How to make a line wait for function return

Question

I have 2 files

//init.js
var x = getMasterList(location);
console.log(x);
console.log("should have printed list");

And

//read.js
function getMasterList(location) {
    var list = [];
   ref.child(company).child("listOfLocations").once("value").then(function(snapshot) {
        snapshot.forEach(function(childSnapshot) {
          var item = childSnapshot.val();
          list.push(item);
        })
    });
    console.log("In function");
    return list;
}

The console's output is

undefined
should have printed list
In function

The problem is, because of javascript's asynchronous nature, it prints out 'x' before assigning it. I've looked at promises and it doesn't seem like it can help me since it didn't change the async behaviour. I've also looked at callbacks and I can't seem to make them work for multiple files.

Any ways of getting around this so that it only prints after 'x' has a value?

You need to learn how to use promises or callbacks. What don't you understand? — SLaks, Feb 22 '18 at 17:54
In JS, to use asychronous functions, use callbacks, promises, or async/await — frozen, Feb 22 '18 at 17:54
This is not supposed to happen with the code you provided. Please provide the exact code that you used. JS is synchronous unless you explicitly tell it to invoke a function asynchronously. — Derek 朕會功夫, Feb 22 '18 at 17:55
Promises don't work, the output is the same. Can you use callbacks when a function is in another file? — Astrodude, Feb 22 '18 at 17:56
_"because of javascript's asynchronous nature"_ -> https://stackoverflow.com/a/32456239/402037 — Andreas, Feb 22 '18 at 17:56
@Astrodude: That means you're using them wrong. Yes; files don't matter. — SLaks, Feb 22 '18 at 17:59

score 0 · Accepted Answer · answered Feb 22 '18 at 18:03

Using callbacks, you can achieve this quite easily.

read.js

function getMasterList(location, callback) {
    var list = [];
    //do stuff
    console.log("In function");
    callback(list);
}

init.js

var x = getMasterList(location, function(x){
    console.log(x);
    console.log("should have printed list");
});

How to make a line wait for function return

1 Answers1