key.quote;
            self.key = make_node(AST_SymbolMethod, self.key, {
                name: self.key.value
            });
        }
    }
    return self;
}

def_optimize(AST_ObjectProperty, lift_key);

def_optimize(AST_ConciseMethod, function(self, compressor) {
    lift_key(self, compressor);
    // p(){return x;} ---> p:()=>x
    if (compressor.option("arrows")
        && compressor.parent() instanceof AST_Object
        && !self.is_generator
        && !self.value.uses_arguments
        && !self.value.pinned()
        && self.value.body.length == 1
        && self.value.body[0] instanceof AST_Return
        && self.value.body[0].value
        && !self.value.contains_this()) {
        var arrow = make_node(AST_Arrow, self.value, self.value);
        arrow.async = self.async;
        arrow.is_generator = self.is_generator;
        return make_node(AST_ObjectKeyVal, self, {
            key: self.key instanceof AST_SymbolMethod ? self.key.name : self.key,
            value: arrow,
            quote: self.quote,
        });
    }
    return self;
});

def_optimize(AST_ObjectKeyVal, function(self, compressor) {
    lift_key(self, compressor);
    // p:function(){} ---> p(){}
    // p:function*(){} ---> *p(){}
    // p:async function(){} ---> async p(){}
    // p:()=>{} ---> p(){}
    // p:async()=>{} ---> async p(){}
    var unsafe_methods = compressor.option("unsafe_methods");
    if (unsafe_methods
        && compressor.option("ecma") >= 2015
        && (!(unsafe_methods instanceof RegExp) || unsafe_methods.test(self.key + ""))) {
        var key = self.key;
        var value = self.value;
        var is_arrow_with_block = value instanceof AST_Arrow
            && Array.isArray(value.body)
            && !value.contains_this();
        if ((is_arrow_with_block || value instanceof AST_Function) && !value.name) {
            return make_node(AST_ConciseMethod, self, {
                async: value.async,
                is_generator: value.is_generator,
                key: key instanceof AST_Node ? key : make_node(AST_SymbolMethod, self, {
                    name: key,
                }),
                value: make_node(AST_Accessor, value, value),
                quote: self.quote,
            });
        }
    }
    return self;
});

def_optimize(AST_Destructuring, function(self, compressor) {
    if (compressor.option("pure_getters") == true
        && compressor.option("unused")
        && !self.is_array
        && Array.isArray(self.names)
        && !is_destructuring_export_decl(compressor)
        && !(self.names[self.names.length - 1] instanceof AST_Expansion)) {
        var keep = [];
        for (var i = 0; i < self.names.length; i++) {
            var elem = self.names[i];
            if (!(elem instanceof AST_ObjectKeyVal
                && typeof elem.key == "string"
                && elem.value instanceof AST_SymbolDeclaration
                && !should_retain(compressor, elem.value.definition()))) {
                keep.push(elem);
            }
        }
        if (keep.length != self.names.length) {
            self.names = keep;
        }
    }
    return self;

    function is_destructuring_export_decl(compressor) {
        var ancestors = [/^VarDef$/, /^(Const|Let|Var)$/, /^Export$/];
        for (var a = 0, p = 0, len = ancestors.length; a < len; p++) {
            var parent = compressor.parent(p);
            if (!parent) return false;
            if (a === 0 && parent.TYPE == "Destructuring") continue;
            if (!ancestors[a].test(parent.TYPE)) {
                return false;
            }
            a++;
        }
        return true;
    }

    function should_retain(compressor, def) {
        if (def.references.length) return true;
        if (!def.global) return false;
        if (compressor.toplevel.vars) {
             if (compressor.top_retain) {
                 return compressor.top_retain(def);
             }
             return false;
        }
        return true;
    }
});

export {
    Compressor,
};