fix #3322: avoid temporaries before "use strict"

CHANGELOG.md

@@ -23,6 +23,32 @@
 
     Note that this bug only affected code using the `local-css` loader. It did not affect code using the `css` loader.
 
+* Avoid inserting temporary variables before `use strict` ([#3322](https://github.com/evanw/esbuild/issues/3322))
+
+    This release fixes a bug where esbuild could incorrectly insert automatically-generated temporary variables before `use strict` directives:
+
+    ```js
+    // Original code
+    function foo() {
+      'use strict'
+      a.b?.c()
+    }
+
+    // Old output (with --target=es6)
+    function foo() {
+      var _a;
+      "use strict";
+      (_a = a.b) == null ? void 0 : _a.c();
+    }
+
+    // New output (with --target=es6)
+    function foo() {
+      "use strict";
+      var _a;
+      (_a = a.b) == null ? void 0 : _a.c();
+    }
+    ```
+
 * Adjust TypeScript `enum` output to better approximate `tsc` ([#3329](https://github.com/evanw/esbuild/issues/3329))
 
     TypeScript enum values can be either number literals or string literals. Numbers create a bidirectional mapping between the name and the value but strings only create a unidirectional mapping from the name to the value. When the enum value is neither a number literal nor a string literal, TypeScript and esbuild both default to treating it as a number:

internal/js_parser/js_parser.go

@@ -8377,7 +8377,23 @@ func (p *parser) visitStmtsAndPrependTempRefs(stmts []js_ast.Stmt, opts prependT
 		}
 	}
 	if len(decls) > 0 {
-		stmts = append([]js_ast.Stmt{{Data: &js_ast.SLocal{Kind: js_ast.LocalVar, Decls: decls}}}, stmts...)
+		// Skip past leading directives and comments
+		split := 0
+		for split < len(stmts) {
+			switch stmts[split].Data.(type) {
+			case *js_ast.SComment, *js_ast.SDirective:
+				split++
+				continue
+			}
+			break
+		}
+		stmts = append(
+			append(
+				append(
+					[]js_ast.Stmt{},
+					stmts[:split]...),
+				js_ast.Stmt{Data: &js_ast.SLocal{Kind: js_ast.LocalVar, Decls: decls}}),
+			stmts[split:]...)
 	}
 
 	p.tempRefsToDeclare = oldTempRefs

internal/js_parser/js_parser_lower_test.go

@@ -99,6 +99,10 @@ func TestLowerNullishCoalescing(t *testing.T) {
 		"function foo() {\n  var _a, _b;\n  if (x) {\n    (_b = (_a = a()) != null ? _a : b()) != null ? _b : c();\n  }\n}\n")
 	expectPrintedTarget(t, 2019, "() => a ?? b", "() => a != null ? a : b;\n")
 	expectPrintedTarget(t, 2019, "() => a() ?? b()", "() => {\n  var _a;\n  return (_a = a()) != null ? _a : b();\n};\n")
+
+	// Temporary variables should not come before "use strict"
+	expectPrintedTarget(t, 2019, "function f() { /*! @license */ 'use strict'; a = b.c ?? d }",
+		"function f() {\n  /*! @license */\n  \"use strict\";\n  var _a;\n  a = (_a = b.c) != null ? _a : d;\n}\n")
 }
 
 func TestLowerNullishCoalescingAssign(t *testing.T) {
@@ -152,6 +156,10 @@ class Foo {
 }
 _x = new WeakMap();
 `)
+
+	// Temporary variables should not come before "use strict"
+	expectPrintedTarget(t, 2019, "function f() { /*! @license */ 'use strict'; a.b ??= c.d }",
+		"function f() {\n  /*! @license */\n  \"use strict\";\n  var _a;\n  (_a = a.b) != null ? _a : a.b = c.d;\n}\n")
 }
 
 func TestLowerLogicalAssign(t *testing.T) {
@@ -704,6 +712,10 @@ func TestLowerOptionalChain(t *testing.T) {
 	expectPrintedTarget(t, 2020, "(x?.y)``", "(x?.y)``;\n")
 	expectPrintedTarget(t, 2019, "(x?.y)``", "var _a;\n(x == null ? void 0 : x.y).call(x, _a || (_a = __template([\"\"])));\n")
 	expectPrintedTarget(t, 5, "(x?.y)``", "var _a;\n(x == null ? void 0 : x.y).call(x, _a || (_a = __template([\"\"])));\n")
+
+	// Temporary variables should not come before "use strict"
+	expectPrintedTarget(t, 2019, "function f() { /*! @license */ 'use strict'; a.b?.c() }",
+		"function f() {\n  /*! @license */\n  \"use strict\";\n  var _a;\n  (_a = a.b) == null ? void 0 : _a.c();\n}\n")
 }
 
 func TestLowerOptionalCatchBinding(t *testing.T) {