drop-side-effect-free.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>

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import {
    AST_Accessor,
    AST_Array,
    AST_Arrow,
    AST_Assign,
    AST_Binary,
    AST_Call,
    AST_Chain,
    AST_Class,
    AST_ClassProperty,
    AST_ConciseMethod,
    AST_Conditional,
    AST_Constant,
    AST_Dot,
    AST_Expansion,
    AST_Function,
    AST_Node,
    AST_Number,
    AST_Object,
    AST_ObjectGetter,
    AST_ObjectKeyVal,
    AST_ObjectProperty,
    AST_ObjectSetter,
    AST_PropAccess,
    AST_Scope,
    AST_Sequence,
    AST_Sub,
    AST_SymbolRef,
    AST_TemplateSegment,
    AST_TemplateString,
    AST_This,
    AST_Unary,
} from "../ast.js";
import { make_node, return_null, return_this } from "../utils/index.js";
import { first_in_statement } from "../utils/first_in_statement.js";

import { pure_prop_access_globals } from "./native-objects.js";
import { lazy_op, unary_side_effects, is_nullish_shortcircuited } from "./inference.js";
import { WRITE_ONLY, set_flag, clear_flag } from "./compressor-flags.js";
import { make_sequence, is_func_expr, is_iife_call } from "./common.js";

// AST_Node#drop_side_effect_free() gets called when we don't care about the value,
// only about side effects. We'll be defining this method for each node type in this module
//
// Examples:
// foo++ -> foo++
// 1 + func() -> func()
// 10 -> (nothing)
// knownPureFunc(foo++) -> foo++

function def_drop_side_effect_free(node, func) {
    node.DEFMETHOD("drop_side_effect_free", func);
}

// Drop side-effect-free elements from an array of expressions.
// Returns an array of expressions with side-effects or null
// if all elements were dropped. Note: original array may be
// returned if nothing changed.
function trim(nodes, compressor, first_in_statement) {
    var len = nodes.length;
    if (!len)  return null;

    var ret = [], changed = false;
    for (var i = 0; i < len; i++) {
        var node = nodes[i].drop_side_effect_free(compressor, first_in_statement);
        changed |= node !== nodes[i];
        if (node) {
            ret.push(node);
            first_in_statement = false;
        }
    }
    return changed ? ret.length ? ret : null : nodes;
}

def_drop_side_effect_free(AST_Node, return_this);
def_drop_side_effect_free(AST_Constant, return_null);
def_drop_side_effect_free(AST_This, return_null);

def_drop_side_effect_free(AST_Call, function (compressor, first_in_statement) {
    if (is_nullish_shortcircuited(this, compressor)) {
        return this.expression.drop_side_effect_free(compressor, first_in_statement);
    }

    if (!this.is_callee_pure(compressor)) {
        if (this.expression.is_call_pure(compressor)) {
            var exprs = this.args.slice();
            exprs.unshift(this.expression.expression);
            exprs = trim(exprs, compressor, first_in_statement);
            return exprs && make_sequence(this, exprs);
        }
        if (is_func_expr(this.expression)
            && (!this.expression.name || !this.expression.name.definition().references.length)) {
            var node = this.clone();
            node.expression.process_expression(false, compressor);
            return node;
        }
        return this;
    }

    var args = trim(this.args, compressor, first_in_statement);
    return args && make_sequence(this, args);
});

def_drop_side_effect_free(AST_Accessor, return_null);

def_drop_side_effect_free(AST_Function, return_null);

def_drop_side_effect_free(AST_Arrow, return_null);

def_drop_side_effect_free(AST_Class, function (compressor) {
    const with_effects = [];
    const trimmed_extends = this.extends && this.extends.drop_side_effect_free(compressor);
    if (trimmed_extends)
        with_effects.push(trimmed_extends);
    for (const prop of this.properties) {
        const trimmed_prop = prop.drop_side_effect_free(compressor);
        if (trimmed_prop)
            with_effects.push(trimmed_prop);
    }
    if (!with_effects.length)
        return null;
    return make_sequence(this, with_effects);
});

def_drop_side_effect_free(AST_Binary, function (compressor, first_in_statement) {
    var right = this.right.drop_side_effect_free(compressor);
    if (!right)
        return this.left.drop_side_effect_free(compressor, first_in_statement);
    if (lazy_op.has(this.operator)) {
        if (right === this.right)
            return this;
        var node = this.clone();
        node.right = right;
        return node;
    } else {
        var left = this.left.drop_side_effect_free(compressor, first_in_statement);
        if (!left)
            return this.right.drop_side_effect_free(compressor, first_in_statement);
        return make_sequence(this, [left, right]);
    }
});

def_drop_side_effect_free(AST_Assign, function (compressor) {
    if (this.logical)
        return this;

    var left = this.left;
    if (left.has_side_effects(compressor)
        || compressor.has_directive("use strict")
        && left instanceof AST_PropAccess
        && left.expression.is_constant()) {
        return this;
    }
    set_flag(this, WRITE_ONLY);
    while (left instanceof AST_PropAccess) {
        left = left.expression;
    }
    if (left.is_constant_expression(compressor.find_parent(AST_Scope))) {
        return this.right.drop_side_effect_free(compressor);
    }
    return this;
});

def_drop_side_effect_free(AST_Conditional, function (compressor) {
    var consequent = this.consequent.drop_side_effect_free(compressor);
    var alternative = this.alternative.drop_side_effect_free(compressor);
    if (consequent === this.consequent && alternative === this.alternative)
        return this;
    if (!consequent)
        return alternative ? make_node(AST_Binary, this, {
            operator: "||",
            left: this.condition,
            right: alternative
        }) : this.condition.drop_side_effect_free(compressor);
    if (!alternative)
        return make_node(AST_Binary, this, {
            operator: "&&",
            left: this.condition,
            right: consequent
        });
    var node = this.clone();
    node.consequent = consequent;
    node.alternative = alternative;
    return node;
});

def_drop_side_effect_free(AST_Unary, function (compressor, first_in_statement) {
    if (unary_side_effects.has(this.operator)) {
        if (!this.expression.has_side_effects(compressor)) {
            set_flag(this, WRITE_ONLY);
        } else {
            clear_flag(this, WRITE_ONLY);
        }
        return this;
    }
    if (this.operator == "typeof" && this.expression instanceof AST_SymbolRef)
        return null;
    var expression = this.expression.drop_side_effect_free(compressor, first_in_statement);
    if (first_in_statement && expression && is_iife_call(expression)) {
        if (expression === this.expression && this.operator == "!")
            return this;
        return expression.negate(compressor, first_in_statement);
    }
    return expression;
});

def_drop_side_effect_free(AST_SymbolRef, function (compressor) {
    const safe_access = this.is_declared(compressor)
        || pure_prop_access_globals.has(this.name);
    return safe_access ? null : this;
});

def_drop_side_effect_free(AST_Object, function (compressor, first_in_statement) {
    var values = trim(this.properties, compressor, first_in_statement);
    return values && make_sequence(this, values);
});

def_drop_side_effect_free(AST_ObjectProperty, function (compressor, first_in_statement) {
    const computed_key = this instanceof AST_ObjectKeyVal && this.key instanceof AST_Node;
    const key = computed_key && this.key.drop_side_effect_free(compressor, first_in_statement);
    const value = this.value && this.value.drop_side_effect_free(compressor, first_in_statement);
    if (key && value) {
        return make_sequence(this, [key, value]);
    }
    return key || value;
});

def_drop_side_effect_free(AST_ClassProperty, function (compressor) {
    const key = this.computed_key() && this.key.drop_side_effect_free(compressor);

    const value = this.static && this.value
        && this.value.drop_side_effect_free(compressor);

    if (key && value)
        return make_sequence(this, [key, value]);
    return key || value || null;
});

def_drop_side_effect_free(AST_ConciseMethod, function () {
    return this.computed_key() ? this.key : null;
});

def_drop_side_effect_free(AST_ObjectGetter, function () {
    return this.computed_key() ? this.key : null;
});

def_drop_side_effect_free(AST_ObjectSetter, function () {
    return this.computed_key() ? this.key : null;
});

def_drop_side_effect_free(AST_Array, function (compressor, first_in_statement) {
    var values = trim(this.elements, compressor, first_in_statement);
    return values && make_sequence(this, values);
});

def_drop_side_effect_free(AST_Dot, function (compressor, first_in_statement) {
    if (is_nullish_shortcircuited(this, compressor)) {
        return this.expression.drop_side_effect_free(compressor, first_in_statement);
    }
    if (this.expression.may_throw_on_access(compressor)) return this;

    return this.expression.drop_side_effect_free(compressor, first_in_statement);
});

def_drop_side_effect_free(AST_Sub, function (compressor, first_in_statement) {
    if (is_nullish_shortcircuited(this, compressor)) {
        return this.expression.drop_side_effect_free(compressor, first_in_statement);
    }
    if (this.expression.may_throw_on_access(compressor)) return this;

    var expression = this.expression.drop_side_effect_free(compressor, first_in_statement);
    if (!expression)
        return this.property.drop_side_effect_free(compressor, first_in_statement);
    var property = this.property.drop_side_effect_free(compressor);
    if (!property)
        return expression;
    return make_sequence(this, [expression, property]);
});

def_drop_side_effect_free(AST_Chain, function (compressor, first_in_statement) {
    return this.expression.drop_side_effect_free(compressor, first_in_statement);
});

def_drop_side_effect_free(AST_Sequence, function (compressor) {
    var last = this.tail_node();
    var expr = last.drop_side_effect_free(compressor);
    if (expr === last)
        return this;
    var expressions = this.expressions.slice(0, -1);
    if (expr)
        expressions.push(expr);
    if (!expressions.length) {
        return make_node(AST_Number, this, { value: 0 });
    }
    return make_sequence(this, expressions);
});

def_drop_side_effect_free(AST_Expansion, function (compressor, first_in_statement) {
    return this.expression.drop_side_effect_free(compressor, first_in_statement);
});

def_drop_side_effect_free(AST_TemplateSegment, return_null);

def_drop_side_effect_free(AST_TemplateString, function (compressor) {
    var values = trim(this.segments, compressor, first_in_statement);
    return values && make_sequence(this, values);
});