fix #3657: empty enum returns {} not undefined

CHANGELOG.md

@@ -1,5 +1,25 @@
 # Changelog
 
+## Unreleased
+
+* Empty enums should behave like an object literal ([#3657](https://github.com/evanw/esbuild/issues/3657))
+
+    TypeScript allows you to create an empty enum and add properties to it at run time. While people usually use an empty object literal for this instead of a TypeScript enum, esbuild's enum transform didn't anticipate this use case and generated `undefined` instead of `{}` for an empty enum. With this release, you can now use an empty enum to generate an empty object literal.
+
+    ```ts
+    // Original code
+    enum Foo {}
+
+    // Old output (with --loader=ts)
+    var Foo = /* @__PURE__ */ ((Foo2) => {
+    })(Foo || {});
+
+    // New output (with --loader=ts)
+    var Foo = /* @__PURE__ */ ((Foo2) => {
+      return Foo2;
+    })(Foo || {});
+    ```
+
 ## 0.20.1
 
 * Fix a bug with the CSS nesting transform ([#3648](https://github.com/evanw/esbuild/issues/3648))

internal/js_parser/ts_parser.go

@@ -1930,19 +1930,17 @@ func (p *parser) generateClosureForTypeScriptEnum(
 
 	// Generate the body of the closure, including a return statement at the end
 	stmtsInsideClosure := []js_ast.Stmt{}
-	if len(exprsInsideClosure) > 0 {
-		argExpr := js_ast.Expr{Loc: nameLoc, Data: &js_ast.EIdentifier{Ref: argRef}}
-		if p.options.minifySyntax {
-			// "a; b; return c;" => "return a, b, c;"
-			joined := js_ast.JoinAllWithComma(exprsInsideClosure)
-			joined = js_ast.JoinWithComma(joined, argExpr)
-			stmtsInsideClosure = append(stmtsInsideClosure, js_ast.Stmt{Loc: joined.Loc, Data: &js_ast.SReturn{ValueOrNil: joined}})
-		} else {
-			for _, expr := range exprsInsideClosure {
-				stmtsInsideClosure = append(stmtsInsideClosure, js_ast.Stmt{Loc: expr.Loc, Data: &js_ast.SExpr{Value: expr}})
-			}
-			stmtsInsideClosure = append(stmtsInsideClosure, js_ast.Stmt{Loc: argExpr.Loc, Data: &js_ast.SReturn{ValueOrNil: argExpr}})
+	argExpr := js_ast.Expr{Loc: nameLoc, Data: &js_ast.EIdentifier{Ref: argRef}}
+	if p.options.minifySyntax {
+		// "a; b; return c;" => "return a, b, c;"
+		joined := js_ast.JoinAllWithComma(exprsInsideClosure)
+		joined = js_ast.JoinWithComma(joined, argExpr)
+		stmtsInsideClosure = append(stmtsInsideClosure, js_ast.Stmt{Loc: joined.Loc, Data: &js_ast.SReturn{ValueOrNil: joined}})
+	} else {
+		for _, expr := range exprsInsideClosure {
+			stmtsInsideClosure = append(stmtsInsideClosure, js_ast.Stmt{Loc: expr.Loc, Data: &js_ast.SExpr{Value: expr}})
 		}
+		stmtsInsideClosure = append(stmtsInsideClosure, js_ast.Stmt{Loc: argExpr.Loc, Data: &js_ast.SReturn{ValueOrNil: argExpr}})
 	}
 
 	// Try to use an arrow function if possible for compactness

internal/js_parser/ts_parser_test.go

@@ -1804,7 +1804,7 @@ func TestTSDeclare(t *testing.T) {
 	expectPrintedTS(t, "declare\nconst foo = 0", "declare;\nconst foo = 0;\n")
 	expectPrintedTS(t, "declare\nfunction foo() {}", "declare;\nfunction foo() {\n}\n")
 	expectPrintedTS(t, "declare\nclass Foo {}", "declare;\nclass Foo {\n}\n")
-	expectPrintedTS(t, "declare\nenum Foo {}", "declare;\nvar Foo = /* @__PURE__ */ ((Foo) => {\n})(Foo || {});\n")
+	expectPrintedTS(t, "declare\nenum Foo {}", "declare;\nvar Foo = /* @__PURE__ */ ((Foo) => {\n  return Foo;\n})(Foo || {});\n")
 	expectPrintedTS(t, "class Foo { declare \n foo }", "class Foo {\n  declare;\n  foo;\n}\n")
 
 	expectPrintedTS(t, "declare;", "declare;\n")