Asyncawait.js multiple arguments in callback

Question

I'm getting more and more confortable with the asyncawait library.

If I'm correct, this:

var doSth = async function() {
  var asyncResult = await promiseReturningFunction();
  subFunction(asyncResult);
};

is equivalent to:

var doSth = function() {
  promiseReturningFunction().then(function(asyncResult) {
    subFunction(asyncResult);
  });
};

But what if the callback has two arguments .then(function(asyncResult1, asyncResult2) {})?

In some other languages, I'd write:

  var asyncResult1, asyncResult2 = await promiseReturningFunction();

but I can't do this in JS, can I? Does await returns an array? The first argument?

The asyncawait library and the upcoming `async/await` are two different things. They may work in a very similar way, but you should clarify which one you are talking about. Even though you are linking to the library, your example doesn't seem to use the library. — Felix Kling, Feb 25 '16 at 17:49
Are you using the experimental async/await transpiler, or that fiber-based library? — Bergi, Feb 25 '16 at 17:54

score 8 · Accepted Answer · answered Feb 25 '16 at 17:58

I can't do this in JS, can I?

No, you can't. Promises fulfill with only a single value.

JavaScript doesn't have tuples, but you can use an array for that. With destructuring, you can make it look almost like in those other languages:

async function promiseReturningFunction() {
    await …;
    …
    return [result1, result2];
}

// elsewhere (in an async function):
var [asyncResult1, asyncResult2] = await promiseReturningFunction();

score 2 · Answer 2 · answered Feb 25 '16 at 18:10

You can easily use the destructuring on a bunch of promises like that:

var [r1, r2, r3] = await Promise.all([p1, p2, p3]);

or

var promises = [p1(), p2(), p3()];
$q.all(promises)
  .then(function ([p1Result, p2Result, p3Result]) {
      // ...
  });

So you will have:

async function doSth() {
    var [asyncResult1, asyncResult1] = await promiseReturningFunction();
    var asyncLastResult = await subFunction(asyncResult1);

    return asyncLastResult;
}

doSth();

score 0 · Answer 3 · edited May 23 '17 at 10:30

Promises can only pass one argument to the fulfillment callback. As Benjamin Gruenbaum writes:

[...] Just the first parameter will be treated as resolution value in the promise constructor. You can resolve with a composite value like an object or array.

(Here is the section in the Promises/A+ specification)

So your case with two arguments doesn't occur in JavaScript (at least not with standard promise implementations). Just use an array or an object (depending on your data structure).

Asyncawait.js multiple arguments in callback

3 Answers3