JavaScript模拟实现Promise功能的示例代码

模拟Promise的功能,  按照下面的步骤,一步一步

1. 新建是个构造函数

2. 传入一个可执行函数 函数的入参第一个为 fullFill函数 第二个为 reject函数; 函数立即执行, 参数函数异步执行

3. 状态一旦更改就不可以变更 只能 pending => fulfilled 或者 pending => rejected

4. then 的时候要处理入参的情况 successCallback 和failCallback 均可能为非函数

默认的 failCallback 一定要将异常抛出, 这样下一个promise便可将其捕获 异常冒泡的目的

5. then 中执行回调的时候要捕获异常 将其传给下一个promise

  • 如果promise状态未变更 则将回调方法添加到对应队列中
  • 如果promise状态已经变更 需要异步处理成功或者失败回调
  • 因为可能出现 回调结果和当前then返回的Promise一致 从而导致死循环问题

6. catch只是then的一种特殊的写法 方便理解和使用

7. finally 特点

  • 不过resolve或者reject都会执行
  • 回调没有参数
  • 返回一个Promise 且值可以穿透到下一个then或者catch

8. Promise.resolve, Promise.reject 根据其参数返回对应的值 或者状态的Promise即可

9. Proise.all 特点

  • 返回一个Promise
  • 入参是数组 resolve的情况下出参也是数组 且结果顺序和调用顺序一致
  • 所有的值或者promise都完成才能resolve 所有要计数
  • 只要有一个为reject 返回的Promise便reject

10. Proise.race 特点

  • 返回一个Promise
  • 入参是数组 那么出参根据第一个成功或者失败的参数来确定
  • 只要有一个resolve 或者reject 便更改返回Promise的状态
const PENDING = 'pending' //等待
const FULFILLED = 'fulfilled' //成功
const REJECTED = 'rejected' //失败

const fulfilledCallback = [] //成功回调函数
const rejectedCallback = [] //失败回调函数

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

 status = PENDING//promise的状态
 value = undefined//成功之后的值
 reason = undefined//失败之后的值
 fulfilledCallback = [] //成功回调函数
 rejectedCallback = [] //失败回调函数
 resolve = value => {
 //如果状态不是等待, 阻止程序继续往下执行
 if(this.status !== PENDING){
 return
 }
 this.status = FULFILLED
 this.value = value
 //判断成功回调是否存在,如果存在, 调用
 // this.fulfilledCallback && this.fulfilledCallback(this.value)
 while(this.fulfilledCallback.length) this.fulfilledCallback.shift()()

 }
 reject = reason => {
 //如果状态不是等待, 阻止程序继续往下执行
 if(this.status !== PENDING){
 return
 }
 this.status = REJECTED
 this.reason = reason
 //判断失败回调是否存在,如果存在, 调用
 // this.rejectedCallback && this.rejectedCallback(this.reason)
 while(this.rejectedCallback.length) this.rejectedCallback.shift()()
 }
 then = (fulfilledCallback, rejectedCallback) => {
 fulfilledCallback = fulfilledCallback ? fulfilledCallback : value => value
 rejectedCallback = rejectedCallback ? rejectedCallback : reason => {throw reason}
 let promise2 = new MyPromise((resolve, reject) => {
 
 //判断状态
 if(this.status === FULFILLED){
 setTimeout(() => {
 try {
 //判断x 传过来的可能是promise,先查看promise返回的结果,在决定是用resolve还是reject,
 //如果是普通值,直接调用resolve
 let x = fulfilledCallback(this.value)
 //外面加载完,才能获取到promise2,用异步解决
 resolvePromise(promise2,x,resolve, reject)
 } catch (error) {
 reject(error)
 }
 
 },0)
 
 }else if(this.status === REJECTED){
 
 setTimeout(() => {
 try {
 //判断x 传过来的可能是promise,先查看promise返回的结果,在决定是用resolve还是reject,
 //如果是普通值,直接调用resolve
 let x = rejectedCallback(this.reason)
 //外面加载完,才能获取到promise2,用异步解决
 resolvePromise(promise2,x,resolve, reject)
 } catch (error) {
 reject(error)
 }
 
 },0)
 }else{
 //等待状态
 this.fulfilledCallback.push(() => {
 setTimeout(() => {
 try {
 //判断x 传过来的可能是promise,先查看promise返回的结果,在决定是用resolve还是reject,
 //如果是普通值,直接调用resolve
 let x = fulfilledCallback(this.value)
 //外面加载完,才能获取到promise2,用异步解决
 resolvePromise(promise2,x,resolve, reject)
 } catch (error) {
 reject(error)
 }
 
 },0)
 });
 this.rejectedCallback.push(() => {
 setTimeout(() => {
 try {
 //判断x 传过来的可能是promise,先查看promise返回的结果,在决定是用resolve还是reject,
 //如果是普通值,直接调用resolve
 let x = rejectedCallback(this.reason)
 //外面加载完,才能获取到promise2,用异步解决
 resolvePromise(promise2,x,resolve, reject)
 } catch (error) {
 reject(error)
 }
 
 },0)
 });
 }
 })
 return promise2
 }

 static all (array) {
 let result= []
 let index = 0;

 return new MyPromise((resolve, reject) => {

 function addData (key, value) {
 result[key] = value
 index++
 if (index === array.length) {
 resolve(result)
 }
 }

 for (let i = 0; i < array.length; i++) {
 
 if (array[i] instanceof MyPromise) {
 //promise对象
 array[i].then(value => addData(i, value), reason => reject(reason))
 } else {
 //普通值, 放到数组里
 addData(i, array[i])
 }
 }
 })

 }

 static resolve (value) {
 if(value instanceof MyPromise) return value
 return new MyPromise(resolve => resolve(value))

 }

 finally (callback) {
 return this.then(value => {
 return MyPromise.resolve(callback()).then(() => value)
 }, reason => {
 return MyPromise.resolve(callback()).then(() => {throw reason})
 })

 }

 catch (rejectedCallback) {
 return this.then(undefined, rejectedCallback)
 }

}

function resolvePromise(promise2,x,resolve, reject){

 if(x === promise2){
 return reject(new TypeError('啦啦啦啦'))
 }
 if(x instanceof MyPromise){
 //是promise
 //往下直接传
 x.then(resolve,reject)
 }else{
 //普通值
 resolve(x)
 }
}

module.exports = MyPromise;
作者:菜鸟小于原文地址:https://www.cnblogs.com/Young111/p/16940017.html

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