console.log(Object.assign({"a":undefined}, {"b":undefined}))
Object { a: undefined, b: undefined }

          case ConstructorId_assign:
          {
            if (args.length < 1) {
              throw ScriptRuntime.typeError1("msg.incompat.call", "assign");
            }
            Scriptable targetObj = ScriptRuntime.toObject(cx, thisObj, args[0]);
            for (int i = 1; i < args.length; i++) {
              if ((args[i] == null) || Undefined.isUndefined(args[i])) {
                continue;
              }
              Scriptable sourceObj = ScriptRuntime.toObject(cx, thisObj, args[i]);
              Object[] ids = sourceObj.getIds();
              for (Object key : ids) {
                if (key instanceof String) {
                  Object val = sourceObj.get((String) key, sourceObj);
                  if ((val != Scriptable.NOT_FOUND) && !Undefined.isUndefined(val)) { // <=== HERE
                    targetObj.put((String) key, targetObj, val);
                  }
                } else if (key instanceof Number) {
                  int ii = ScriptRuntime.toInt32(key);
                  Object val = sourceObj.get(ii, sourceObj);
                  if ((val != Scriptable.NOT_FOUND) && !Undefined.isUndefined(val)) {  // <=== AND HERE
                    targetObj.put(ii, targetObj, val);
                  }
                }
              }
            }
            return targetObj;
          }