从零手撕Promise，掌握Promise的实现原理(12)之Promise的catch、resolve、reject方法

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传送门

从零手撕Promise，掌握Promise的实现原理(1)之promise基本结构的实现

从零手撕Promise，掌握Promise的实现原理(2)之基础版本的promise实现

从零手撕Promise，掌握Promise的实现原理(3)之回调地狱是什么

从零手撕Promise，掌握Promise的实现原理(4)之then方法链式调用的初步实现

从零手撕Promise，掌握Promise的实现原理(5)之then方法链式调用的进阶实现

从零手撕Promise，掌握Promise的实现原理(6)之then方法的回调为什么是异步微任务

从零手撕Promise，掌握Promise的实现原理(7)then方法链式调用之核心方法resolvePromise

从零手撕Promise，掌握Promise的实现原理(8)then方法链式调用之核心方法resolvePromise再探究

从零手撕Promise，掌握Promise的实现原理(9)then方法链式调用之核心方法resolvePromise完全体

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

从零手撕Promise，掌握Promise的实现原理(11)之测试完全体Promise是否符合PromiseA+

回顾

经过前几篇文章的介绍，我们的Promise已经经过了，Promise A+规范的测试，现在我们的Promise,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){
    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
  }
}

`Promsie`的其他方法

我们完成了，Promsie的主体后，还有一些其他的方法(这些方法并不属于Promsie A+中规定的)，例如catch、reject、resolve等等，这章节主要介绍这三个方法。

`catch`方法

catch方法很简单，catch方法是原型上的方法，它的作用主要是捕获Promise失败的，其实内部实现就是调用then方法的传入第二个参数（第一个参数传null），具体实现如下：

catch(onRejected){
  return this.then(null, onRejected)
}

`resolve`方法

resolve方法是静态方法，它的作用是返回一个成功的Promsie。

注意但是并不是说resolve方法返回的一定是成功，他也会返回失败的Promise,看下面这张图片。假如说给resolve传递了一个Promise，那resolve返回的Promise会由嵌套的这个Promise，去决定。

resolve实现如下，我们还需要改动我们Promsie的resolve方法。


const resolve = (value) => {
   //如果发现value是一个Promise，我们需要调用value的then方法去递归解析
  if(value instanceof Promise){
      return value.then(resolve,reject); // 递归解析
  }
  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())
  }
}


static rsolve(value){
  return new Promise((resolve,reject)=>{
            resolve(value);
  })
}

`reject`方法

reject方法十分的简单，他也是静态方法，它的作用是返回一个失败的Promise,实现如下。

static reject(reason){
    return new Promise((resolve,reject)=>{
        reject(reason);
    })
}

截至目前的`Promise`

现在我们的Promise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) => {
       //如果发现value是一个Promise，我们需要调用value的then方法去递归解析
      if(value instanceof Promise){
          return value.then(resolve,reject); // 递归解析
      }
      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){
    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
  }
  catch(onRejected){
    return this.then(null, onRejected)
  }
  static rsolve(value){
    return new Promise((resolve,reject)=>{
              resolve(value);
    })
  }
  static reject(reason){
      return new Promise((resolve,reject)=>{
          reject(reason);
      })
  }
}