sean2078 sean2078 - 6 months ago 11
Javascript Question

How to "await" for a callback to return in JavaScript ES7?

When using ES7 and 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' );
}

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 api.on( '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
}
Comments