allow "in" inside template inside "for" (#835)

CHANGELOG.md

@@ -1,5 +1,13 @@
 # Changelog
 
+## Unreleased
+
+* Fix some JavaScript parsing edge cases ([#835](https://github.com/evanw/esbuild/issues/835))
+
+    This release fixes the following edge cases, which were detected by a fuzzer:
+
+    * Code using `in` inside a template literal inside a for loop initializer such as ``for (let x = `${a in b ? '0' : '1'}`; false; );`` is now allowed. Previously the `in` operator was incorrectly considered to be part of a for-in loop.
+
 ## 0.8.47
 
 * Release native binaries for the Apple M1 chip ([#550](https://github.com/evanw/esbuild/issues/550))

internal/js_parser/js_parser.go

@@ -4095,6 +4095,11 @@ func (p *parser) parseJSXElement(loc logger.Loc) js_ast.Expr {
 
 func (p *parser) parseTemplateParts(includeRaw bool) []js_ast.TemplatePart {
 	parts := []js_ast.TemplatePart{}
+
+	// Allow "in" inside template literals
+	oldAllowIn := p.allowIn
+	p.allowIn = true
+
 	for {
 		p.lexer.Next()
 		value := p.parseExpr(js_ast.LLowest)
@@ -4111,6 +4116,9 @@ func (p *parser) parseTemplateParts(includeRaw bool) []js_ast.TemplatePart {
 			break
 		}
 	}
+
+	p.allowIn = oldAllowIn
+
 	return parts
 }
 

internal/js_parser/js_parser_test.go

@@ -562,6 +562,7 @@ func TestFor(t *testing.T) {
 	expectPrinted(t, "for (var x = a + b in c);", "x = a + b;\nfor (var x in c)\n  ;\n")
 
 	// Make sure "in" rules are disabled
+	expectPrinted(t, "for (var x = `${y in z}`;;);", "for (var x = `${y in z}`; ; )\n  ;\n")
 	expectPrinted(t, "for (var {[x in y]: z} = {};;);", "for (var {[x in y]: z} = {}; ; )\n  ;\n")
 	expectPrinted(t, "for (var {x = y in z} = {};;);", "for (var {x = y in z} = {}; ; )\n  ;\n")
 	expectPrinted(t, "for (var [x = y in z] = {};;);", "for (var [x = y in z] = {}; ; )\n  ;\n")