手写一个符合Promise/A+规范的Promise

Promise/A+规范
参考文献这个博主真的写的特别好
首先我们应该知道的事情：
在Promise中：

• 首先我们在调用Promise的时候会返回一个Promise对象
• 构建Promise时，会给其传入一个函数参数executor,这个函数立即执行
• 当这个函数运行成功之后，会调用resolve函数，如果运行失败，会调用rejected函数
• Promise状态不可逆，不能停止，同时调用 resolve 函数和 reject 函数，默认会采取第一次调用的结果。

    const PENDING = "PENDING";
    const FULFILLED = "FULFILLED";
    const REJECTED = "REJECTED"
    class Promise {
      constructor(executor) {
        this.status = PENDING;
        this.value = undefined;
        this.reason = undefined;
        this.onResolvedCallbacks = [] // 专门存放成功的回调函数
        this.onRejectedCallbacks = [] // 专门存放失败的回调函数
        //成功调用的函数
        let resolved = (value) => {
          if (this.status === PENDING) {
            this.status = FULFILLED;
            this.value = value;
            this.onResolvedCallbacks.forEach(fn => { fn() })
          }
        }
        //失败调用的函数
        let rejected = (reason) => {
          if (this.status === PENDING) {
            this.status = REJECTED;
            this.reason = reason;
            this.onRejectedCallbacks.forEach(fn => { fn() })// 需要让成功的方法依次执行
          }
        }
        //立即执行executor
        try {
          executor(resolved, rejected)
        } catch (error) {
          rejected(error)
        }
        //then方法


      }
      then(onFulfilled, onRejected) {
        if (this.status === FULFILLED) {
          onFulfilled(this.value)
        }
        if (this.status === REJECTED) {
          onRejected(this.reason)
        }
        if (this.status === PENDING) {//如果状态是等待，就将回调函数push到专门存放成功/失败的回调函数
          this.onResolvedCallbacks.push(() => { onFulfilled(this.value) })
          this.onRejectedCallbacks.push(() => { onRejected(this.reason) })
        }
      }
    }
    const promise = new Promise((resolve, reject) => {
      setTimeout(() => {
        resolve('成功');
      }, 1000);
    }).then(
      (data) => {
        console.log('success', data)
      },
      (err) => {
        console.log('faild', err)
      }
    )
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