fix(ecmascript): treat this as potentially having side effects (#21297)

fixes https://github.com/oxc-project/oxc/issues/21296 by treating `this` as potentially having side effects.
We can reduce the case that is bailed out by checking whether `this` is written inside `constructor` or not in the future.

crates/oxc_ecmascript/src/side_effects/expressions.rs

@@ -26,7 +26,6 @@ impl<'a> MayHaveSideEffects<'a> for Expression<'a> {
             | Expression::NullLiteral(_)
             | Expression::RegExpLiteral(_)
             | Expression::MetaProperty(_)
-            | Expression::ThisExpression(_)
             | Expression::ArrowFunctionExpression(_)
             | Expression::FunctionExpression(_)
             | Expression::Super(_) => false,

crates/oxc_minifier/tests/ecmascript/may_have_side_effects.rs

@@ -471,9 +471,11 @@ fn test_identifier_reference() {
 fn test_simple_expressions() {
     test("1n", false);
     test("true", false);
-    test("this", false);
     test("import.meta", false);
     test("(() => {})", false);
+
+    // referencing `this` in a derived class before `super()` is called causes a ReferenceError
+    test("this", true);
 }
 
 #[test]

crates/oxc_minifier/tests/peephole/esbuild.rs

@@ -1477,7 +1477,7 @@ fn test_remove_dead_expr() {
     test("0; 'abc'", "");
     test("'abc'; 'use strict'", "'abc';");
     test("function f() { 'abc'; 'use strict' }", "function f() { 'abc'; }");
-    test("this", "");
+    // test("this", "");
     test("/regex/", "");
     test("(function() {})", "");
     test("(() => {})", "");

crates/oxc_minifier/tests/peephole/inline_single_use_variable.rs

@@ -65,7 +65,8 @@ fn test_inline_single_use_variable() {
         }
     ",
     );
-    test(
+    // Ideally we should merge it to `this.#foo = foo;`
+    test_same(
         "
         class Foo {
             #foo;
@@ -75,14 +76,6 @@ fn test_inline_single_use_variable() {
             }
         }
         ",
-        "
-        class Foo {
-            #foo;
-            static {
-                this.#foo = foo;
-            }
-        }
-    ",
     );
     test(
         "
@@ -105,6 +98,47 @@ fn test_inline_single_use_variable() {
         }
     ",
     );
+    // need to avoid merging to `this.value = await ...` to avoid ReferenceError (https://github.com/oxc-project/oxc/issues/21296)
+    test(
+        "
+        class Base {
+            constructor(obj) {
+                obj.start()
+            }
+        }
+        class Wrapped extends Base {
+            constructor() {
+                super({
+                    start: async () => {
+                        let result = await new Promise(resolve => { setTimeout(() => { resolve(0) }, 0) })
+                        this.value = result
+                    }
+                })
+            }
+        }
+        let wrapped = new Wrapped()
+        setTimeout(() => { console.log('value', wrapped.value) }, 10)
+    ",
+        "
+        class Base {
+            constructor(obj) {
+                obj.start()
+            }
+        }
+        class Wrapped extends Base {
+            constructor() {
+                super({
+                    start: async () => {
+                        let result = await new Promise(resolve => { setTimeout(() => { resolve(0) }, 0) })
+                        this.value = result
+                    }
+                })
+            }
+        }
+        let wrapped = new Wrapped()
+        setTimeout(() => { console.log('value', wrapped.value) }, 10)
+    ",
+    );
     test(
         "function wrapper(arg0, arg1) { let x = arg0; return new arg1(...x);}",
         "function wrapper(arg0, arg1) { return new arg1(...arg0);}",

crates/oxc_minifier/tests/peephole/manual_pure_functions.rs

@@ -75,6 +75,7 @@ mod terser_tests {
         );
     }
 
+    // we can remove `this` to reduce code size further
     #[test]
     fn babel() {
         test(
@@ -88,7 +89,9 @@ mod terser_tests {
                 };
             "#,
             r"
-                export var Foo = function() {};
+                export var Foo = function() {
+                    this;
+                };
             ",
             &["_classCallCheck"],
         );

crates/oxc_minifier/tests/peephole/remove_unused_expression.rs

@@ -13,7 +13,7 @@ fn test_remove_unused_expression() {
     test("1", "");
     test("1n", "");
     test(";'s'", "");
-    test("this", "");
+    // test("this", "");
     test("/asdf/", "");
     test("(function () {})", "");
     test("(() => {})", "");

tasks/minsize/minsize.snap

@@ -9,19 +9,19 @@ Original   | minified   | minified   | gzip       | gzip       | Iterations | Fi
 
 342.15 kB  | 116.98 kB  | 118.14 kB  | 43.17 kB   | 44.37 kB   |          2 | vue.js     
 
-544.10 kB  | 71.03 kB   | 72.48 kB   | 25.77 kB   | 26.20 kB   |          2 | lodash.js  
+544.10 kB  | 71.04 kB   | 72.48 kB   | 25.77 kB   | 26.20 kB   |          2 | lodash.js  
 
-555.77 kB  | 267.38 kB  | 270.13 kB  | 88.00 kB   | 90.80 kB   |          2 | d3.js      
+555.77 kB  | 267.41 kB  | 270.13 kB  | 88.02 kB   | 90.80 kB   |          2 | d3.js      
 
 1.01 MB    | 439.37 kB  | 458.89 kB  | 122.05 kB  | 126.71 kB  |          2 | bundle.min.js 
 
 1.25 MB    | 642.65 kB  | 646.76 kB  | 159.39 kB  | 163.73 kB  |          2 | three.js   
 
-2.14 MB    | 711.09 kB  | 724.14 kB  | 160.42 kB  | 181.07 kB  |          2 | victory.js 
+2.14 MB    | 711.10 kB  | 724.14 kB  | 160.42 kB  | 181.07 kB  |          2 | victory.js 
 
-3.20 MB    | 1.00 MB    | 1.01 MB    | 322.57 kB  | 331.56 kB  |          3 | echarts.js 
+3.20 MB    | 1.00 MB    | 1.01 MB    | 322.61 kB  | 331.56 kB  |          2 | echarts.js 
 
-6.69 MB    | 2.22 MB    | 2.31 MB    | 458.42 kB  | 488.28 kB  |          4 | antd.js    
+6.69 MB    | 2.22 MB    | 2.31 MB    | 458.43 kB  | 488.28 kB  |          4 | antd.js    
 
-10.95 MB   | 3.33 MB    | 3.49 MB    | 853.27 kB  | 915.50 kB  |          4 | typescript.js 
+10.95 MB   | 3.33 MB    | 3.49 MB    | 853.29 kB  | 915.50 kB  |          4 | typescript.js 
 

tasks/track_memory_allocations/allocs_minifier.snap

@@ -6,9 +6,9 @@ cal.com.tsx                | 1.06 MB        ||          20214 |             46 |
 
 RadixUIAdoptionSection.jsx | 2.52 kB        ||             29 |              6 ||             30 |              6
 
-pdf.mjs                    | 567.30 kB      ||           4052 |            417 ||          47464 |           7734
+pdf.mjs                    | 567.30 kB      ||           4052 |            417 ||          47472 |           7746
 
-antd.js                    | 6.69 MB        ||           7702 |           1657 ||         331673 |          69344
+antd.js                    | 6.69 MB        ||           7702 |           1657 ||         331684 |          69349
 
 binder.ts                  | 193.08 kB      ||             66 |             23 ||           7075 |            824