test.js

const PENDING = 'PENDING',
  FULFILLED = 'FULFILLED',
  REJECTED = 'REJECTED'

class BasePromise {

  constructor(executor) {
    this.status = PENDING
    this.value = undefined
    this.reason = undefined
    this.resolve = this.resolve.bind(this)
    this.reject = this.reject.bind(this)
    this.onResolvedCallbacks = []
    this.onRejectedCallbacks = []
    try {
      executor(this.resolve, this.reject)
    } catch (error) {
      this.reject(error)
    }
  }

  resolve(value) {
    if (this.status === PENDING) {
      this.value = value
      this.status = FULFILLED
      this.onResolvedCallbacks.forEach((fn) => {
        fn(this.value)
      })
    }
  }

  reject(reason) {
    if (this.status === PENDING) {
      
      this.reason = reason
      this.status = REJECTED
      this.onRejectedCallbacks.forEach((fn) => {
        fn(this.reason)
      })
    }
  }
  
  
  
  // 这里只能处理包含一个then方法,并接收两个参数onFulfilled, onRejected
  then(onFulfilled, onRejected) {
    // 成功提交
    if (this.status === FULFILLED) {
      onFulfilled(this.value)
    } else if (this.status === REJECTED) {
      // 失败提交
      onRejected(this.reason)
    } else if (this.status === PENDING) {
      // 如果promise的状态是PENDING，需要将onFulfilled和onRejected函数存放起来，等待状态确定后，再依次执行对应的函数
      this.onResolvedCallbacks.push(() => {
        this.value = onFulfilled(this.value)
      })
      this.onRejectedCallbacks.push(() => {
        this.reason = onRejected(this.reason)
      })
    }
  }

  catch() {}
}

// let data = new BasePromise((res,rej) => {
//   console.log(112)
//   res(333)
// })

const promise = new BasePromise((resolve, reject) => {
  setTimeout(() => {
    console.log('success')
    resolve('success')
  }, 2000); 
})

promise.then(() => {
  return new BasePromise((res, rej) => {
    setTimeout(() => {
      console.log('success2')
      resolve('success2')
    }, 2000); 
  })
}).then(() => {
  console.log('end')
})

// promise.then(value => {
//   console.log('resolve', value)
// }, reason => {
//   console.log('reject', reason)
// })