fix evanw#610: minify now drops overwritten functions

CHANGELOG.md

@@ -25,6 +25,22 @@
     (0, import_fs.readFile)();
     ```
 
+* Strip overwritten function declarations when minifying ([#610](https://github.com/evanw/esbuild/issues/610))
+
+    JavaScript allows functions to be re-declared, with each declaration overwriting the previous declaration. This type of code can sometimes be emitted by automatic code generators. With this release, esbuild now takes this behavior into account when minifying to drop all but the last declaration for a given function:
+
+    ```js
+    // Original code
+    function foo() { console.log(1) }
+    function foo() { console.log(2) }
+
+    // Old output (with --minify)
+    function foo(){console.log(1)}function foo(){console.log(2)}
+
+    // New output (with --minify)
+    function foo(){console.log(2)}
+    ```
+
 * Allow whitespace around `:` in JSX elements ([#1877](https://github.com/evanw/esbuild/issues/1877))
 
     This release allows you to write the JSX `<rdf:Description rdf:ID="foo" />` as `<rdf : Description rdf : ID="foo" />` instead. Doing this is not forbidden by [the JSX specification](https://facebook.github.io/jsx/). While this doesn't work in TypeScript, it does work with other JSX parsers in the ecosystem, so support for this has been added to esbuild.

internal/js_ast/js_ast.go

@@ -1561,6 +1561,14 @@ const (
 	//   Foo.#foo = Foo;
 	//
 	PrivateSymbolMustBeLowered
+
+	// This is used to remove the all but the last function re-declaration if a
+	// function is re-declared multiple times like this:
+	//
+	//   function foo() { console.log(1) }
+	//   function foo() { console.log(2) }
+	//
+	RemoveOverwrittenFunctionDeclaration
 )
 
 func (flags SymbolFlags) Has(flag SymbolFlags) bool {

internal/js_parser/js_parser.go

@@ -1426,7 +1426,7 @@ func (p *parser) canMergeSymbols(scope *js_ast.Scope, existing js_ast.SymbolKind
 	if new.IsHoistedOrFunction() && existing.IsHoistedOrFunction() &&
 		(scope.Kind == js_ast.ScopeEntry || scope.Kind == js_ast.ScopeFunctionBody ||
 			(new.IsHoisted() && existing.IsHoisted())) {
-		return mergeKeepExisting
+		return mergeReplaceWithNew
 	}
 
 	// "get #foo() {} set #foo() {}"
@@ -1491,6 +1491,11 @@ func (p *parser) declareSymbol(kind js_ast.SymbolKind, loc logger.Loc, name stri
 		case mergeReplaceWithNew:
 			symbol.Link = ref
 
+			// If these are both functions, remove the overwritten declaration
+			if p.options.mangleSyntax && kind.IsFunction() && symbol.Kind.IsFunction() {
+				symbol.Flags |= js_ast.RemoveOverwrittenFunctionDeclaration
+			}
+
 		case mergeBecomePrivateGetSetPair:
 			ref = existing.Ref
 			symbol.Kind = js_ast.SymbolPrivateGetSetPair
@@ -9485,6 +9490,11 @@ func (p *parser) visitAndAppendStmt(stmts []js_ast.Stmt, stmt js_ast.Stmt) []js_
 	case *js_ast.SFunction:
 		p.visitFn(&s.Fn, s.Fn.OpenParenLoc)
 
+		// Strip this function declaration if it was overwritten
+		if p.symbols[s.Fn.Name.Ref.InnerIndex].Flags.Has(js_ast.RemoveOverwrittenFunctionDeclaration) && !s.IsExport {
+			return stmts
+		}
+
 		// Handle exporting this function from a namespace
 		if s.IsExport && p.enclosingNamespaceArgRef != nil {
 			s.IsExport = false

internal/js_parser/js_parser_test.go

@@ -1342,6 +1342,21 @@ func TestFunction(t *testing.T) {
 	expectPrintedMangle(t, "function* foo() { return undefined }", "function* foo() {\n}\n")
 	expectPrintedMangle(t, "async function foo() { return undefined }", "async function foo() {\n}\n")
 	expectPrintedMangle(t, "async function* foo() { return undefined }", "async function* foo() {\n  return void 0;\n}\n")
+
+	// Strip overwritten function declarations
+	expectPrintedMangle(t, "function f() { x() } function f() { y() }", "function f() {\n  y();\n}\n")
+	expectPrintedMangle(t, "function f() { x() } function *f() { y() }", "function* f() {\n  y();\n}\n")
+	expectPrintedMangle(t, "function *f() { x() } function f() { y() }", "function f() {\n  y();\n}\n")
+	expectPrintedMangle(t, "function *f() { x() } function *f() { y() }", "function* f() {\n  y();\n}\n")
+	expectPrintedMangle(t, "function f() { x() } async function f() { y() }", "async function f() {\n  y();\n}\n")
+	expectPrintedMangle(t, "async function f() { x() } function f() { y() }", "function f() {\n  y();\n}\n")
+	expectPrintedMangle(t, "async function f() { x() } async function f() { y() }", "async function f() {\n  y();\n}\n")
+	expectPrintedMangle(t, "var f; function f() {}", "var f;\nfunction f() {\n}\n")
+	expectPrintedMangle(t, "function f() {} var f", "function f() {\n}\nvar f;\n")
+	expectPrintedMangle(t, "var f; function f() { x() } function f() { y() }", "var f;\nfunction f() {\n  y();\n}\n")
+	expectPrintedMangle(t, "function f() { x() } function f() { y() } var f", "function f() {\n  y();\n}\nvar f;\n")
+	expectPrintedMangle(t, "function f() { x() } var f; function f() { y() }", "function f() {\n  x();\n}\nvar f;\nfunction f() {\n  y();\n}\n")
+	expectPrintedMangle(t, "export function f() { x() } function f() { y() }", "export function f() {\n  x();\n}\nfunction f() {\n  y();\n}\n")
 }
 
 func TestClass(t *testing.T) {