express promise async await promisify 回调函数

Promise 概述

Promise 是 JavaScript 中异步编程解决方案，可以解决回调函数方案中的回调地狱问题可以将 Promise 理解为容器，用于包裹异步 API的容器，当容器中的异步 API执行完成后，Promise 允许我们在容器的外面获取异步API的执行结果，从而避免回调函数嵌套。Promise 翻译为承若，表示它承若帮我们做一些事情，既然它承若了它就要去做，做就会有一个过程、就会有一个结果，结果要么是成功要么是失败。
所以在 Promise 中有三种状态,分别为等待(pending)，成功(fulfilled)，失败(rejected)。默认状态为等待，等待可以变为成功，等待可以变为失败状态一旦更改不可改变，成功不能变回等待，失败不能变回等待，成功不能变成失败，失败不能变成成功。

Promise示例代码

示例1

let myPro = new Promise((resolve, reject)=>{
    setTimeout(()=>{
        resolve(5)
    },1000)
}).then(() => {
    console.log(123)
})

输出

123

示例2

let myPro = new Promise((resolve, reject)=>{
    const currentTime = new Date();
    const formattedTime = `${currentTime.getHours()}:${currentTime.getMinutes()}:${currentTime.getSeconds()}:${currentTime.getMilliseconds()}`;
    console.log(formattedTime);
    for (let i = 0; i < 1000000000; i++) {

    }
    resolve()
}).then(() => {

})
const currentTime = new Date();
const formattedTime = `${currentTime.getHours()}:${currentTime.getMinutes()}:${currentTime.getSeconds()}:${currentTime.getMilliseconds()}`;
console.log(formattedTime);
console.log(123)

输出：

20:18:50:45
20:18:50:486
123

示例3

let p1 = new Promise((resolve, reject) => setTimeout(reject, 300, "失败"))
let p2 = new Promise(resolve => setTimeout(resolve, 100, 2))
let p3 = 3
const currentTime = new Date();
const formattedTime = `${currentTime.getHours()}:${currentTime.getMinutes()}:${currentTime.getSeconds()}:${currentTime.getMilliseconds()}`;
console.log(formattedTime);
Promise.all([p1, p2, p3]).then(value => {
    const currentTime = new Date();
    const formattedTime = `${currentTime.getHours()}:${currentTime.getMinutes()}:${currentTime.getSeconds()}:${currentTime.getMilliseconds()}`;
    console.log(formattedTime);
    console.log(value)
})

输出

20:43:12:671
(node:25908) UnhandledPromiseRejectionWarning: 失败                    
(Use `node --trace-warnings ...` to show where the warning was created)
(node:25908) UnhandledPromiseRejectionWarning: Unhandled promise rejection. This error originated either by throwing inside of an async function without a catch blo
ck, or by rejecting a promise which was not handled with .catch(). To terminate the node process on unhandled promise rejection, use the CLI flag `--unhandled-rejec
tions=strict` (see https://nodejs.org/api/cli.html#cli_unhandled_rejections_mode). (rejection id: 1)
(node:25908) [DEP0018] DeprecationWarning: Unhandled promise rejections are deprecated. In the future, promise rejections that are not handled will terminate the No
de.js process with a non-zero exit code.

Promise.all 并发操作

示例1

Promise.all([
    readFile('D:\\work\\jjBest\\0.txt'),
    readFile('D:\\work\\jjBest\\1.txt'),
    readFile('D:\\work\\jjBest\\2.txt')
]).then(function (res) {
    console.log(res)
})

示例2

let p1 = new Promise(resolve => setTimeout(resolve, 300, 1))
let p2 = new Promise(resolve => setTimeout(resolve, 100, 2))
let p3 = 3
const currentTime = new Date();
const formattedTime = `${currentTime.getHours()}:${currentTime.getMinutes()}:${currentTime.getSeconds()}:${currentTime.getMilliseconds()}`;
console.log(formattedTime);
Promise.all([p1, p2, p3]).then(value => {
    const currentTime = new Date();
    const formattedTime = `${currentTime.getHours()}:${currentTime.getMinutes()}:${currentTime.getSeconds()}:${currentTime.getMilliseconds()}`;
    console.log(formattedTime);
    console.log(value)
})

输出

20:39:25:180
20:39:25:483
[ 1, 2, 3 ]

Promise.allSettled

示例1

let p1 = new Promise((resolve, reject) => setTimeout(reject, 300, "失败"))
let p2 = new Promise(resolve => setTimeout(resolve, 100, 2))
let p3 = 3
const currentTime = new Date();
const formattedTime = `${currentTime.getHours()}:${currentTime.getMinutes()}:${currentTime.getSeconds()}:${currentTime.getMilliseconds()}`;
console.log(formattedTime);
Promise.allSettled([p1, p2, p3]).then(value => {
    const currentTime = new Date();
    const formattedTime = `${currentTime.getHours()}:${currentTime.getMinutes()}:${currentTime.getSeconds()}:${currentTime.getMilliseconds()}`;
    console.log(formattedTime);
    console.log(value)
})

输出

20:45:57:235
20:45:57:543
[                                        
  { status: 'rejected', reason: '失败' },
  { status: 'fulfilled', value: 2 },     
  { status: 'fulfilled', value: 3 }      
] 

async

加上async就是异步函数了，返回值类型为Promise
返回值会自动包裹在promise中

示例1

async function getTitle(){
    return "标题"
}
console.log(getTitle())

Promise { '标题' }

示例2

async function getTitle() {
    return "标题"
}

getTitle().then(value => console.log(value))

输出

标题

await

异步函数关键字 await

示例1

function readFile(path){
    return new Promise(function (resolve, reject) {
        fs.readFile(path,'utf-8',function (err, data) {
            if (err){
                reject(err)
            }else {
                resolve(data)
            }
        })
    })
}

// 加上async就是异步函数了
async function run(){
    let x = await readFile('D:\\work\\jjBest\\0.txt');
    console.log(x)
    let y = await readFile('D:\\work\\jjBest\\1.txt');
    console.log(y)
    return [x,y]
}
run().then(function (res) {
    console.log(res)
})

示例2

function testWait() {
    return new Promise((resolve, reject) => {
        setTimeout(function () {
            console.log("testWait");
            resolve();
        }, 1000);
    })
}

async function testAwaitUse(){
    await testWait()
    console.log("hello")
    return "test"
}
console.log(testAwaitUse())

输出

Promise { <pending> }
testWait
hello

示例3

function testWait() {
    return new Promise((resolve, reject) => {
        setTimeout(function () {
            console.log("testWait");
            resolve();
        }, 1000);
    })
}

async function testAwaitUse(){
    testWait()
    console.log("hello")
    return "test"
}
console.log(testAwaitUse())

输出

hello             
Promise { 'test' }
testWait

promisify

使用promisify后，不用声明readFile的promise函数了。

const promisify = require('util').promisify
const readFile = promisify(fs.readFile)

// 加上async就是异步函数了
async function run() {
    let x = await readFile('D:\\work\\jjBest\\0.txt', "utf-8");
    console.log(x)
    let y = await readFile('D:\\work\\jjBest\\3.txt', "utf-8");
    console.log(y)
    return [x, y]
}

run().then(function (res) {
    console.log(res)
}).catch(function (err) {
    console.log(err)
})