前端面试时被问到JS异步执行的问题：有A、B、C三个任务，要求：A和B异步执行，二者都完成后执行C

Question

前端面试时被问到JS异步执行的问题：有A、B、C三个任务，要求：A和B异步执行，二者都完成后执行C

Answer 1

方案一，Promise.all形式：var promise1 = new Promise((resolve, reject) => {    setTimeout(() => {        console.log(1);        resolve()    }, 0);    });var promise2 = new Promise((resolve, reject) => {    setTimeout(() => {        console.log(2);        resolve()    }, 0);});Promise.all([promise1, promise2]).then(function(res) {  console.log(3)});方案二，callback形式：var index = 0function C(){    console.log(3);}setTimeout(() => {    console.log(1);    index++;    if(index === 2){        C()    }}, 0);setTimeout(() => {    console.log(2);    index++;    if(index === 2){        C()    }}, 0);

Answer 2

var a = Promise.resolve('a')var b = new Promise((resolve, reject) => {  setTimeout(() => {    resolve('b')  }, 1000)})var c = function() {  console.log('c')}Promise.all([a, b]).then(res => {  res.forEach(console.log)  c()})

Answer 3

const A = async () =>  await 'A';const B = async () =>  await 'B';const C = () =>  'C';(async function All() {    await Promise.all([A(), B()]);    C();})();

Answer 4

async function A(){}async function B(){}function C(){}Promise.all([A(),B()]).then(C)

Answer 5

给定一个计数器为2，A，B任务完成后，把计数器减去1，如果为0，则去指定C