Promise Js: Wait till promise fulfilled

Question

I'm working with Bluebird for promises in Node.Js, and was wondering how can I make a function return when the promise is fulfilled(completed).The behavior I want is :

function getItem(){
  functionReturningPromise.then(function(result){
    //do some operation on result
    return result;
  });
}

However, the above implementation won't return anything since the promise isn't completed at the time of execution. What would be the best workaround around this?

The purpose of getItem is to modify whatever functionReturningPromise returns, and then return it

You can't. If your operation is async, then the result of that operation cannot be returned synchronously, ever. Simply return your promise and have the caller use `.then()` on the promise to get the result. That's how async development works in node.js. — jfriend00, Mar 03 '16 at 23:35

georg · Accepted Answer · 2016-03-03T23:42:53.173

15

You can't, var value = someAsyncFunction() is not possible (until we have generators, at least). Promises are viral, once you use a Promise in a function, its caller must use then and the caller of the caller too, etc up to your main program. So, in the function that calls getItem you have to change this

item = getItem();
do something with item

into this

getItem().then(function(item) {
    do something with item
})

and getItem should return a Promise as well

function getItem(){
  return functionReturningPromise().then(function(result){
    //do some operation on result
    return result;
  });
}

(Theoretically it can also be a callback, but I prefer to always return promises).

edited Mar 03 '16 at 23:42

answered Mar 03 '16 at 23:32

georg

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+1 but... you don't really need `Promise.resolve(result)`. Just return `result` since you're inside a `then`. and please don't suggest callbacks when using promises. – Onur Yıldırım Mar 03 '16 at 23:41
1

@OnurYıldırım: right, it's late here ;) – georg Mar 03 '16 at 23:43
Returning the promise does what I was looking for. – Ashwini Khare Mar 04 '16 at 00:10

Dimitris Karagiannis · Answer 2 · 2016-03-04T08:25:52.647

-2

The purpose of getItem is to modify whatever functionReturningPromise returns, and then return it

Then, I think you could simply do

functionReturningPromise.then(getItem);

where

function getItem(valueFromPromiseResolution){
    //do some operation on valueFromPromiseResolution
    return whatever;
}

edited Mar 04 '16 at 08:25

answered Mar 03 '16 at 23:25

Dimitris Karagiannis

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Here you're just passing a function in as the callback, you can't return that value outside of the promise. – richardpringle Dec 01 '16 at 19:08
@richardpringle You are right. Still, not going to delete the answer even if it's wrong, let the downvotes come; I deserve them. – Dimitris Karagiannis Dec 02 '16 at 13:18
this was actually helpful cause that's what I was doing lol. – Aditya Kendre Mar 09 '21 at 23:16

Promise Js: Wait till promise fulfilled

2 Answers2