Javascript return Promise from function - promise chain vs variable

Question

Is there a difference between these two ways for returning promise from a function

var promise;
promise = callAsync();
promise.then(doSomething).then(doSomethingElse);
return promise;

vs

var promise;
promise = callAsync();
return promise.then(doSomething).then(doSomethingElse);

I thought both approaches are same, but in a mocha test case, only second approach is working.

only the second approach is correct, the first returns the callAsync promise only — Jaromanda X, Mar 11 '17 at 06:22
@JaromandaX would't the promise initiated by callAsync() would have created the promise chain in both cases ? — Jithin, Mar 11 '17 at 06:34

score 4 · Accepted Answer · edited May 23 '17 at 12:09

They are certainly not the same. Each .then() returns a new promise. So,

return promise;

is returning the original promise, but:

return promise.then(doSomething).then(doSomethingElse);

is returning a new promise that is the result of calling both .then() methods. The difference is that the latter one is returning a promise that is influenced by the functions called in those .then() methods, whereas the first promise is only influence by callAsync() and has nothing to do with the other .then() handlers.

The first promise is only monitoring callAsync(). It has absolutely nothing to do with what happens in other .then() handlers. The key to understanding .then() is that it returns a new promise and it is that new promise that is influenced by what happens in the .then() handlers.

For more info, read this answer: Is there a difference between promise.then.then vs promise.then; promise.then to understand the difference between chaining and branching.

Javascript return Promise from function - promise chain vs variable

1 Answers1