/**
 * Set a property on an object. Adds the new property and
 * triggers change notification if the property doesn't
 * already exist.
 */
export function set (target: Array<any> | Object, key: any, val: any): any {
  if (process.env.NODE_ENV !== 'production' &&
    (isUndef(target) || isPrimitive(target))
  ) {
    warn(`Cannot set reactive property on undefined, null, or primitive value: ${(target)}`)
  }
  if (Array.isArray(target) && isValidArrayIndex(key)) {
    target.length = Math.max(target.length, key)
    target.splice(key, 1, val)
    return val
  }
  if (key in target && !(key in Object.prototype)) { 
    target[key] = val
    return val
  }
  const ob = target.__ob__
  if (target._isVue || (ob && ob.vmCount)) {
    process.env.NODE_ENV !== 'production' && warn(
      'Avoid adding reactive properties to a Vue instance or its root $data ' +
      'at runtime - declare it upfront in the data option.'
    )
    return val
  }
  if (!ob) {
    target[key] = val
    return val
  }
  defineReactive(ob.value, key, val)
  // 主動通知觸發(fā)更新
  ob.dep.notify()
  return val
}

• vue的$set方法默認傳入三個參數(shù)，需要修改的對象引用target，對象的鍵值（數(shù)組的index）key，要修改的值val。如果傳入的target為vue本身、 $data,或者boolean,string,number,symbol等原始數(shù)據(jù)類型，則修改無效。
• 對于數(shù)組或者對象已有屬性的修改，直接修改相應(yīng)的值。
• 對于新的屬性值，如果操作對象是可觀測數(shù)據(jù)，則將屬性添加為可觀測屬性值，并主動觸發(fā)通知。如果是普通對象，則直接修改相應(yīng)的屬性值。