4

I don't understand how to work with asynchronous functions.

Why does the code below stop the main thread?

async function foo() {
  for (;;) {}
}
foo();
Improve this question
7
  • Check this stackoverflow.com/questions/11488014/… Commented Feb 1, 2019 at 11:11
  • 3
    Asynchronous code does not mean multithreaded, the code still runs in one thread. Async is just some kind of cooperative multitasking, but not multithreading. Commented Feb 1, 2019 at 11:11
  • 1
    @Ivar — Most modern JS implementations support a form of threads these days. It's just that the async keyword isn't that. Commented Feb 1, 2019 at 11:15
  • browsers have an event loop. when you calling an async function, the function will be scheduled to be run in the next loop, but still is single threaded. Commented Feb 1, 2019 at 11:27
  • @behzadbesharati calling an async function does not schedule is execution. It will be executed immediatly (as you can see in this fiddle ). The returned Promise is resolve at a later state. Commented Feb 2, 2019 at 20:05

2 Answers 2

Reset to default
13

The async keyword, and promises in general, don't make synchronous code asynchronous, slow running code fast, or blocking code non-blocking.

async just makes the function return a promise and provides (with the await keyword) a mechanism to interact with other promises as if there were synchronous.

Your function starts a loop, and then just goes around and around.

It doesn't get to the end of the function, which would end the function and resolve the promise it returned.

It doesn't reach an await keyword and pause while it waits for the awaited promise to be resolved.

It just goes around and around.

If you were actually doing something in the loop which was computationally expensive and you wanted to push off into the background, then you could use a Node.js Worker Thread or a browser-based Web Worker to do it.

Improve this answer
Sign up to request clarification or add additional context in comments.

Comments

-3

Putting the async keyword before the function only implies that this is asynchronous function. You need to include the keyword await before the function that you want to actually wait for. Just like this:

async function hashPin(pin){
    const hashedPin = await bcrypt.hash(pin, saltRounds);
}

That's just the example from one of my projects (the redundant code has been removed before posting)

Improve this answer

4 Comments

Good, this one works for me.
I'm not sure why this would work because an infinite loop will still be blocking.
async in front of the function serves two purposes. 1. it enables usage of await in the function. 2. it tells the caller that it is guaranteed that this function returns a Promise. But neither async nor await do magically resolve blocking issues. So if bcrypt.hash has some blocking code in it, then await in front of it would not resolve that blocking issue. And removing the await and writing async function hashPin(pin){ return bcrypt.hash(pin, saltRounds).then( hashedPin => /*...*/)} would not make a difference regarding blocking.
You guys are right, I totally missed the bit about infinite loop and only provided an answer to how to work with asynchronous functions. My bad