fix #313: parenthesis bug with arrow functions

CHANGELOG.md

@@ -6,6 +6,10 @@
 
     Console output on Windows now uses color instead of being monochrome. This should make log messages easier to read.
 
+* Parenthesize destructuring assignment in arrow function expressions ([#313](https://github.com/evanw/esbuild/issues/313))
+
+    This fixes a bug where `() => ({} = {})` was incorrectly printed as `() => ({}) = {}`, which is a syntax error. This case is now printed correctly.
+
 ## 0.6.15
 
 * Support symlinks with absolute paths in `node_modules` ([#310](https://github.com/evanw/esbuild/issues/310))

internal/printer/printer.go

@@ -1834,7 +1834,7 @@ func (p *printer) printExpr(expr ast.Expr, level ast.L, flags int) {
 		wrap := level >= entry.Level || (e.Op == ast.BinOpIn && (flags&forbidIn) != 0)
 
 		// Destructuring assignments must be parenthesized
-		if p.stmtStart == len(p.js) {
+		if n := len(p.js); p.stmtStart == n || p.arrowExprStart == n {
 			if _, ok := e.Left.Data.(*ast.EObject); ok {
 				wrap = true
 			}

internal/printer/printer_test.go

@@ -325,6 +325,14 @@ func TestTemplate(t *testing.T) {
 
 func TestObject(t *testing.T) {
 	expectPrinted(t, "let x = {'(':')'}", "let x = {\"(\": \")\"};\n")
+	expectPrinted(t, "({})", "({});\n")
+	expectPrinted(t, "({}.x)", "({}).x;\n")
+	expectPrinted(t, "({} = {})", "({} = {});\n")
+	expectPrinted(t, "(x, {} = {})", "x, {} = {};\n")
+	expectPrinted(t, "let x = () => ({})", "let x = () => ({});\n")
+	expectPrinted(t, "let x = () => ({}.x)", "let x = () => ({}).x;\n")
+	expectPrinted(t, "let x = () => ({} = {})", "let x = () => ({} = {});\n")
+	expectPrinted(t, "let x = () => (x, {} = {})", "let x = () => (x, {} = {});\n")
 }
 
 func TestFor(t *testing.T) {
@@ -574,7 +582,7 @@ func TestWhitespace(t *testing.T) {
 
 	expectPrintedMinify(t, "()=>({})", "()=>({});")
 	expectPrintedMinify(t, "()=>({}[1])", "()=>({})[1];")
-	expectPrintedMinify(t, "()=>({}+0)", "()=>({})+0;")
+	expectPrintedMinify(t, "()=>({}+0)", "()=>({}+0);")
 	expectPrintedMinify(t, "()=>function(){}", "()=>function(){};")
 
 	expectPrintedMinify(t, "(function(){})", "(function(){});")