javascript - async/await and forEach issues

Question

Method 1: No problem

    (async function () {
        for (let i = 0; i < triggerArr.length; ++i) {
            await sleep();
            triggerArr[i]();
        }
    })();

Method 2: They are output together, why? (No waiting)

    const test = async function (item) {
            await sleep();
            item();
    };

    triggerArr.forEach(test);

All codes

function signalLamp(singalArr) {
    function tic(sign, delay) {
        return () => new Promise((res, rej) => {
            setTimeout(() => {
                res();
                console.log(sign);
            }, delay || 1000);
        });
    }

    const rawArr = singalArr.slice();
    const triggerArr = rawArr.reduce(function (prev, item) {
        return prev.concat([tic(item, 1000)]);
    }, []);

    const sleep = () => new Promise((res, rej) => setTimeout(res, 1000));


    /* Method 1 */
    (async function () {
        for (let i = 0; i < triggerArr.length; ++i) {
            await sleep();
            triggerArr[i]();
        }
    })();
    /* Method 2 */
    // const test = async function (item) {
    //         await sleep();
    //         item();
    // };
    // triggerArr.forEach(test);
}

signalLamp(['red', 'green', 'yellow']);

Answer 1

Let me tell you.
await can only be used in the function context of async declaration. As shown below, in forEach, await cannot be used directly.

let array = [0,1,2,3,4,5];
(async ()=>{
  array.forEach(function(item){
    console.log(item);
    await wait(1000);//这是错误的写法
  });
})();
//因await只能用于 async 声明的函数上下文中, 故不能写在forEach内.下面我们来看正确的写法
(async ()=>{
  for(let i=0,len=array.length;i<len;i++){
    console.log(array[i]);
    await wait(1000);
  }
})();

Looking carefully, I found that your problem is another situation.
If you pass test as a callback function, the sleep method is executed synchronously, and await is still effective, but at the same time. Therefore, subsequent functions will be executed together after waiting for the same time.

Answer 2

async When doing asynchronous loops it is best to use for ... of ... or Promise.all()