code: function del (target, key) {
    if (Array.isArray(target) && typeof key === 'number') {
      target.splice(key, 1);
      return
    }
    var ob = (target ).__ob__;
    if (target._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(target, key)) {
      return
    }
    delete target[key];
    if (!ob) {
      return
    }
    ob.dep.notify();
  }
docs: Delete a property and trigger change if necessary.