手写Promise的实现（链式编程）

五毛

December 6, 2020

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个人笔记

上一次实现了一个简单的promise,这次继续写他的链式编程,仅仅是return出一个值的情况

### 特点

1. promise的then方法会继续返回一个promise还能继续then
2. 如果直接return一个值会在下一个then的成功处回调出来
3. 有两种情况会在下一个失败的回调输出，一个是第一个then里面的方法报错了，还有一个是返回的一个promise里面的方法报错了。

```javascript
const PENDING = 'PENDING'
const FULFILLED = 'FULFILLED'
const REJECTED = 'REJECTED'
class Promise {
  constructor(executor) {
    this.value = ''
    this.reason = ''
    this.status = PENDING
    this.onFulfilledCBArr = []
    this.onRejectedCBArr = []
    executor(resolve.bind(this), reject.bind(this))
    function resolve(params) {
      this.status = FULFILLED
      this.value = params
      this.onFulfilledCBArr.forEach(fn => fn())
    }
    function reject(params) {
      this.status = REJECTED
      this.reason = params
      this.onRejectedCBArr.forEach(fn => fn())
    }
  }
  then(onFulfilledCB, onRejectedCB) {
    return new Promise((resolve, reject) => {
      if (this.status === FULFILLED) {
        try {
          let r = onFulfilledCB(this.value)
          resolve(r)
        } catch (error) {
          reject(error)
        }
      }
      if (this.status === REJECTED) {
        try {
          let r = onRejectedCB(this.reason)
          resolve(r)
        } catch (error) {
          reject(error)
        }
      }
      if (this.status === PENDING) {
        this.onFulfilledCBArr.push(() => {
          try {
            let r = onFulfilledCB(this.value)
            resolve(r)
          } catch (error) {
            reject(error)
          }
        })
        this.onRejectedCBArr.push(() => {
          try {
            let r = onRejectedCB(this.reason)
            resolve(r)
          } catch (error) {
            reject(error)
          }
        })
      }
    })
  }
}

module.exports = Promise
```

Last modification：December 6th, 2020 at 11:50 pm

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手写Promise的实现（链式编程）