Array.forEach是异步的吗？

Question

According to JavaScript, Node.js: is Array.forEach asynchronous? , Array.forEach is synchronous. However, for my code below:

function wait5() {
    return new Promise(resolve => 
    setTimeout(resolve, 5000));
}

async function main() {
   console.log("Start");
   [1,2].forEach(async (e) => {
      const d = await wait5().then(()=> console.log("5s later") )
   })
   console.log("This should come last!");
}

main();

The output is:

Start
This should come last!
5s later
5s later

with the two "5s later" coming out in rapid succession.

Why is this the case?

If I use a normal for loop:

async function main() {
   console.log("Start");
   for (let i=0;i<2;i++) {
      const d = await wait5().then(()=> console.log("5s later") )
   }
   console.log("This should come last!");
}

then the result is what I wanted:

Start
5s later
5s later
This should come last!

Answer 1

forEach is synchronous. Your particular callback function, however, is not. So forEach synchronously calls your function, which starts its work, once for each entry in the array. Later, the work that started finishes asynchronously, long after forEach has returned.

The issue is that your callback is async , not that forEach is asynchronous.

In general, when you're using an API like forEach that doesn't do anything with the return value (or doesn't expect a promise as a return value), either:

1. Don't pass it an async function, or

2. Ensure that you handle errors within the function itself

Otherwise, you'll get unhandled errors if something goes wrong in the function.

Or of course:

3. Use a try / catch block within the async function to catch and handle/report errors within the function itself.

Answer 2

It looks like you're declaring an async function inside of a caller that really doesn't care for that sort of thing, forEach . Declaring a function async makes it promise-like, but that promise is only useful if acted on.

If you need a promise-aware forEach , that's something you could implement, though Promise.each in libraries like Bluebird already do that.