Using Promises To Await Triggered Events
Solution 1:
Don't do any await
ing inside the Promise
constructor - you only should do the promisification of the asynchronous callback in there:
asyncfunctionfn() {
awaita();
awaitb();
awaitc(); // this causes myEventreturnnewPromise(function(resolve, reject) {
thing.once('myEvent', function(e) {
resolve(e.data); // done
});
});
}
The thing that starts the process which eventually causes the event to be emitted is usually called inside the Promise
executor callback as well (to catch synchronous exceptions), but usually it doesn't return a promise like your c
function does.
Maybe this expresses the intent better:
asyncfunctionfn() {
awaita();
awaitb();
const {data} = awaitnewPromise(resolve => {
thing.once('myEvent', resolve);
thing.c(); // this causes myEvent
});
return data;
}
Of course this assumes that you only need to start listening to the event when you've called the other ones. If you expect the event to fire before that, you essentially have a race with parallel execution - I'd recommend to use Promise.all
in that case:
asyncfunctionfn() {
awaita();
awaitb();
const [{data}, cResult] = awaitPromise.all([
newPromise(resolve => thing.once('myEvent', resolve)),
c()
]);
return data;
}
If you have node v11.13.0 or higher you can use the events.once
method so that you don't have to build the promise yourself - and it also handles error events correctly:
import { once } from'events';
async function fn () {
await a()
await b()
await c()
await once(thing, 'myEvent')
}
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