1. 同步与异步API

回调函数

通过回调函数返回异步API的值

代码执行顺序

2. promise 解决回调地狱问题

const fs = require('fs');


function p1() {
    return new Promise((resolve, reject) => {
        fs.readFile('./1.txt','utf8',(err,result) => {
            resolve(result)
        })
    });
}

function p2() {
    return new Promise((resolve, reject) => {
        fs.readFile('./2.txt','utf8',(err,result) => {
            resolve(result)
        })
    });
}

function p2() {
    return new Promise((resolve, reject) => {
        fs.readFile('./2.txt','utf8',(err,result) => {
            resolve(result)
        })
    });
}

p1().then((r1)=> {
    console.log(r1);
    return p2();
})
.then((r2)=> {
    console.log(r2);
    return p3();
})
.then((r3) => {
    console.log(r3)
})

3.异步函数

他是异步编程语法的终极解决方案

async

async function p1 () {
    return 'p1';
}
async function p2 () {
    return 'p2';
}
async function p3 () {
    return 'p3';
}

async function run(){
    let r1 = await p1()
    let r2 = await p2()
    let r3 = await p3()
    console.log(r1)
    console.log(r2)
    console.log(r3)
}

run();

***

const fs = require('fs');
const promisify = require('util').promisify;
const readFile = promisify(fs.readFile);
async function run(){
    let r1 = await readFile('./1.txt','utf8')
    let r2 = await readFile('./2.txt','utf8')
    let r3 = await readFile('./3.txt','utf8')
    console.log(r1);
    console.log(r2);
    console.log(r3);
}

4.全局对象global