fix #4018: fix for async as boolean edge case

CHANGELOG.md

@@ -6,6 +6,18 @@
 
     Due to a copy+paste typo, the binary published to `@esbuild/netbsd-arm64` was not actually for `arm64`, and didn't run in that environment. This release should fix running esbuild in that environment (NetBSD on 64-bit ARM). Sorry about the mistake.
 
+* Fix esbuild incorrectly rejecting valid TypeScript edge case ([#4027](https://github.com/evanw/esbuild/issues/4027))
+
+    The following TypeScript code is valid:
+
+    ```ts
+    export function open(async?: boolean): void {
+      console.log(async as boolean)
+    }
+    ```
+
+    Before this version, esbuild would fail to parse this with a syntax error as it expected the token sequence `async as ...` to be the start of an async arrow function expression `async as => ...`. This edge case should be parsed correctly by esbuild starting with this release.
+
 ## 2024
 
 All esbuild versions published in the year 2024 (versions 0.19.12 through 0.24.2) can be found in [CHANGELOG-2024.md](./CHANGELOG-2024.md).

internal/js_parser/js_parser.go

@@ -2879,9 +2879,10 @@ func (p *parser) parseAsyncPrefixExpr(asyncRange logger.Range, level js_ast.L, f
 		// "async x => {}"
 		case js_lexer.TIdentifier:
 			if level <= js_ast.LAssign {
-				// See https://github.com/tc39/ecma262/issues/2034 for details
 				isArrowFn := true
 				if (flags&exprFlagForLoopInit) != 0 && p.lexer.Identifier.String == "of" {
+					// See https://github.com/tc39/ecma262/issues/2034 for details
+
 					// "for (async of" is only an arrow function if the next token is "=>"
 					isArrowFn = p.checkForArrowAfterTheCurrentToken()
 
@@ -2891,6 +2892,18 @@ func (p *parser) parseAsyncPrefixExpr(asyncRange logger.Range, level js_ast.L, f
 						p.log.AddError(&p.tracker, r, "For loop initializers cannot start with \"async of\"")
 						panic(js_lexer.LexerPanic{})
 					}
+				} else if p.options.ts.Parse && p.lexer.Token == js_lexer.TIdentifier {
+					// Make sure we can parse the following TypeScript code:
+					//
+					//   export function open(async?: boolean): void {
+					//     console.log(async as boolean)
+					//   }
+					//
+					// TypeScript solves this by using a two-token lookahead to check for
+					// "=>" after an identifier after the "async". This is done in
+					// "isUnParenthesizedAsyncArrowFunctionWorker" which was introduced
+					// here: https://github.com/microsoft/TypeScript/pull/8444
+					isArrowFn = p.checkForArrowAfterTheCurrentToken()
 				}
 
 				if isArrowFn {

internal/js_parser/ts_parser_test.go

@@ -2307,7 +2307,7 @@ func TestTSArrow(t *testing.T) {
 
 	expectPrintedTS(t, "async (): void => {}", "async () => {\n};\n")
 	expectPrintedTS(t, "async (a): void => {}", "async (a) => {\n};\n")
-	expectParseErrorTS(t, "async x: void => {}", "<stdin>: ERROR: Expected \"=>\" but found \":\"\n")
+	expectParseErrorTS(t, "async x: void => {}", "<stdin>: ERROR: Expected \";\" but found \"x\"\n")
 
 	expectPrintedTS(t, "function foo(x: boolean): asserts x", "")
 	expectPrintedTS(t, "function foo(x: boolean): asserts<T>", "")
@@ -2331,6 +2331,11 @@ func TestTSArrow(t *testing.T) {
 	expectParseErrorTargetTS(t, 5, "return check ? (hover = 2, bar) : baz()", "")
 	expectParseErrorTargetTS(t, 5, "return check ? (hover = 2, bar) => 0 : baz()",
 		"<stdin>: ERROR: Transforming default arguments to the configured target environment is not supported yet\n")
+
+	// https://github.com/evanw/esbuild/issues/4027
+	expectPrintedTS(t, "function f(async?) { g(async in x) }", "function f(async) {\n  g(async in x);\n}\n")
+	expectPrintedTS(t, "function f(async?) { g(async as boolean) }", "function f(async) {\n  g(async);\n}\n")
+	expectPrintedTS(t, "function f() { g(async as => boolean) }", "function f() {\n  g(async (as) => boolean);\n}\n")
 }
 
 func TestTSSuperCall(t *testing.T) {