How we can end an async
function from a callback passed to an event emitter without promisifing the event emitter?
Also without using external modules, just plain NodeJS 7.x/8.x (Which supports Es6 syntax and async/await
.
We want basically to mix an async function ...
with an event emitter, so that it resolves when the event emitter signals end
.
Also bear in mind that we won't start with the event emitter until done with some other async functions, using await
.
If we had a "new Promise(...)" we would call resolve(); and the headache would be over, but in 'async' there's no 'resolve', plus we cannot use 'return' because we're inside a callback.
/*
* Example of mixing Events + async/await.
*/
// Supose a random pomise'd function like:
function canIHazACheezBurger () {
return new Promise((resolve, reject) => {
setTimeout(() => {
resolve(Math.random() > 0.5);
}, 500 + Math.random() * 500)
});
}
/**
* Then, we want to mix an event emitter with this logic,
* what we want is that this function resolves the promise
* when the event emitter signals 'end' (for example).
* Also bear in mind that we won't start with the event emitter
* until done with the above function.
* If I had a "new Promise(...)" I would call resolve(); and the
* headache would be over, but in 'async' there's no 'resolve',
* plus I cannot use 'return' because I'm inside a callback.
*/
async function bakeMeSomeBurgers () {
let canIHave = await canIHazACheezBurger();
// Do something with the result, as an example.
if (canIHave) {
console.log('Hehe, you can have...');
} else {
console.log('NOPE');
}
// Here invoke our event emitter:
let cook = new BurgerCooking('cheez');
// Assume that is a normal event emitter, like for handling a download.
cook.on('update', (percent) => {
console.log(`The burger is ${percent}% done`);
});
// Here lies the problem:
cook.on('end', () => {
console.log('I\'ve finished the burger!');
if (canIHave) {
console.log('Here, take it :)');
} else {
console.log('Too bad you can\'t have it >:)');
}
// So, now... What?
// resolve(); ? nope
// return; ?
});
}
I want to apologize if this question is already done somewhere. The research done shows questions related to mix async with sync logic, but I've found nothing about this.
A similar question in title is this 'write async function with EventEmitter' but it has nothing to do with this question.
Can we end an async function from a callback passed to an event emitter without promisifing the event emitter?
No. async
/await
syntax is just sugar for then
calls and relies on promises.
async function bakeMeSomeBurgers () {
let canIHave = await canIHazACheezBurger();
if (canIHave)
console.log('Hehe, you can have...');
else
console.log('NOPE');
// Here we create and await our promise:
await new Promise((resolve, reject) => {
// Here invoke our event emitter:
let cook = new BurgerCooking('cheez');
// a normal event callback:
cook.on('update', percent => {
console.log(`The burger is ${percent}% done`);
});
cook.on('end', resolve); // call resolve when its done
cook.on('error', reject); // don't forget this
});
console.log('I\'ve finished the burger!');
if (canIHave)
console.log('Here, take it :)');
else
console.log('Too bad, you can\'t have it >:)');
}
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