从零手撕Promise，掌握Promise的实现原理(10)then方法完全体「这是我参与11月更文挑战的第10天，活动

「这是我参与11月更文挑战的第10天，活动详情查看：2021最后一次更文挑战」。

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经过前面9篇文章的介绍，我们的Promise已经接近尾声了，现在它长这样

const PENDING = 'pending'
const FULFILLED = 'fulfilled'
const REJECTED = 'rejected'
const resolvePromise = (promise, x, resolve, reject) => {
  //*******************************************************判断x与promsie是否是同一个promise，防止进入死循环*********************//////*********
  if(x === promise) throw new TypeError('Chaining cycle detected for promise #<Promise>')
  // 1.首先判断`x`是基础类型数据，还是引用类型，基础类型的数据直接`resolve`,即可。  
  if(x !== null && /^(object|function)$/.test(typeof x)){
    let then 
    // 2. 如果是引用类型的数据，尝试获取`x`上的`then`属性(`x.then`)，如果在获取属性的时候报异常则`reject`
    try{
      then = x.then
    }catch(e){
      reject(e)
    }
    //3. 判断`then`是否是函数，如果是一个函数则我们认定它为`Promise`,如果不是则`resolve`
    if(typeof then === 'function'){
      let called = false //**************************************这里加了变量*************************//////
      try{
        then.call(x, (y) => {
          if(called) return //**************************************这里加了变量*************************//////
          called = true
          resolvePromise(promise, y, resolve, reject)
        },(r) => {
          if(called) return//**************************************这里加了变量*************************//////
          reject(r)
        })
      }catch(e){
        if(called) return//**************************************这里加了变量*************************//////
        reject(e)
      }
    }else{
      resolve(x)
    }
  }else{
    //基础类型数据直接resolve
    resolve(x)
  }
}
class Promise{
  constructor(executor){

    this.state = PENDING
    this.value = undefined
    this.reason = undefined
    //存放onFulfilled
    this.onResolvedCallbacks = []
    //存放onRejected
    this.onRejectedCallbacks = []
    const resolve = (value) => {
      if (this.state === PENDING) {
        this.value = value
        this.state = FULFILLED
        //promise实例状态改变后调用暂存的onFulfilled
        this.onResolvedCallbacks.forEach(fn => fn())
      }
    }

    const reject = (reason) => {
      if (this.state === PENDING) {
        this.reason = reason
        this.state = REJECTED
        //promise实例状态改变后调用的onRejected
        this.onRejectedCallbacks.forEach(fn => fn())
      }
    }
    try {
      //executor函数执行过程中出错，将会导致Promise失败
      executor(resolve,reject)
    } catch (error) {
      reject(error)
    }
  }
  then(onFulfilled, onRejected){

    let promise = new Promise((resolve, reject) => {
        
        switch(this.state){
          case FULFILLED:
              setTimeout(() => {
                try{
                  let x = onFulfilled(this.value)
                  resolvePromise(promise, x, resolve, reject)
                } catch(e){
                  reject(e)
                }
              })   
              break
          case REJECTED:
              setTimeout(() => {
                try{
                  let x = onRejected(this.reason)
                  resolvePromise(promise, x, resolve, reject)
                } catch(e){
                  reject(e)
                }
              })   
              break
          default:
            this.onResolvedCallbacks.push(() => {
              setTimeout(() => {
                try{
                  let x = onFulfilled(this.value)
                  resolvePromise(promise, x, resolve, reject)
                } catch(e){
                  reject(e)
                }
              })
            })
            this.onRejectedCallbacks.push(() => {
              setTimeout(() => {
                try{
                  let x = onRejected(this.reason)
                  resolvePromise(promise, x, resolve, reject)
                } catch(e){
                  reject(e)
                }
              })
            })
        }
    })
    return promise
  }
}

`then`方法小瑕疵解决

有这样一种情况，我们的then方法，是无法满足链式调用的，例如下面这段代码，用户在调用then方法的时候不传值，或者传递了空，我们需要判断处理一下，让then函数能够继续链式调用。

let p = new Promise((resolve,reject) => {
  resolve('成功')
})

p.then().then(value => {
console.log('value',value);
},reason => {
console.log('reason',reason);
})
// p.then(null, null).then(value => {
// console.log('value',value);
// },reason => {
// console.log('reason',reason);
// })

then方法改造如下如果用户在使用then方法的时候没有传递onFulfilled/onRejected，或者传递不是一个函数，那我们要给onFulfilled/onRejected赋予默认值，这样就可以链式调用了。

then(onFulfilled, onRejected){
  
    onFulfilled = typeof onFulfilled === 'function' ? onFulfilled : v => v
    onRejected = typeof onRejected === 'function' ? onRejected : err => { throw err }

    let promise = new Promise((resolve, reject) => {
        
        switch(this.state){
          case FULFILLED:
              setTimeout(() => {
                try{
                  let x = onFulfilled(this.value)
                  resolvePromise(promise, x, resolve, reject)
                } catch(e){
                  reject(e)
                }
              })   
              break
          case REJECTED:
              setTimeout(() => {
                try{
                  let x = onRejected(this.reason)
                  resolvePromise(promise, x, resolve, reject)
                } catch(e){
                  reject(e)
                }
              })   
              break
          default:
            this.onResolvedCallbacks.push(() => {
              setTimeout(() => {
                try{
                  let x = onFulfilled(this.value)
                  resolvePromise(promise, x, resolve, reject)
                } catch(e){
                  reject(e)
                }
              })
            })
            this.onRejectedCallbacks.push(() => {
              setTimeout(() => {
                try{
                  let x = onRejected(this.reason)
                  resolvePromise(promise, x, resolve, reject)
                } catch(e){
                  reject(e)
                }
              })
            })
        }
    })
    return promise
  }

下篇文章将会测试我们的成果，看看是否符合`Promsie A+`规范

传送门

从零手撕Promise，掌握Promise的实现原理(10)then方法完全体

回顾

then方法小瑕疵解决

下篇文章将会测试我们的成果，看看是否符合Promsie A+规范

`then`方法小瑕疵解决

下篇文章将会测试我们的成果，看看是否符合`Promsie A+`规范