support default values (#4442)

lib/output.js

@@ -702,6 +702,8 @@ function OutputStream(options) {
             // (false, true) ? (a = 10, b = 20) : (c = 30)
             // ==> 20 (side effect, set a := 10 and b := 20)
             || p instanceof AST_Conditional
+            // [ a = (1, 2) ] = [] ==> a == 2
+            || p instanceof AST_DefaultValue
             // { [(1, 2)]: 3 }[2] ==> 3
             // { foo: (1, 2) }.foo ==> 2
             || p instanceof AST_DestructuredKeyVal
@@ -1218,6 +1220,15 @@ function OutputStream(options) {
         }
     });
 
+    DEFPRINT(AST_DefaultValue, function(output) {
+        var self = this;
+        self.name.print(output);
+        output.space();
+        output.print("=");
+        output.space();
+        self.value.print(output);
+    });
+
     /* -----[ other expressions ]----- */
     function print_call_args(self, output) {
         if (self.expression instanceof AST_Call || self.expression instanceof AST_Lambda) {