code: function del (obj, key) {
    var ob = obj.__ob__;
    if (obj._isVue || (ob && ob.vmCount)) {
      process.env.NODE_ENV !== 'production' && warn(
        'Avoid deleting properties on a Vue instance or its root $data ' +
        '- just set it to null.'
      );
      return
    }
    if (!hasOwn(obj, key)) {
      return
    }
    delete obj[key];
    if (!ob) {
      return
    }
    ob.dep.notify();
  }
docs: Delete a property and trigger change if necessary.