lhs_constants option for #1359 (#1361)

* First pass at #1359

* Adding test case for lhs_constants

README.md

@@ -708,7 +708,8 @@ If you happen to need the source map as a raw object, set `sourceMap.asObject` t
 
 - `comparisons` (default: `true`) -- apply certain optimizations to binary nodes,
   e.g. `!(a <= b) → a > b` (only when `unsafe_comps`), attempts to negate binary
-  nodes, e.g. `a = !b && !c && !d && !e → a=!(b||c||d||e)` etc.
+  nodes, e.g. `a = !b && !c && !d && !e → a=!(b||c||d||e)` etc. Note: `comparisons`
+  works best with `lhs_constants` enabled.
 
 - `computed_props` (default: `true`) -- Transforms constant computed properties
   into regular ones: `{["computed"]: 1}` is converted to `{computed: 1}`.
@@ -775,6 +776,9 @@ If you happen to need the source map as a raw object, set `sourceMap.asObject` t
 - `keep_infinity` (default: `false`) -- Pass `true` to prevent `Infinity` from
   being compressed into `1/0`, which may cause performance issues on Chrome.
 
+- `lhs_constants` (default: `true`) -- Moves constant values to the left-hand side
+  of binary nodes. `foo == 42 → 42 == foo`
+
 - `loops` (default: `true`) -- optimizations for `do`, `while` and `for` loops
   when we can statically determine the condition.
 

lib/compress/index.js

@@ -245,6 +245,7 @@ class Compressor extends TreeWalker {
             keep_fargs    : true,
             keep_fnames   : false,
             keep_infinity : false,
+            lhs_constants : !false_by_default,
             loops         : !false_by_default,
             module        : false,
             negate_iife   : !false_by_default,
@@ -2131,7 +2132,7 @@ def_optimize(AST_Binary, function(self, compressor) {
             self.right = tmp;
         }
     }
-    if (commutativeOperators.has(self.operator)) {
+    if (compressor.option("lhs_constants") && commutativeOperators.has(self.operator)) {
         if (self.right.is_constant()
             && !self.left.is_constant()) {
             // if right is a constant, whatever side effects the
@@ -2161,6 +2162,9 @@ def_optimize(AST_Binary, function(self, compressor) {
         // void 0 == x => null == x
         if (!is_strict_comparison && is_undefined(self.left, compressor)) {
             self.left = make_node(AST_Null, self.left);
+        // x == void 0 => x == null
+        } else if (!is_strict_comparison && is_undefined(self.right, compressor)) {
+            self.right = make_node(AST_Null, self.right);
         } else if (compressor.option("typeofs")
             // "undefined" == typeof x => undefined === x
             && self.left instanceof AST_String
@@ -2174,6 +2178,19 @@ def_optimize(AST_Binary, function(self, compressor) {
                 self.left = make_node(AST_Undefined, self.left).optimize(compressor);
                 if (self.operator.length == 2) self.operator += "=";
             }
+        } else if (compressor.option("typeofs")
+            // typeof x === "undefined" => x === undefined
+            && self.left instanceof AST_UnaryPrefix
+            && self.left.operator == "typeof"
+            && self.right instanceof AST_String
+            && self.right.value == "undefined") {
+            var expr = self.left.expression;
+            if (expr instanceof AST_SymbolRef ? expr.is_declared(compressor)
+                : !(expr instanceof AST_PropAccess && compressor.option("ie8"))) {
+                self.left = expr;
+                self.right = make_node(AST_Undefined, self.right).optimize(compressor);
+                if (self.operator.length == 2) self.operator += "=";
+            }
         } else if (self.left instanceof AST_SymbolRef
             // obj !== obj => false
             && self.right instanceof AST_SymbolRef

test/compress/asm.js

@@ -12,6 +12,7 @@ asm_mixed: {
         join_vars: true,
         keep_fargs: true,
         keep_fnames: false,
+        lhs_constants: true,
         loops: true,
         negate_iife: true,
         properties: true,

test/compress/class-properties.js

@@ -182,11 +182,11 @@ static_means_execution: {
         let x = 0;
         // Does not get inlined as it contains an immediate side effect
         class WithStaticProps {
-            static prop = (x = 0 === x ? 1 : "FAIL")
+            static prop = (x = x === 0 ? 1 : "FAIL")
         }
         new class {};
         new class {
-            prop = (x = 1 === x ? "PASS" : "FAIL")
+            prop = (x = x === 1 ? "PASS" : "FAIL")
         };
         new WithStaticProps();
 

test/compress/collapse_vars.js

@@ -473,7 +473,7 @@ collapse_vars_do_while: {
     expect: {
         function f1(y) {
             var c = 9;
-            do ; while (77 === c);
+            do ; while (c === 77);
         }
         function f2(y) {
             var c = 5 - y;
@@ -560,7 +560,7 @@ collapse_vars_do_while_drop_assign: {
     expect: {
         function f1(y) {
             var c = 9;
-            do ; while (77 === c);
+            do ; while (c === 77);
         }
         function f2(y) {
             var c = 5 - y;
@@ -2002,7 +2002,7 @@ var_side_effects_1: {
     expect: {
         var print = console.log.bind(console);
         function foo(x) {
-            print('Foo:', 2 * x);
+            print('Foo:', x * 2);
         }
         foo(10);
     }
@@ -2025,7 +2025,7 @@ var_side_effects_2: {
     expect: {
         var print = console.log.bind(console);
         function foo(x) {
-            var twice = 2 * x.y;
+            var twice = x.y * 2;
             print('Foo:', twice);
         }
         foo({ y: 10 });
@@ -2051,7 +2051,7 @@ var_side_effects_3: {
     expect: {
         var print = console.log.bind(console);
         function foo(x) {
-            print('Foo:', 2 * x.y);
+            print('Foo:', x.y * 2);
         }
         foo({ y: 10 });
     }

test/compress/comparing.js

@@ -116,6 +116,7 @@ self_comparison_2: {
 issue_2857_1: {
     options = {
         comparisons: true,
+        lhs_constants: true,
     }
     input: {
         function f1(a) {
@@ -166,6 +167,7 @@ issue_2857_1: {
 issue_2857_2: {
     options = {
         comparisons: true,
+        lhs_constants: true,
     }
     input: {
         function f(a, p) {
@@ -196,6 +198,7 @@ issue_2857_2: {
 issue_2857_3: {
     options = {
         comparisons: true,
+        lhs_constants: true,
     }
     input: {
         function f(a, p) {
@@ -226,6 +229,7 @@ issue_2857_3: {
 issue_2857_4: {
     options = {
         comparisons: true,
+        lhs_constants: true,
     }
     input: {
         function f(a, p) {
@@ -256,6 +260,7 @@ issue_2857_4: {
 issue_2857_5: {
     options = {
         comparisons: true,
+        lhs_constants: true,
     }
     input: {
         function f(a, p) {
@@ -286,6 +291,7 @@ issue_2857_5: {
 issue_2857_6: {
     options = {
         comparisons: true,
+        lhs_constants: true,
         pure_getters: "strict",
         reduce_vars: true,
     }

test/compress/conditionals.js

@@ -1270,7 +1270,7 @@ issue_2994: {
         }
         let aValue = 2, anotherValue = 3;
         for (let i = 0; i < 8; ++i)
-            console.log(f(4 & i, 2 & i, 1 & i));
+            console.log(f(i & 4, i & 2, i & 1));
     }
     expect_stdout: [
         "undefined",

test/compress/const.js

@@ -98,7 +98,7 @@ issue_1396: {
             console.log(3);
             console.log(2);
             console.log(1);
-            console.log(1 & a);
+            console.log(a & 1);
         }
         function bar() {
             const s = "01234567890123456789";

test/compress/dead-code.js

@@ -1185,7 +1185,7 @@ issue_2860_1: {
     }
     expect: {
         console.log(function(a) {
-            return 1 ^ a;
+            return a ^ 1;
         }());
     }
     expect_stdout: "1"

test/compress/drop-unused.js

@@ -1663,7 +1663,7 @@ issue_2516_1: {
         function foo() {
             function bar(x) {
                 var value = (4 - 1) * (x || never_called());
-                console.log(6 == value ? "PASS" : value);
+                console.log(value == 6 ? "PASS" : value);
             }
             Baz = function(x) {
                 bar.call(null, x);
@@ -1704,7 +1704,7 @@ issue_2516_2: {
         function foo() {
             function bar (x) {
                 var value = (4 - 1) * (x || never_called());
-                console.log(6 == value ? "PASS" : value);
+                console.log(value == 6 ? "PASS" : value);
             }
             Baz = function(x) {
                 bar.call(null, x);
@@ -2496,7 +2496,7 @@ issue_t161_top_retain_7: {
     }
     expect: {
         var y = 3;
-        console.log(2, y, 4, 2 * y, 8, 4 * y);
+        console.log(2, y, 4, 2 * y, 8, y * 4);
     }
     expect_stdout: "2 3 4 6 8 12"
 }

test/compress/functions.js

@@ -434,7 +434,7 @@ inner_ref: {
         console.log(function (a) {
             return a + 1;
         }(1), function (a) {
-            return void 0 === a;
+            return a === void 0;
         }());
     }
     expect_stdout: "2 true"

test/compress/harmony.js

@@ -1906,7 +1906,7 @@ issue_t80: {
     }
     expect: {
         function foo(data = []) {
-            if (1 == arguments.length)
+            if (arguments.length == 1)
                 data = [data];
             return data;
         }

test/compress/hoist_props.js

@@ -418,7 +418,7 @@ hoist_class: {
         }
         var o_p = class Foo {
             constructor(value) {
-                this.value = 10 * value;
+                this.value = value * 10;
             }
         };
         console.log(o_p.name, true, run(o_p, 1), run(o_p, 2));
@@ -455,7 +455,7 @@ hoist_class_with_new: {
     expect: {
         var o_p = class Foo {
             constructor(value) {
-                this.value = 10 * value;
+                this.value = value * 10;
             }
         };
         console.log(o_p.name, true, new o_p(1).value, new o_p(2).value);
@@ -825,7 +825,7 @@ issue_2519: {
     }
     expect: {
         function testFunc() {
-            return 1 * ((6 + 5) / 2);
+            return ((6 + 5) / 2) * 1;
         }
         console.log(testFunc());
     }

test/compress/identity.js

@@ -206,10 +206,10 @@ inline_identity_lose_this: {
         const id = x => x;
 
         const func_bag = {
-            func: function () { return void 0 === this ? "PASS" : "FAIL"; }
+            func: function () { return this === void 0 ? "PASS" : "FAIL"; }
         };
 
-        func_bag.func2 = function () { return void 0 === this ? "PASS" : "FAIL"; };
+        func_bag.func2 = function () { return this === void 0 ? "PASS" : "FAIL"; };
 
         console.log((0, func_bag.func)());
         console.log((0, func_bag.func2)());

test/compress/ie8.js

@@ -143,7 +143,7 @@ do_screw_try_catch_undefined: {
                 console.log("caught: "+o)
             }
             console.log("undefined is " + void 0);
-            return void 0===o
+            return o===void 0
         }
     }
     expect_stdout: true

test/compress/if_return.js

@@ -479,7 +479,7 @@ issue_2747: {
     expect: {
         "use strict";
         function f(baz) {
-            if (0 === baz) return null;
+            if (baz === 0) return null;
             let r;
             return r = baz > 2 ? 4 : 5, r;
         }

test/compress/issue-1007.js

@@ -11,7 +11,7 @@ optional_chaining_boolean_expr: {
     }
     expect: {
         (function(option) {
-            if ("DIV" !== option.container?.tagName)
+            if (option.container?.tagName !== "DIV")
                 throw new Error("Invalid `container` and/or `viewer` option.");
         })()
     }

test/compress/issue-1043.js

@@ -18,7 +18,7 @@ issue_1043: {
 
     expect: {
         function* range(start = 0, end = null, step = 1) {
-            if (null == end) {
+            if (end == null) {
                 end = start;
                 start = 0;
             }

test/compress/issue-1446.js

@@ -12,9 +12,9 @@ typeof_eq_undefined: {
         var h = "undefined" == typeof i.j;
     }
     expect: {
-        var a = "undefined" != typeof b;
-        b = void 0 !== a;
-        var c = void 0 !== d.e;
+        var a = typeof b != "undefined";
+        b = a !== void 0;
+        var c = d.e !== void 0;
         var f = "undefined" == typeof g;
         g = void 0 === f;
         var h = void 0 === i.j;
@@ -36,9 +36,9 @@ typeof_eq_undefined_ie8: {
         var h = "undefined" == typeof i.j;
     }
     expect: {
-        var a = "undefined" != typeof b;
-        b = void 0 !== a;
-        var c = "undefined" != typeof d.e;
+        var a = typeof b != "undefined";
+        b = a !== void 0;
+        var c =  typeof d.e != "undefined";
         var f = "undefined" == typeof g;
         g = void 0 === f;
         var h = "undefined" == typeof i.j;
@@ -56,7 +56,7 @@ undefined_redefined: {
             return typeof n == "undefined";
         }
     }
-    expect_exact: "function f(undefined){var n=1;return void 0===n}"
+    expect_exact: "function f(undefined){var n=1;return n===void 0}"
 }
 
 undefined_redefined_mangle: {
@@ -71,5 +71,5 @@ undefined_redefined_mangle: {
             return typeof n == "undefined";
         }
     }
-    expect_exact: "function f(n){var r=1;return void 0===r}"
+    expect_exact: "function f(n){var r=1;return r===void 0}"
 }

test/compress/issue-2871.js

@@ -19,10 +19,10 @@ comparison_with_undefined: {
         void 0 !== a;
     }
     expect: {
-        null == a;
-        null != a;
-        void 0 === a;
-        void 0 !== a;
+        a == null;
+        a != null;
+        a === void 0;
+        a !== void 0;
 
         null == a;
         null != a;

test/compress/issue-368.js

@@ -38,10 +38,10 @@ collapse: {
     }
     expect: {
         function f1() {
-            return void 0 !== ('function' === typeof b ? b() : b) && c();
+            return (typeof b === 'function' ? b() : b) !== void 0 && c();
         }
         function f2(b) {
-            return 'stirng' == typeof ('function' === typeof (b = c()) ? b() : b) && d();
+            return 'stirng' == typeof (typeof (b = c()) === 'function' ? b() : b) && d();
         }
         function f3(c) {
             var a;