fix #2139: dangling else minify label edge case

Author: evanw

CHANGELOG.md

@@ -2,6 +2,26 @@
 
 ## Unreleased
 
+* Fix a minification bug with a double-nested `if` inside a label followed by `else` ([#2139](https://github.com/evanw/esbuild/issues/2139))
+
+    This fixes a minification bug that affects the edge case where `if` is followed by `else` and the `if` contains a label that contains a nested `if`. Normally esbuild's AST printer automatically wraps the body of a single-statement `if` in braces to avoid the "dangling else" `if`/`else` ambiguity common to C-like languages (where the `else` accidentally becomes associated with the inner `if` instead of the outer `if`). However, I was missing automatic wrapping of label statements, which did not have test coverage because they are a rarely-used feature. This release fixes the bug:
+
+    ```js
+    // Original code
+    if (a)
+      b: {
+        if (c) break b
+      }
+    else if (d)
+      e()
+
+    // Old output (with --minify)
+    if(a)e:if(c)break e;else d&&e();
+
+    // New output (with --minify)
+    if(a){e:if(c)break e}else d&&e();
+    ```
+
 * Fix edge case regarding `baseUrl` and `paths` in `tsconfig.json` ([#2119](https://github.com/evanw/esbuild/issues/2119))
 
     In `tsconfig.json`, TypeScript forbids non-relative values inside `paths` if `baseUrl` is not present, and esbuild does too. However, TypeScript checked this after the entire `tsconfig.json` hierarchy was parsed while esbuild incorrectly checked this immediately when parsing the file containing the `paths` map. This caused incorrect warnings to be generated for `tsconfig.json` files that specify a `baseUrl` value and that inherit a `paths` value from an `extends` clause. Now esbuild will only check for non-relative `paths` values after the entire hierarchy has been parsed to avoid generating incorrect warnings.

internal/js_parser/js_parser_test.go

@@ -3192,6 +3192,16 @@ func TestMangleIf(t *testing.T) {
 	expectPrintedMangle(t, "(x ? 1 : y) || foo();", "x || y || foo();\n")
 }
 
+func TestMangleWrapToAvoidAmbiguousElse(t *testing.T) {
+	expectPrintedMangle(t, "if (a) { if (b) return c } else return d", "if (a) {\n  if (b)\n    return c;\n} else\n  return d;\n")
+	expectPrintedMangle(t, "if (a) while (1) { if (b) return c } else return d", "if (a) {\n  for (; ; )\n    if (b)\n      return c;\n} else\n  return d;\n")
+	expectPrintedMangle(t, "if (a) for (;;) { if (b) return c } else return d", "if (a) {\n  for (; ; )\n    if (b)\n      return c;\n} else\n  return d;\n")
+	expectPrintedMangle(t, "if (a) for (x in y) { if (b) return c } else return d", "if (a) {\n  for (x in y)\n    if (b)\n      return c;\n} else\n  return d;\n")
+	expectPrintedMangle(t, "if (a) for (x of y) { if (b) return c } else return d", "if (a) {\n  for (x of y)\n    if (b)\n      return c;\n} else\n  return d;\n")
+	expectPrintedMangle(t, "if (a) with (x) { if (b) return c } else return d", "if (a) {\n  with (x)\n    if (b)\n      return c;\n} else\n  return d;\n")
+	expectPrintedMangle(t, "if (a) x: { if (b) return c } else return d", "if (a) {\n  x:\n    if (b)\n      return c;\n} else\n  return d;\n")
+}
+
 func TestMangleOptionalChain(t *testing.T) {
 	expectPrintedMangle(t, "let a; return a != null ? a.b : undefined", "let a;\nreturn a?.b;\n")
 	expectPrintedMangle(t, "let a; return a != null ? a[b] : undefined", "let a;\nreturn a?.[b];\n")

internal/js_printer/js_printer.go

@@ -2901,6 +2901,9 @@ func wrapToAvoidAmbiguousElse(s js_ast.S) bool {
 		case *js_ast.SWith:
 			s = current.Body.Data
 
+		case *js_ast.SLabel:
+			s = current.Stmt.Data
+
 		default:
 			return false
 		}