Non-destructuring default parameters

lib/ast.js

@@ -422,6 +422,13 @@ var AST_ArrowParametersOrSeq = DEFNODE("ArrowParametersOrSeq", "expressions", {
                     is_array: true,
                     names: ex.elements.map(to_fun_args)
                 });
+            } else if (ex instanceof AST_Assign && ex.left instanceof AST_Symbol) {
+                return new AST_SymbolFunarg({
+                    name: ex.left.name,
+                    default: ex.right,
+                    start: ex.start,
+                    end: ex.end
+                });
             } else {
                 croak("Invalid function parameter", ex.start.line, ex.start.col);
             }
@@ -1026,8 +1033,16 @@ var AST_SymbolConst = DEFNODE("SymbolConst", null, {
     $documentation: "A constant declaration"
 }, AST_SymbolDeclaration);
 
-var AST_SymbolFunarg = DEFNODE("SymbolFunarg", null, {
+var AST_SymbolFunarg = DEFNODE("SymbolFunarg", "default", {
     $documentation: "Symbol naming a function argument",
+    $propdoc: {
+        default: "[AST_Expression] The default for this parameter. For example, `= 6`"
+    },
+    _walk: function (visitor) {
+        return visitor._visit(this, function() {
+            if (this.default) this.default._walk(visitor);
+        });
+    }
 }, AST_SymbolVar);
 
 var AST_SymbolDefun = DEFNODE("SymbolDefun", null, {

lib/output.js

@@ -1275,9 +1275,21 @@ function OutputStream(options) {
         output.space();
         self.value.print(output);
     });
-    DEFPRINT(AST_Symbol, function(self, output){
-        var def = self.definition();
-        output.print_name(def ? def.mangled_name || def.name : self.name);
+    AST_Symbol.DEFMETHOD("_do_print", function(output){
+        var def = this.definition();
+        output.print_name(def ? def.mangled_name || def.name : this.name);
+    });
+    DEFPRINT(AST_Symbol, function (self, output) {
+        self._do_print(output);
+    });
+    DEFPRINT(AST_SymbolFunarg, function(self, output){
+        self._do_print(output);
+        if (self.default) {
+            output.space();
+            output.print('=');
+            output.space();
+            self.default.print(output)
+        }
     });
     DEFPRINT(AST_ObjectSymbol, function(self, output){
         var name = self.mangled_key || self.symbol.name;

test/compress/harmony.js

@@ -132,6 +132,14 @@ destructuring_arguments: {
     }
 }
 
+default_arguments: {
+    input: {
+        function x(a = 6) { }
+        function x(a = (6 + 5)) { }
+    }
+    expect_exact: "function x(a=6){}function x(a=6+5){}"
+}
+
 concise_methods: {
     input: {
         x = {