Why does this JavaScript function always returns 'undefined' – and how to fix that?

Question

I'm using phonegap with application preferences plugin and trying to make a helper function to get a value from it. However the function is not returning a correct value. I know this has to do with asynchronous thingy but unfortunately I don't know how to fix it. (I've tried to search help here, and found little, and tried to implement it in helper method)

What I want to achieve is:

function populateList() {
    var a = 1;

    var number = getSettingFromApplicationPreferences('number');
    // number is always undefined

    var letter = getSettingFromApplicationPreferences('letter');
    // letter is always undefined

    number = (number) ? number : 1;
    letter = (letter) ? letter : 'b';

    // Here I'll do some DOM manipulation and use 'number' and 'letter' on it, and 
    // define new variables based on 'number' and 'letter'

}

here's the helper function that I need help with:

function getSettingFromApplicationPreferences(setting) {

    var x = (function () {
        window.plugins.applicationPreferences.get(
            // setting
            setting, 
            // success callback
            function(returnValue) {
                console.log(setting + ': ' + returnValue);
                return returnValue;
            },
            // error callback
            function(error) {
                alert("Failed to get a setting: " + error);
                return false;
            }
        );
    })();

    return x;   
}

Question
How is it possible to return the 'returnValue' from application preferences with that helper function?

Answer 1

The problem is, your callback doesn't actually set a value for x. So, you're going to have some other way to do whatever you're doing, because return values will not work.

Answer 2

You are using an asynchronous function incorrectly, you cannot assign like you are because the function hasn't returned yet and so you get an undefined. You need to use a callback function instead.

That means that inside the success function you would do whatever you need to do with the "returnValue".