实现

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

class MyPromise {
  constructor(executor) {
    executor(this.resolve, this.reject)
  }

  status = PENDING
  value = undefined
  reason = undefined

  resolve = value => {
    if (this.status !== PENDING) return
    this.status = FULFILLED
    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

测试

const MyPromise = require('./03-my-promise')

const promise = new MyPromise((resolve, reject) => {
  resolve('成功！')
  // reject('失败！')
})

promise.then(value => {
  console.log(value)
}, reason => {
  console.log(reason)
})