Promise介绍与基本使用
理解
抽象表达
Promise 是一门新的技术(ES6 规范)
Promise 是 JS 中进行异步编程的新解决方案 (旧方案是单纯使用回调函数)
异步编程:
fs文件操作:require('fs').readFile('./index.html', (err,data)=>{})
数据库操作
Ajax网络请求:$.get('/server', (data)=>{})
定时器:setTimeout(()=>{}, 2000)
具体表达
从语法上来说: Promise 是一个构造函数
从功能上来说: promise 对象用来封装一个异步操作并可以获取其成功或失败的结果值
使用Promise的原因
指定回调函数的方式更加灵活
旧的: 必须在启动异步任务前指定
promise: 启动异步任务 => 返回promise对象 => 给promise对象绑定回调函数(甚至可以在异步任务结束后指定/多个)
支持链式调用, 可以解决回调地狱问题
回调地狱:回调函数嵌套调用, 外部回调函数异步执行的结果是嵌套的回调执行的条件
回调地狱的缺点:
对象属性
状态
实例对象中的一个属性 『PromiseState
』
pending
未决定的
resolved
/ fullfilled
成功
rejected
失败
结果值
实例对象中的另一个属性 『PromiseResult
』,保存着异步任务『成功/失败』的结果
工作流程
基本使用
基本编码
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 61 62 63 64 65 66 67 68 <!DOCTYPE html > <html > <head > <meta charset ="UTF-8" > <meta http-equiv ="X-UA-Compatible" content ="IE=edge" > <meta name ="viewport" content ="width=device-width, initial-scale=1.0" > <title > Promise初体验</title > <link crossorigin ='anonymous' href ="https://cdn.bootcss.com/twitter-bootstrap/3.3.7/css/bootstrap.min.css" rel ="stylesheet" > </head > <body > <div class ="container" > <h2 class ="page-header" > Promise 初体验</h2 > <button id ="btn" class ="btn btn-primary" > 点击抽奖</button > </div > <script > function rand (m, n ) { return Math .ceil(Math .random() * (n-m+1 )) + m-1 } const btn = document .getElementById('btn' ) btn.addEventListener('click' , function ( ) { const p = new Promise ((resolve, reject ) => { setTimeout (() => { let n = rand(1 , 100 ) if (n <= 30 ) { resolve(n) } else { reject(n) } }, 1000 ) }) p.then((value ) => { alert('恭喜中奖,号码为' + value) }, (reason ) => { alert('再接再厉,编号为' + reason) }) }) </script > </body > </html >
fs模块读取文件
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 const fs = require ('fs' )let p = new Promise ((resolve, reject ) => { fs.readFile('./resource/content.txt' , (err, data ) => { if (err) reject(err) resolve(data) }) }) p.then(value => { console .log(value.toString()) }, reason => { console .log(reason) })
发送Ajax请求
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 <!DOCTYPE html > <html > <head > <meta charset ="UTF-8" > <meta http-equiv ="X-UA-Compatible" content ="IE=edge" > <meta name ="viewport" content ="width=device-width, initial-scale=1.0" > <title > Ajax请求</title > <link crossorigin ='anonymous' href ="https://cdn.bootcss.com/twitter-bootstrap/3.3.7/css/bootstrap.min.css" rel ="stylesheet" > </head > <body > <div class ="container" > <h2 class ="page-header" > Promise Ajax请求</h2 > <button id ="btn" class ="btn btn-primary" > 发送请求</button > </div > <script > const btn = document .getElementById('btn' ) btn.addEventListener('click' , function ( ) { const p = new Promise ((resolve, reject ) => { const xhr = new XMLHttpRequest() xhr.open('GET' , 'https://api.apiopen.top/getJoke' ) xhr.send() xhr.onreadystatechange = function ( ) { if (xhr.readyState === 4 ) { if (xhr.status >=200 && xhr.status < 300 ) { resolve(xhr.response) } else { reject(xhr.status) } } } }) p.then((value ) => { console .log(value) }, (reason ) => { console .log(reason) }) }) </script > </body > </html >
封装fs模块
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 function myReadFile (path ) { return new Promise ((resolve, reject ) => { require ('fs' ).readFile(path, (err, data ) => { if (err) reject(err) resolve(data) }) }) } myReadFile('./resource/content.txt' ).then((value ) => { console .log(value.toString()) }, (reason ) => { console .log(reason) })
util.promisify
1 2 3 4 5 6 7 8 9 10 11 12 13 14 const util = require ('util' )const fs = require ('fs' )let myReadFile = util.promisify(fs.readFile)myReadFile('./resource/content.txt' ).then((value ) => { console .log(value.toString()) }, (reason ) => { console .log(reason) })
封装Ajax
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 <!DOCTYPE html > <html > <head > <meta charset ="UTF-8" > <meta http-equiv ="X-UA-Compatible" content ="IE=edge" > <meta name ="viewport" content ="width=device-width, initial-scale=1.0" > <title > Promise封装AJAX操作</title > </head > <body > <script > function sendAjax (url ) { return new Promise ((resolve, reject ) => { const xhr = new XMLHttpRequest() xhr.responseType = 'json' xhr.open('GET' , url) xhr.send() xhr.onreadystatechange = function ( ) { if (xhr.readyState === 4 ) { if (xhr.status >=200 && xhr.status < 300 ) { resolve(xhr.response) } else { reject(xhr.status) } } } }) } sendAjax('https://api.apiopen.top/getJoke' ).then((value ) => { console .log(value) }, (reason ) => { console .log(reason) }) </script > </body > </html >
Promise API
Promise
构造函数
Promise (excutor) {}
executor
函数: 执行器 (resolve, reject) => {}
resolve
函数: 内部定义成功时我们调用的函数 value => {}
reject
函数: 内部定义失败时我们调用的函数 reason => {}
说明: executor
会在Promise
内部立即同步调用,异步操作在执行器中执行
Promise.prototype.then
方法
(onResolved, onRejected) => {}
onResolved
函数: 成功的回调函数 (value) => {}
onRejected
函数: 失败的回调函数 (reason) => {}
说明: 指定用于得到成功 value
的成功回调和用于得到失败 reason
的失败回调返回一个新的 Promise
对象
Promise.prototype.catch
方法
(onRejected) => {}
onRejected
函数: 失败的回调函数 (reason) => {}
说明: then()
的语法糖, 相当于: then(undefined, onRejected)
Promise.resolve
方法
(value) => {}
,属于Promise函数对象,不属于实例对象
value
: 成功的数据或 Promise
对象
如果传入的参数为 非Promise
类型的对象, 则返回的结果为成功Promise
对象
如果传入的参数为 Promise
对象, 则参数的结果决定了 resolve
的结果
1 2 3 4 5 6 7 8 9 10 11 12 let p1 = Promise .resolve(521 ) console .log(p1) let p2 = Promise .resolve(new Promise ((resolve, reject ) => { reject('Error' ) })); console .log(p2) p2.catch(reason => { })
Promise.reject
方法
(reason) => {}
,属于Promise函数对象,不属于实例对象
reason
: 失败的原因
说明: 返回一个失败的 promise
对象
Promise.all
方法
(promises) => {}
promises
: 包含 n
个 promise
的数组
说明: 返回一个新的 promise
,只有所有的 promise
都成功才成功, 只要有一个失败了就直接失败
1 2 3 4 5 6 7 8 9 10 11 12 let p1 = new Promise ((resolve, reject ) => { resolve('OK' ); }) let p2 = Promise .resolve('Success' )let p3 = Promise .resolve('Oh Yeah' )let p4 = Promise .reject('Error' )const result = Promise .all([p1, p2, p3])const fResult = Promise .all([p1, p2, p3, p4])console .log(result);console .log(fResult)
Promise.race
方法
(promises) => {}
promises
: 包含 n
个 promise
的数组
说明: 返回一个新的 promise
,第一个完成 的 promise
的结果状态就是最终的结果状态
使用场景:ajax请求和超时回调,ajax在超时回调执行前完成,显示回调结果,否则显示请求超时
1 2 3 4 5 6 7 8 9 10 11 12 let p1 = new Promise ((resolve, reject ) => { setTimeout (() => { resolve('OK' ) }, 1000 ); }) let p2 = Promise .resolve('Success' )let p3 = Promise .resolve('Oh Yeah' )const result = Promise .race([p1, p2, p3])console .log(result)
Promise关键问题
如何改变 promise 的状态?
resolve(value)
:如果当前是 pending
就会变为 resolved
reject(reason)
:如果当前是 pending
就会变为 rejected
throw 'error_str'
抛出异常:如果当前是 pending
就会变为 rejected
(异步不能throw
,捕获不到)
一个 promise
指定多个成功/失败回调函数, 都会调用吗?(用then()
方法为一个promise
对象指定多个回调)
改变 promise
状态和指定回调函数谁先谁后?
promise.then()
返回的新 promise
的结果状态由什么决定?
简单表达: 由 then()
指定的回调函数执行的结果决定
详细表达:
如果抛出异常,新 promise
变为 rejected
,reason
为抛出的异常
如果返回的是非promise
的任意值,新promise
变为resolved
,value
为返回的值
如果返回的是另一个新promise
,此promise
的结果就会成为新promise
的结果
promise
如何串连多个操作任务
promise
的 then()
返回一个新的 promise
,可以开成 then()
的链式调用
通过 then
的链式调用串连多个同步/异步任务
let p = new Promise((resolve, reject) => {
setTimeout(() => {
resolve('OK')
}, 1000)
})
p.then(value => {
return new Promise((resolve, reject) => {
resolve("success")
})
}).then(value => {
console.log(value)
}).then(value => {
console.log(value)
})
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 ![image.png](https://s2.loli.net/2022/04/11/5Itb3GgPznDi9vB.png) 6. `promise`异常穿透 * 当使用 `promise` 的 `then` 链式调用时,可以在最后指定失败的回调 ```js p.then(value => { // console.log(111) throw '失败啦!'; }).then(value => { console.log(222) }).then(value => { console.log(333) }).catch(reason => { console.warn(reason) });
前面任何操作出了异常,都会传到最后失败的回调中处理
如何中断 promise
链
当使用 promise
的 then
链式调用时, 在中间中断, 不再调用后面的回调函数
办法: 在回调函数中返回一个 pendding
状态的 promise 对象
1 2 3 4 5 6 7 8 9 10 11 p.then(value => { console .log(111 ) return new Promise (() => {}) }).then(value => { console .log(222 ) }).then(value => { console .log(333 ) }).catch(reason => { console .warn(reason) });
Promise自定义封装(手写Promise)
初始结构搭建
1 2 3 4 5 6 7 8 9 function Promise (executor ) { } Promise .prototype.then = function (onResolved, onRejected ) {}
resolve与reject
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 function Promise (executor ) { this .PromsieState = 'pending' this .PromiseResult = null const self = this function resolve (data ) { self.PromsieState = 'fulfilled' self.PromiseResult = data } function reject (data ) { self.PromsieState = 'rejected' self.PromiseResult = data } executor(resolve, reject) }
throw抛出异常改变状态
1 2 3 4 5 6 try { executor(resolve, reject) } catch (e) { reject(e) }
设置对象状态只能修改一次
在resolve()
和reject()
添加状态判断
1 2 if (self.PromsieState !== 'pending' ) return
then方法执行回调
1 2 3 4 5 6 7 8 9 Promise .prototype.then = function (onResolved, onRejected ) { if (this .PromsieState === 'fulfilled' ) { onResolved(this .PromiseResult) } if (this .PromsieState === 'rejected' ) { onRejected(this .PromiseResult) } }
异步任务回调的执行
为Promise对象增加属性:this.callback = {}
在then方法中判断pending状态:
1 2 3 4 5 6 7 8 if (this .PromiseState === 'pending' ) { this .callback = { onResolved : onResolved, onRejected : onRejected } }
resolve函数和reject函数分别调用对应的回调函数:
1 2 3 4 5 6 7 8 9 if (self.callback.onResolved) { self.callback.onResolved(data) } if (self.callback.onRejected) { self.callback.onRejected(data) }
指定多个回调的实现
将callback修改为数组:this.callbacks = []
修改then方法中判断pending状态:
1 2 3 4 5 6 7 if (this .PromiseState === 'pending' ) { this .callbacks.push({ onResolved, onRejected }) }
resolve函数和reject函数分别调用对应的回调函数:
1 2 3 4 5 6 7 8 9 self.callbacks.forEach(item => { item.onResolved(data) }) self.callbacks.forEach(item => { item.onRejected(data) })
修改状态then返回结果
同步任务
判断onResolved
的返回值,是Promise对象则then,非Promise对象则resolve,使用try-catch抛出异常
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 Promise .prototype.then = function (onResolved, onRejected ) { return new Promise ((resolve, reject ) => { if (this .PromiseState === 'fulfilled' ) { try { let result = onResolved(this .PromiseResult) if (result instanceof Promise ) { result.then(v => { resolve(v) }, r => { reject(r) }) } else { resolve(result) } } catch (e) { reject(e) } } if (this .PromiseState === 'rejected' ) { try { let result = onRejected(this .PromiseResult) if (result instanceof Promise ) { result.then(v => { resolve(v) }, r => { reject(r) }) } else { resolve(result) } } catch (e) { reject(e) } } if (this .PromiseState === 'pending' ) { this .callbacks.push({ onResolved, onRejected }) } }) }
异步任务
状态为pending时
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 if (this .PromiseState === 'pending' ) { this .callbacks.push({ onResolved : function ( ) { try { let result = onResolved(self.PromiseResult) if (result instanceof Promise ) { result.then(v => { resolve(v) }, r => { reject(r) }) } else { resolve(result) } } catch (e) { reject(e) } }, onRejected : function ( ) { try { let result = onRejected(self.PromiseResult) if (result instanceof Promise ) { result.then(v => { resolve(v) }, r => { reject(r) }) } else { resolve(result) } } catch (e) { reject(e) } } }) }
代码优化
将重复的代码块进行封装
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 Promise .prototype.then = function (onResolved, onRejected ) { const self = this return new Promise ((resolve, reject ) => { function callback (type ) { try { let result = type(self.PromiseResult) if (result instanceof Promise ) { result.then(v => { resolve(v) }, r => { reject(r) }) } else { resolve(result) } } catch (e) { reject(e) } } if (this .PromiseState === 'fulfilled' ) { callback(onResolved) } if (this .PromiseState === 'rejected' ) { callback(onRejected) } if (this .PromiseState === 'pending' ) { this .callbacks.push({ onResolved : function ( ) { callback(onResolved) }, onRejected : function ( ) { callback(onRejected) } }) } }) }
catch方法
catch方法
1 2 3 4 Promise .prototype.catch = function (onRejected ) { return this .then(undefined , onRejected) }
异常穿透与值传递
then方法中判断回调函数参数
1 2 3 4 5 6 7 8 9 10 if (onRejected !== 'function' ) { onRejected = reason => { throw reason } } if (onResolved !== 'function' ) { onResolved = value => value }
Promise.resolve与Promise.reject封装
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 Promise .resolve = function (value ) { return new Promise ((resolve, reject ) => { if (value instanceof Promise ) { value.then(v => { resolve(v) }, r => { reject(r) }) } else { resolve(value) } }) } Promise .reject = function (reason ) { return new Promise ((resolve, reject ) => { reject(reason) }) }
Promise.all封装
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 Promise .all = function (promises ) { return new Promise ((resolve, reject ) => { let count = 0 let arr = [] for (let i = 0 ; i < promises.length; i++) { promises[i].then(v => { count++ arr[i] = v if (count === promises.length) { resolve(arr) } }, r => { reject(r) }) } }) }
Promise.race封装
1 2 3 4 5 6 7 8 9 10 11 12 Promise .race = function (promises ) { return new Promise ((resolve, reject ) => { for (let i = 0 ; i < promises.length; i++) { promises[i].then(v => { resolve(v) }, r => { reject(r) }) } }) }
回调函数『异步执行』
“回调函数『异步执行』”指then方法指定的回调函数是异步执行的
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 <!DOCTYPE html > <html lang ="zh" > <head > <meta charset ="UTF-8" > <meta name ="viewport" content ="width=device-width, initial-scale=1.0" > <title > Promise回调函数『异步执行』</title > </head > <body > <script > let p1 = new Promise ((resolve, reject ) => { resolve('OK' ) console .log('111' ) }) p1.then(value => { console .log('222' ) }) console .log('333' ) </script > </body > </html >
执行结果:
then方法中的回调是异步执行的,需要同步执行完毕后再执行
对then方法与构造函数中的resolve、reject在调用回调时,添加定时器:
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 function Promise (executor ) { ... function resolve (data ) { ... setTimeout (() => { self.callbacks.forEach(item => { item.onResolved(data) }) }) } function reject (data ) { ... setTimeout (() => { self.callbacks.forEach(item => { item.onRejected(data) }) }) } ... } Promise .prototype.then = function (onResolved, onRejected ) { ... return new Promise ((resolve, reject ) => { ... if (this .PromiseState === 'fulfilled' ) { setTimeout (() => { callback(onResolved) }) } if (this .PromiseState === 'rejected' ) { setTimeout (() => { callback(onResolved) }) } ... }) }
完整代码
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 61 62 63 64 65 66 67 68 69 70 71 72 73 74 75 76 77 78 79 80 81 82 83 84 85 86 87 88 89 90 91 92 93 94 95 96 97 98 99 100 101 102 103 104 105 106 107 108 109 110 111 112 113 114 115 116 117 118 119 120 121 122 123 124 125 126 127 128 129 130 131 132 133 134 135 136 137 138 139 140 141 142 143 144 145 146 147 148 149 150 151 152 153 154 155 156 157 158 159 160 161 162 163 164 165 166 167 168 169 170 171 172 173 function Promise (executor ) { this .PromiseState = 'pending' this .PromiseResult = null this .callbacks = [] const self = this function resolve (data ) { if (self.PromiseState !== 'pending' ) return self.PromiseState = 'fulfilled' self.PromiseResult = data setTimeout (() => { self.callbacks.forEach(item => { item.onResolved(data) }) }) } function reject (data ) { if (self.PromiseState !== 'pending' ) return self.PromiseState = 'rejected' self.PromiseResult = data setTimeout (() => { self.callbacks.forEach(item => { item.onRejected(data) }) }) } try { executor(resolve, reject) } catch (e) { reject(e) } } Promise .prototype.then = function (onResolved, onRejected ) { const self = this if (typeof onRejected !== 'function' ) { onRejected = reason => { throw reason } } if (typeof onResolved !== 'function' ) { onResolved = value => value } return new Promise ((resolve, reject ) => { function callback (type ) { try { let result = type(self.PromiseResult) if (result instanceof Promise ) { result.then(v => { resolve(v) }, r => { reject(r) }) } else { resolve(result) } } catch (e) { reject(e) } } if (this .PromiseState === 'fulfilled' ) { setTimeout (() => { callback(onResolved) }) } if (this .PromiseState === 'rejected' ) { setTimeout (() => { callback(onResolved) }) } if (this .PromiseState === 'pending' ) { this .callbacks.push({ onResolved : function ( ) { callback(onResolved) }, onRejected : function ( ) { callback(onRejected) } }) } }) } Promise .prototype.catch = function (onRejected ) { return this .then(undefined , onRejected) } Promise .resolve = function (value ) { return new Promise ((resolve, reject ) => { if (value instanceof Promise ) { value.then(v => { resolve(v) }, r => { reject(r) }) } else { resolve(value) } }) } Promise .reject = function (reason ) { return new Promise ((resolve, reject ) => { reject(reason) }) } Promise .all = function (promises ) { return new Promise ((resolve, reject ) => { let count = 0 let arr = [] for (let i = 0 ; i < promises.length; i++) { promises[i].then(v => { count++ arr[i] = v if (count === promises.length) { resolve(arr) } }, r => { reject(r) }) } }) } Promise .race = function (promises ) { return new Promise ((resolve, reject ) => { for (let i = 0 ; i < promises.length; i++) { promises[i].then(v => { resolve(v) }, r => { reject(r) }) } }) }
封装成类
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 61 62 63 64 65 66 67 68 69 70 71 72 73 74 75 76 77 78 79 80 81 82 83 84 85 86 87 88 89 90 91 92 93 94 95 96 97 98 99 100 101 102 103 104 105 106 107 108 109 110 111 112 113 114 115 116 117 118 119 120 121 122 123 124 125 126 127 128 129 130 131 132 133 134 135 136 137 138 139 140 141 142 143 144 145 146 147 148 149 150 151 152 153 154 155 156 157 158 159 160 161 162 163 164 165 166 167 class Promise { constructor (executor ) { this .PromiseState = 'pending' this .PromiseResult = null this .callbacks = [] const self = this function resolve (data ) { if (self.PromiseState !== 'pending' ) return self.PromiseState = 'fulfilled' self.PromiseResult = data setTimeout (() => { self.callbacks.forEach(item => { item.onResolved(data) }) }) } function reject (data ) { if (self.PromiseState !== 'pending' ) return self.PromiseState = 'rejected' self.PromiseResult = data setTimeout (() => { self.callbacks.forEach(item => { item.onRejected(data) }) }) } try { executor(resolve, reject) } catch (e) { reject(e) } } then (onResolved, onRejected ) { const self = this if (typeof onRejected !== 'function' ) { onRejected = reason => { throw reason } } if (typeof onResolved !== 'function' ) { onResolved = value => value } return new Promise ((resolve, reject ) => { function callback (type ) { try { let result = type(self.PromiseResult) if (result instanceof Promise ) { result.then(v => { resolve(v) }, r => { reject(r) }) } else { resolve(result) } } catch (e) { reject(e) } } if (this .PromiseState === 'fulfilled' ) { setTimeout (() => { callback(onResolved) }) } if (this .PromiseState === 'rejected' ) { setTimeout (() => { callback(onResolved) }) } if (this .PromiseState === 'pending' ) { this .callbacks.push({ onResolved : function ( ) { callback(onResolved) }, onRejected : function ( ) { callback(onRejected) } }) } }) } catch (onRejected) { return this .then(undefined , onRejected) } static resolve (value ) { return new Promise ((resolve, reject ) => { if (value instanceof Promise ) { value.then(v => { resolve(v) }, r => { reject(r) }) } else { resolve(value) } }) } static reject (reason ) { return new Promise ((resolve, reject ) => { reject(reason) }) } static all (promises ) { return new Promise ((resolve, reject ) => { let count = 0 let arr = [] for (let i = 0 ; i < promises.length; i++) { promises[i].then(v => { count++ arr[i] = v if (count === promises.length) { resolve(arr) } }, r => { reject(r) }) } }) } static race (promises ) { return new Promise ((resolve, reject ) => { for (let i = 0 ; i < promises.length; i++) { promises[i].then(v => { resolve(v) }, r => { reject(r) }) } }) } }
async与await
async函数 await表达式
async函数
函数的返回值为Promise对象:
返回一个非Promise类型的数据:返回值的状态为成功、结果值为该值
返回一个Promise对象:返回值的状态由该对象决定
抛出异常:返回值的状态为失败,结果值为抛出的异常
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 async function main ( ) { throw "Oh NO" ; } let result = main();console .log(result);
await表达式
await右侧的表达式一般为Promise对象,但也可以是其他的值:
如果表达式是promise对象,await返回的是promise成功的值
如果表达式是其他值,直接将此值作为await的返回信息
注意:
await必须写在async函数中,但async函数中可以没有await表达式
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 async function main ( ) { let p = Promise .resolve('ok' ) let res = await p console .log(res) res = await 20 console .log(res) try { let p2 = Promise .reject('error' ) res = await p2 } catch (e) { console .log(e) } }
案例:读取文件
使用fs模块从三个文件中读取内容并拼接
回调函数方式
1 2 3 4 5 6 7 8 9 10 11 12 13 const fs = require ('fs' )fs.readFile('./resource/1.html' , (err, data1 ) => { if (err) throw err fs.readFile('./resource/2.html' , (err, data2 ) => { if (err) throw err fs.readFile('./resource/3.html' , (err, data3 ) => { if (err) throw err console .log(data1 + data2 + data3) }) }) })
async与await结合
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 const fs = require ('fs' )const util = require ('util' )const myReadFile = util.promisify(fs.readFile)async function main ( ) { try { let data1 = await myReadFile('./resource/1.html' ) let data2 = await myReadFile('./resource/2.html' ) let data3 = await myReadFile('./resource/3.html' ) console .log(data1 + data2 + data3) } catch (e) { console .log(e) } } main()
案例:发送Ajax请求
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 <!DOCTYPE html > <html > <head > <meta charset ="UTF-8" > <title > Ajax</title > </head > <body > <button > 点击获取段子</button > <script > function sendAjax (url ) { return new Promise ((resolve, reject ) => { const xhr = new XMLHttpRequest() xhr.responseType = 'json' xhr.open('GET' , url) xhr.send() xhr.onreadystatechange = function ( ) { if (xhr.readyState === 4 ) { if (xhr.status >=200 && xhr.status < 300 ) { resolve(xhr.response) } else { reject(xhr.status) } } } }) } const btn = document .querySelector('button' ) btn.addEventListener('click' , async function ( ) { let result = await sendAjax('https://api.apiopen.top/getJoke' ) console .log(result) }) </script > </body > </html >