async/awiat 实例详解

背景：

异步回调：callback hell
Promise than catch 链式调用，也是基于回调函数.
async/await是用同步语法写异步

function loadImg(src) {
    return new Promise(
        //参数 resolve reject 均是函数
        (resolve, reject) => {
            const img = document.createElement('img')
            img.onload = () => {
                resolve(img)
            }
            img.onerror = () => {
                const err = new Error(`图片加载失败！${src}`)
                reject(err)
            }
            img.src = src
        }
    )
}
const url1 = 'https://img3.mukewang.com/5686a790000124f701000100-100-100.jpg'
const url2 = 'https://img3.mukewang.com/5686a790000124f701000100-100-100.jpg'

!(async function(){
    console.log('start')
    const img1 = await loadImg(url1)
    console.log(img1.width)
    const img2 = await loadImg(url2)
    console.log(img2.height)
})()//立即执行函数

async/await与Promise的关系 *****

1. 执行async函数，返回的是Promise对象
   await相当于Promise的then

async function fn1() {
    return 100
}
console.log(fn1()) //Promise {: 100}
fn1().then(data => {
    console.log(data) //100
})
// 执行async函数，返回的是Promise对象


!(async function(){
    const data = await Promise.resolve(300) // await相当于Promise的then
    console.log(data)//300
    const data2 = await 400 
    console.log(data2) //400
    const data3 = await fn1() //执行async函数 返回的promise对象
    console.log(data3) //100
})()

2. try..catch可捕获异常，代替了Promise的catch

!(async function(){
    const p=Promise.reject('err')
    try{
        const res =await p
        console.log(res)
    }catch(ex){
        console.error(ex) //err : try..catch可捕获异常，代替了Promise的catch
    }
})()

async/await是语法糖，异步的本质

async/await是语法层面解决异步的回调
JS还是单线程，还是得有异步，还得是基于event loop
async/await只是一个语法糖

async function async1() {
    console.log('async1 start') //2
    await async2() //立马执行async2 返回undefined
    //await后面，都可以看作是calback里的内容，即异步
    console.log('async1 end')//5 异步代码
}
async function async2() {
    console.log('async2')//3
}
console.log('script start')//1
async1() //立马执行
console.log('script end')//4

// script start
// async1 start
// async2
// script end
// async1 end

// ----------------------------------------------
async function async1() {
    console.log('async1 start')//2
    await async2()
        // 以下三行都是 async2 的异步回调callback的内容
        console.log('async1 end') 
        await async3()
            //以下一行是async3 的异步回调callback的内容
            console.log('async1 end 2')
}
async function async2() {
    console.log('async2') 
}
async function async3() {
    console.log('async3')
}

console.log('script start') //1
async1()
console.log('script end') 

// script start
// async1 start
// async2
// script end
// async1 end
// async3
// async1 end 2

异步的本质是什么？

JS是单线程的,异步需要回调

for of的使用

function muti(num) {
    return new Promise(resolve => {
        setTimeout(() => {
            resolve(num * num)
        }, 1000);
    })
}

const nums = [1, 2, 3]
!(async function () {
    for (const i of nums) {
        const res = await muti(i)
        console.log(res)
    }
})()