Question

106

How to await for a callback to return?

rated 0 times [ 111] [ 5] / answers: 1 / hits: 156093 / 8 Years ago, sun, may 8, 2016, 12:00:00

When using a simple callback such as in the example below:

test() {

  api.on( 'someEvent', function( response ) {

    return response;

  });

}

How can the function be changed to use async / await? Specifically, assuming 'someEvent' is guaranteed to be called once and only once, I'd like the function test to be an async function which does not return until the callback is executed such as:

async test() {

  return await api.on( 'someEvent' );

}

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brodyfrancisi

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3 Years ago

answered 8 Years ago aidan · Accepted Answer

async/await is not magic. An async function is a function that can unwrap Promises for you, so you'll need api.on() to return a Promise for that to work. Something like this:

function apiOn(event) {

  return new Promise(resolve => {

    api.on(event, response => resolve(response));

  });

}

Then

async function test() {

  return await apiOn( 'someEvent' ); // await is actually optional here

                                      // you'd return a Promise either way.

}

But that's a lie too, because async functions also return Promises themselves, so you aren't going to actually get the value out of test(), but rather, a Promise for a value, which you can use like so:

async function whatever() {

  // snip

  const response = await test();

  // use response here

  // snip

}