异步常规操作

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const fn1 = (async () => {
    return 1;
})
fn1().then(res=>{
    console.log( res );
});

等价于

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const fn2 = () => {
    return new Promise(resolve=>{
        resolve(2)
    })
}
fn2().then(res=>{
    console.log( res );
});

异步执行循环

forEach map是同步操作 并行执行
while, do while, for in, for of是串行执行

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const sleep = (time: number) => {
    return new Promise((resolve, reject) => {
        setTimeout(() => {
            resolve(true)
        }, time * 1000)
    })
}

const main = async () => {
    for (let i = 0; i < arr.length; i++) {
        await sleep(1)
        console.log(arr[i])
    }
    console.log('完成')
}