Resolving promises from ES6 Arrow Functions

Question

Reading the docs as I understand it in ES6 the meaning of:

foo => someFun(foo);

is equivalent to:

foo => { return someFun(foo); }

I'm returning a new Promise and within that code using arrow functions to invoke the resolve & reject methods, e.g.

return new Promise(function(resolve, reject)
{
    someFunThatReturnsAPromise()
    .then(data => resolve(data))
    .catch(err => reject(err));
});

As such is the code in the then actually,

.then(data => return resolve(data))

If so, does it matter that the result of resolve (of which I'm not sure of the type of value) and should I instead be slightly less terse and write it using {} to prevent the implicit return

.then(data => { resolve(data); })

A couple of comments have stated accurately that the inner Promise is not needed. As such, please consider the original question but with the then modified to be ".then(data => resolve(data.item))", i.e. performing some additional (albeit it slight) processing — const, Oct 08 '17 at 10:02
In any case, avoid the [`Promise` constructor antipattern](https://stackoverflow.com/q/23803743/1048572?What-is-the-promise-construction-antipattern-and-how-to-avoid-it) and don't pass `resolve`/`reject` as callbacks to a promise at all! — Bergi, Oct 08 '17 at 12:24

score 5 · Answer 1 · answered Oct 08 '17 at 10:41

The resolve function already returns undefined, so it makes absolutely no difference if you implicitly return it with a one-line arrow function or don't return it from a function body at all (since the latter means your function body implicitly returns undefined itself).

Furthermore, since you've wrapped the promise returned by someFunThatReturnsAPromise() in a new promise, there's nothing that handles the return anyway so it wouldn't make any difference even if did return something.

More importantly, the way you've wrapped a promise in a new promise is an anti-pattern. The new Promise() construct is only for dealing with asynchronous processes that aren't already promise based.
Since someFunThatReturnsAPromise() already return a promise, you don't need to wrap it in a new one, simply use the one you got!
For your example that would simply mean returning it:

return someFunThatReturnsAPromise()

If you want to do some processing of the data, such as only returning a part of the data (the status property in the below example), you do that in a then callback:

return someFunThatReturnsAPromise().then(data => data.status)

When you return in a then callback, it will in turn return a new promise that gets resolved with the data you returned (unless you returned another promise, in which case it will resolve when that promise resolves).
This is how promises are designed to work, by chaining the asynchronous processes and their results.

That's great. I realised from your other comment that the outer Promise is not needed. In my actual code I've been able to remove a bunch of these this has reduced the code size. It has also resolved issues where the outer Promise changes the binding of this which led to various const self = this which have now been removed. Thanks! — const, Oct 08 '17 at 11:10

score 3 · Answer 2 · answered Oct 08 '17 at 09:50

3

If you just want to return data, and reject incase of an error then you don't need a then()

return new Promise(function(resolve, reject)
{
    someFunThatReturnsAPromise()
    .then(data => resolve(data))
    .catch(err => reject(err));
});

would be equivalent to

return someFunThatReturnsAPromise()

unless you want to do some processing on data

answered Oct 08 '17 at 09:50

marvel308

10,288
1
21
32

shouldn't it be `return new Promise ( (reslove, reject) = > { ... } )` – Stavm Oct 08 '17 at 09:53
someFunThatReturnsAPromise() already returns a promise, why would you want to wrap it in another promise ? – marvel308 Oct 08 '17 at 09:54
That's a valid question. In the example code it's not necessarily needed. Perhaps the example would have been a little better if the then line was: .then(data => resolve(data.item)) say – const Oct 08 '17 at 09:59
@const unless you want to do some multi line processing then both are equivalent. – marvel308 Oct 08 '17 at 10:04
1

@const `return someFunThatReturnsAPromise().then(data => data.item)` – Lennholm Oct 08 '17 at 10:20

Resolving promises from ES6 Arrow Functions

2 Answers2