making a .then() function return a promise to be used by another function

Question

I am using this native script firebase plugin, which makes me use this function:

getValue() {
     var value;
     firebase.getValue('/companies')
         .then(result => {
             console.log(JSON.stringify(result));
             value = result;
             return result;

         })
         .catch(error => {
             console.log("Error: " + error);
             return error;
         });
     return value;
 }

If I import it in another file and use it like this:

var myValue = this.myFirebaseService.getValue();

If I run it the first time it returns undefined but the second time it returns the value.

My question is how do I make the firebase function return a promise so I can make my second statement wait for it.

`firebase.getValue()` already returns Promise. What you are not doing is returning that promise from `getValue()`. — Roamer-1888, Apr 29 '18 at 18:20

nashcheez · Answer 1 · 2018-04-29T18:41:30.943

0

You can initialize myValue after the result has been received from the then block.

Something like this:

var myValue;

function getValue() {
  return firebase.getValue('/companies');
}

myValue = getValue().then(result => {
    const value = JSON.stringify(result);
    console.log(value);
    return value;
  })
  .catch(error => {
    console.log("Error: " + error);
  });
}

edited Apr 29 '18 at 18:41

answered Apr 29 '18 at 18:05

nashcheez

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thanks...but what if i want to use it in another file. lets say i want import this function in another file and use the value it returns – Stel Dare Apr 29 '18 at 18:08
be sure to import the second file so that the function invoked within the `then` doesnt return undefined ;) – versvs Apr 29 '18 at 18:09
your `function getValue() {` didnt return the promise so `myValue = ` will always be undefined. might wanna update your answer ... `return firebase.getValue(...` – MilesStanfield Apr 29 '18 at 18:10
it returns undefined the first time. – Stel Dare Apr 29 '18 at 18:10
@MilesStanfield Thank you for pointing it out. Updated the answer. StelDare - Check updated answer and follow what versvs says – nashcheez Apr 29 '18 at 18:15
1

This is ugly. Why not just use the returned promise and set the variable value in next then() in chain? – charlietfl Apr 29 '18 at 18:16
3

How can `init()` be called after a return ? – NayoR Apr 29 '18 at 18:18
@nashcheez am importing the getValue() function into another file to use it there. – Stel Dare Apr 29 '18 at 18:22

James Moran · Accepted Answer · 2018-04-29T19:07:18.623

The firebase function you're referring to is returning a promise. That's why you can use .then and .catch after calling the getValue function.

If you return the firebase promise from getValue(), like this:

getValue(){
  return firebase.getValue('/companies');
}

You'll be able to call getValue() whenever you like, but you'll have to execute the code after getValue() (which is a promise) is resolved. Like so:

getValue().then(function(value) {
  console.log(value);
});

I would read up on promises if you don't understand this: https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Promise

making a .then() function return a promise to be used by another function

2 Answers2