release notes for #1067

CHANGELOG.md

@@ -1,5 +1,22 @@
 # Changelog
 
+## Unreleased
+
+* Warn about mutation of private methods ([#1067](https://github.com/evanw/esbuild/pull/1067))
+
+    Mutating a private method in JavaScript is not allowed, and will throw at run-time:
+
+    ```js
+    class Foo {
+      #method() {}
+      mutate() {
+        this.#method = () => {}
+      }
+    }
+    ```
+
+    This is the case both when esbuild passes the syntax through untransformed and when esbuild transforms the syntax into the equivalent code that uses a `WeakSet` to emulate private methods in older browsers. However, it's clear from this code that doing this will always throw, so this code is almost surely a mistake. With this release, esbuild will now warn when you do this. This change was contributed by [@jridgewell](https://github.com/jridgewell).
+
 ## 0.10.2
 
 * Fix a crash that was introduced in the previous release ([#1064](https://github.com/evanw/esbuild/issues/1064))

internal/js_parser/js_parser.go

@@ -10554,7 +10554,7 @@ func (p *parser) visitExprInOut(expr js_ast.Expr, in exprIn) (js_ast.Expr, exprO
 			} else if !p.options.suppressWarningsAboutWeirdCode {
 				if in.assignTarget != js_ast.AssignTargetNone && (kind == js_ast.SymbolPrivateMethod || kind == js_ast.SymbolPrivateStaticMethod) {
 					r := logger.Range{Loc: e.Index.Loc, Len: int32(len(name))}
-					p.log.AddRangeWarning(&p.source, r, fmt.Sprintf("Writing to readonly method %q will throw", name))
+					p.log.AddRangeWarning(&p.source, r, fmt.Sprintf("Writing to read-only method %q will throw", name))
 				} else if in.assignTarget != js_ast.AssignTargetNone && (kind == js_ast.SymbolPrivateGet || kind == js_ast.SymbolPrivateStaticGet) {
 					r := logger.Range{Loc: e.Index.Loc, Len: int32(len(name))}
 					p.log.AddRangeWarning(&p.source, r, fmt.Sprintf("Writing to getter-only property %q will throw", name))

internal/js_parser/js_parser_test.go

@@ -3902,9 +3902,9 @@ func TestPrivateIdentifiers(t *testing.T) {
 
 	// Writing to method warnings
 	expectParseError(t, "class Foo { #x() { this.#x = 1 } }",
-		"<stdin>: warning: Writing to readonly method \"#x\" will throw\n")
+		"<stdin>: warning: Writing to read-only method \"#x\" will throw\n")
 	expectParseError(t, "class Foo { #x() { this.#x += 1 } }",
-		"<stdin>: warning: Writing to readonly method \"#x\" will throw\n")
+		"<stdin>: warning: Writing to read-only method \"#x\" will throw\n")
 
 	expectPrinted(t, `class Foo {
 	#if

scripts/end-to-end-tests.js

@@ -2053,7 +2053,7 @@
         new Foo().bar()
       `,
     }, {
-      expectedStderr: ` > in.js:22:29: warning: Writing to readonly method "#method" will throw
+      expectedStderr: ` > in.js:22:29: warning: Writing to read-only method "#method" will throw
     22 │             expect(() => obj.#method = 1, 'Cannot write to private f...
        ╵                              ~~~~~~~
 
@@ -2065,7 +2065,7 @@
     24 │             expect(() => this.#getter = 1, 'member.set is not a func...
        ╵                               ~~~~~~~
 
- > in.js:25:30: warning: Writing to readonly method "#method" will throw
+ > in.js:25:30: warning: Writing to read-only method "#method" will throw
     25 │             expect(() => this.#method = 1, 'member.set is not a func...
        ╵                               ~~~~~~~