Does async/await blocks event loop?
Contrary to what it seems, await
does not block. It's just syntactic sugar over promises. Nothing is blocked; it may look blocking to allow code to be synchronous, but that's just sugar over promises. For example, this may look synchronous:
const response = await fetch(…);
const json = await response.json();
const foo = JSON.parse(json); // Using json here, even though my request was async!
But it's not. Once you desugar it, all you get are promises, which are nonblocking:
fetch(…)
.then(response => response.json())
.then(json => {
const foo = JSON.parse(json);
});
It would be absolutely catastrophic if await
were blocking. JavaScript runtimes are generally single threaded. That means user interaction and other processes would cease whenever you made a request or some other async operation such as using the filesystem. On a related note, this is, along with dynamic imports, are the main argument against top level await
Aziz
Updated on June 30, 2022Comments
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Aziz almost 2 years
I was reading Don't Block the Event Loop from the Node.js guide. There was a line saying:
You should make sure you never block the Event Loop. In other words, each of your JavaScript callbacks should complete quickly. This of course also applies to your
await
's, yourPromise.then
's, and so on.I started to wonder, what if, some API call to the database which I'm
await
ing is taking some time to resolve, does that mean that I have blocked the event loop with thatawait
call?After that, I started testing some self written codes but after testing I'm still not clear how blocking through
await
works. Here are some testing codes:Assuming, that I'm using express for testing. I understand why making 2 API calls to the
/test
route blocks the event loop in this case.function someHeavyWork() { // like calling pbkdf2 function } app.get('/test', (req, res) => { someHeavyWork(); res.json(data); });
But that doesn't happen in this case.
function fakeDBCall() { return new Promise((resolve, reject) => { setTimeout(() => { resolve(data); }, 5000) }) } app.get('/test', async (req, res) => { const data = await fakeDbCall(); res.json(data); })
This may be because of my lack of understanding of how blocking works in the case of
async/await
. -
Aziz over 5 yearsUnderstood why the second call is not blocking the event loop. One quick question, when the promise resolves does the callback of
then
is pushed to call-stack by event loop, if not then how it got pushed to it? -
Andrew Li over 5 years@MohammadAziz Internally,
await
executes PerformPromiseThen which is basically justPromise#then
. In the operation, a new Job is enqueued (with the callback). And those jobs are executed once the call-stack is empty (so by the event loop, pushing new jobs to the call stack once it is empty). -
JORGE GARNICA about 3 yearsAlso, in the first example, you have the advantaje to manipulate the
response
andjson
objects. I mean, iterating the objects or getting another information. -
avalanche1 over 2 yearsDoesn't answer the question