11 实现一个简洁版的promise

    // 三个常量用于表示状态
    const PENDING = 'pending'
    const RESOLVED = 'resolved'
    const REJECTED = 'rejected'
    
    function MyPromise(fn) {
        const that = this
        this.state = PENDING
    
        // value 变量用于保存 resolve 或者 reject 中传入的值
        this.value = null
    
        // 用于保存 then 中的回调，因为当执行完 Promise 时状态可能还是等待中，这时候应该把 then 中的回调保存起来用于状态改变时使用
        that.resolvedCallbacks = []
        that.rejectedCallbacks = []
    
    
        function resolve(value) {
             // 首先两个函数都得判断当前状态是否为等待中
            if(that.state === PENDING) {
                that.state = RESOLVED
                that.value = value
    
                // 遍历回调数组并执行
                that.resolvedCallbacks.map(cb=>cb(that.value))
            }
        }
        function reject(value) {
            if(that.state === PENDING) {
                that.state = REJECTED
                that.value = value
                that.rejectedCallbacks.map(cb=>cb(that.value))
            }
        }
    
        // 完成以上两个函数以后，我们就该实现如何执行 Promise 中传入的函数了
        try {
            fn(resolve,reject)
        }cach(e){
            reject(e)
        }
    }
    
    // 最后我们来实现较为复杂的 then 函数
    MyPromise.prototype.then = function(onFulfilled,onRejected){
      const that = this
    
      // 判断两个参数是否为函数类型，因为这两个参数是可选参数
      onFulfilled = typeof onFulfilled === 'function' ? onFulfilled : v=>v
      onRejected = typeof onRejected === 'function' ? onRejected : e=>throw e
    
      // 当状态不是等待态时，就去执行相对应的函数。如果状态是等待态的话，就往回调函数中 push 函数
      if(this.state === PENDING) {
          this.resolvedCallbacks.push(onFulfilled)
          this.rejectedCallbacks.push(onRejected)
      }
      if(this.state === RESOLVED) {
          onFulfilled(that.value)
      }
      if(this.state === REJECTED) {
          onRejected(that.value)
      }
    }