async await的串行与并行

创建两个promise

const fn1 = () => {
    return new Promise((resolve,reject)=>{
        setTimeout(()=>{
            resolve(new Date())
        },2000)
    })
}
const fn2 = () => {
    return new Promise((resolve,reject)=>{
        setTimeout(()=>{
            resolve(new Date())
        },4000)
    })
}

async await的串行对应promise的链式调用

输出时间间隔2秒和4秒，说明promise串行执行

//async await的串行
(async ()=>{
    console.log(new Date())
    const time1 = await fn1()
    console.log(time1)
    const time2 = await fn2()
    console.log(time2)
})()

//async await的串行等同与promise的链式调用相同
console.log(new Date())
fn1().then(resolve=>{
    console.log(resolve)
    return fn2()
}).then(resolve=>{
    console.log(resolve)
})

async await的并行对应promise.all

输出时间间隔2秒和2秒，说明promise并行执行

//async await的并行
(async ()=>{
    console.log(new Date())
  	//在promise对象被创建的时候，立即调用（resolve,reject）=>{}函数
    const promise1 = fn1()
    const promise2 = fn2()
    console.log(await promise1)
    console.log(await promise2)
})()

//async await的并行相当于promise.all
console.log(new Date())
Promise.all([fn1(),fn2()]).then(resolve=>{
    console.log(resolve)
})