Promise类核心逻辑实现

1.Promise 就是一个类 在执行这个类的时候 需要传递一个执行器进去 执行器接收参数会立即执行
2.Promise 中有三种状态分别为成功fulfilled 失败rejected 等待 pending
pending -> fulfilled pending -> rejected一旦状态确定就不可更改
3.resolve和reject函数是用来更改状态的
resolve: fulfilled
reject: rejected
4.then方法内部做的事情就判断状态 如果状态是成功 调用成功的回调函数 如果状态是失败调用失败回调函数 then方法是被定义在原型对象上的
5.then成功回调有一个参数 表示成功之后的值 then失败回调有一个参数 表示失败后的原因

myPromise.js
const PENDING = 'pending' // 等待
const FULFILLED = 'fulfilled' // 成功
const REJECTED = 'rejected'// 失败
class MyPromise {
  constructor(executor) {
    executor(this.resolve, this.reject)
  }
  status = PENDING
  value = undefined
  reason = undefined
  resolve = value => {
    if (this.status !== PENDING) return
    // 将状态修改为成功
    this.status = FULFILLED
    // 保存成功之后的值
    this.value = value
  }
  reject = reason => {
    if (this.status !== PENDING) return
    this.status = REJECTED
    this.reason = reason
  }
  then(successCallback, failCallback) {
    if (this.status === FULFILLED) {
      successCallback(this.value)
    } else if (this.status === REJECTED) {
      failCallback(this.reason)
    }
  }
}

module.exports = MyPromise

index.js
const MyPromise = require('./myPromise')
let promise = new MyPromise((resolve, reject) => {
  // resolve('成功')
  reject('失败')
})
promise.then(value => {
  console.log(value)
}, reason => {
  console.log(reason)
})