手写Promise（超详解）

1. 最简单版本

// ES6 ES2015
// https://promisesaplus.com/
const PROMISE_STATUS_PENDING = 'pending'
const PROMISE_STATUS_FULFILLED = 'fulfilled'
const PROMISE_STATUS_REJECTED = 'rejected'

class HYPromise {
  constructor(executor) {
    this.status = PROMISE_STATUS_PENDING
    this.value = undefined
    this.reason = undefined

    const resolve = (value) => {
      if (this.status === PROMISE_STATUS_PENDING) {
        this.status = PROMISE_STATUS_FULFILLED
        queueMicrotask(() => {
          this.value = value
          this.onFulfilled(this.value)
        });
      }
    }

    const reject = (reason) => {
      if (this.status === PROMISE_STATUS_PENDING) {
        this.status = PROMISE_STATUS_REJECTED
        queueMicrotask(() => {
          this.reason = reason
          this.onRejected(this.reason)
        })
      }
    }

    executor(resolve, reject)
  }

  then(onFulfilled, onRejected) {
    this.onFulfilled = onFulfilled
    this.onRejected = onRejected
  }
}

const promise = new HYPromise((resolve, reject) => {
  console.log("状态pending")
  // reject(2222)
  resolve(1111)
})

// 调用then方法
promise.then(res => {
  console.log("res1:", res)
  return 1111
}, err => {
  console.log("err:", err)
}).then(res => {
  console.log("res3:", res)
})

// promise.then(res => {
//   console.log("res2:", res)
// }, err => {
//   console.log("err2:", err)
// })