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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
回顾
经过10篇文章的介绍我们的Promsie
已经完成了,接下来我们准备通过一个第三方的npm包来测试一下我们的Promise
是否符合Promise A+
规范。
现在完全体的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
}
}
测试流程
-
npm init --yes
初始化package.json
-
npm i promise-aplus-tests -D
本地安装测试Promise
的第三方包
-
- 在
index.js
文件中,放入我们要测试的Promise
,并加入以下代码.
- 这段代码很简单,在
Promise
上扩展一个静态的方法,这个方法返回一个对象dfd
,dfd
上有三个属性,一个是Promsie
实例,另外两个分别是其对应的resolve
、reject
。 - 其实就是相当于调用这个方法,可以创建一个
Promise
实例,并把这个实例返回。
Promise.deferred = function(){ let dfd = {} dfd.promise = new Promise((resolve,reject)=>{ dfd.resolve = resolve; dfd.reject = reject; }) return dfd; }
- 在
-
- 最后导出我们的
Promsie
module.exports = Promise
- 最后导出我们的
index.js
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
}
}
Promise.deferred = function(){
let dfd = {}
dfd.promise = new Promise((resolve,reject)=>{
dfd.resolve = resolve;
dfd.reject = reject;
})
return dfd;
}
module.exports = Promise
添加测试的执行脚本
- 在
package.json
中添加下面的脚本
"scripts": {
"test": "promises-aplus-tests index.js"
},
- 在终端输入
npm run test
最终测试完美通过Promise A+
规范,大家可以试试
下一篇文章继续,介绍Promise
的其他方法
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