模拟实现promise|算法专题 | 前端进阶之旅

# 基础版本

设定三个状态 PENDING、FULFILLED、REJECTED ，只能由PENDING改变为FULFILLED、REJECTED，并且只能改变一次
MyPromise接收一个函数executor，executor有两个参数resolve方法和reject方法
resolve将PENDING改变为FULFILLED
reject将PENDING改变为FULFILLED
promise变为FULFILLED状态后具有一个唯一的value
promise变为REJECTED状态后具有一个唯一的reason

    const PENDING = 'pending';
    const FULFILLED = 'fulfilled';
    const REJECTED = 'rejected';

    function MyPromise(executor) {
      this.state = PENDING;
      this.value = null;
      this.reason = null;

      const resolve = (value) => {
        if (this.state === PENDING) {
          this.state = FULFILLED;
          this.value = value;
        }
      }

      const reject = (reason) => {
        if (this.state === PENDING) {
          this.state = REJECTED;
          this.reason = reason;
        }
      }

      try {
        executor(resolve, reject);
      } catch (reason) {
        reject(reason);
      }
    }

@前端进阶之旅: 代码已经复制到剪贴板

# then方法

then方法接受两个参数onFulfilled、onRejected，它们分别在状态由PENDING改变为FULFILLED、REJECTED后调用
一个promise可绑定多个then方法
then方法可以同步调用也可以异步调用
同步调用：状态已经改变，直接调用onFulfilled方法
异步调用：状态还是PENDING，将onFulfilled、onRejected分别加入两个函数数组onFulfilledCallbacks、onRejectedCallbacks，当异步调用resolve和reject时，将两个数组中绑定的事件循环执行。

  function MyPromise(executor) {
      this.state = PENDING;
      this.value = null;
      this.<