async 函数的使用

基本使用

async函数返回的是一个Promise，能够进行Promise的相关操作,函数内部返回的结果会成为then方法回调函数的参数

async function asyncfunc(){
    return "这是一个async函数"
}
asyncfunc()
asyncfunc().then(function(info){console.log(info)})

当函数执行中遇到await，需等异步操作完成，才会执行后面的代码

async function asyncfunc() {
        let now = new Date().getTime();
        await new Promise((resolve, reject) => {
            setTimeout(() => {
                resolve()
            }, 2000)
        })
        console.log(new Date().getTime() - now)
    }

函数内部错处，返回的promise状态为rejected

 async function asyncfunc(){   
    console.log(err)
    }

async函数返回的Promise只有函数内部中await后的异步事件执行完后，才会改变，除非中途return或者出错

(async function() {
        await new Promise((resolve, reject) => {
            console.log("第一个await")
            resolve()
        })
        await new Promise((resolve, reject) => {
            console.log("第二个await")
            resolve()
        })
        await new Promise((resolve, reject) => {
            console.log("第三个await")
            resolve()
        })
    })().then(info=>{console.log("触发then")})

(async function() {
        await new Promise((resolve, reject) => {
            console.log("第一个await")
            resolve()
        })
        return "执行return"
        await new Promise((resolve, reject) => {
            console.log("第二个await")
            resolve()
        })
        await new Promise((resolve, reject) => {
            console.log("第三个await")
            resolve()
        })
    })().then(info=>{console.log("触发then")})

(async function() {
        await new Promise((resolve, reject) => {
            console.log("第一个await")
            resolve()
        })
        throw new Error("出错了")
        await new Promise((resolve, reject) => {
            console.log("第二个await")
            resolve()
        })
        await new Promise((resolve, reject) => {
            console.log("第三个await")
            resolve()
        })
    })()

await后的Promise状态变为rejected时，会被catch接收到

(async function() {
        await new Promise((resolve, reject) => {
            reject("状态变为rejected")
        })
    })().catch(info => {
        console.log(info)
    })

任意一个await后的Promise函数变为rejecte，async函数的执行就会中断，若想继续执行，可使用try{}catch(e){}捕获

(async function() {
        await new Promise((resolve, reject) => {
            reject("状态变为rejected")
        })
        await new Promise((resolve, reject) => {
            console.log("第二个await")
            resolve()
        })
    })().catch(info => {
        console.log(info)
    })

(async function() {
        try {
            await new Promise((resolve, reject) => {
                reject("状态变为rejected")
            })
        } catch (err) {
            console.log("捕获" + err)
        }

        await new Promise((resolve, reject) => {
            console.log("第二个await")
            resolve()
        })
    })().catch(info => {
        console.log(info)
    })

另一种方法是将await后面的Promise添加catch

(async function() {

        await new Promise((resolve, reject) => {
            reject("状态变为rejected")
        }).catch(info => {
            console.log("捕获" + info)
        })
        await new Promise((resolve, reject) => {
            console.log("第二个await")
            resolve()
        })
    })().catch(info => {
        console.log(info)
    })