Return a value from nested function in Javascript

Question

I'm trying to return a value from a method. I have a nested function within the method which should return a value.

However, I can't seem to get it to return the value of title when calling traverseProjects().

Here's my code.

getTitle: function(userId, projectId) {
        u = new Firebase('<firebase.com>/users/' + userId + /projects/ + projectId);
        function traverseProjects() {
            u.once('value', function(snapshot) {
                if (snapshot.key() === projectId) {
                    var obj = snapshot.val();
                    var title = obj.details.title;
                }
                console.log(title);
                return title;
            });
        }

        var title = traverseProjects();
        console.log(title);
        return title;
    }

Use `callback`...Values retrieved after asynchronous operation can not be returned as `return` statement is executed long back when the value was undefined.... — Rayon, Feb 28 '16 at 04:11

Ben · Accepted Answer · 2016-02-28T16:59:03.920

1

Do something like this:

getTitle: function(userId, projectId, callback) {
    u = new Firebase('<firebase.com>/users/' + userId + /projects/ + projectId);
    function traverseProjects(callack) {
        u.once('value', function(snapshot) {
            if (snapshot.key() === projectId) {
                var obj = snapshot.val();
                var title = obj.details.title;
            }
            console.log(title);
            callback(title);
        });
    }

    traverseProjects(function(title) {
      console.log(title);
      callback(title);
    });
}

And call the function with:

getTitle('uid', 'pid', function(title){
    console.log(title);
})

Like @felix-kling suggested, we can simply eliminate the inner function:

getTitle: function(userId, projectId, callback) {
    u = new Firebase('<firebase.com>/users/' + userId + /projects/ + projectId);
    u.once('value', function(snapshot) {
        if (snapshot.key() === projectId) {
            var obj = snapshot.val();
            var title = obj.details.title;
        }
        console.log(title);
        callback(title);
    });
}

edited Feb 28 '16 at 16:59

answered Feb 28 '16 at 04:25

Ben

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Don't really see the purpose of `traverseProjects` here, or why you do `return callback()` instead of `callback()`. Keep it simple! – Felix Kling Feb 28 '16 at 04:27
I see it as good coding style to explicitly return in every function, some people find it easier to read like this :) – Ben Feb 28 '16 at 04:28
I'd argue that it's more confusing when working with asynchronous methods, since it has no effect, but it could make the impression that it does. – Felix Kling Feb 28 '16 at 04:29
True traverseProjects is probably not needed, but for the purpose of the direct problem, I think it can be more clear if we demonstrate how the callback works without too much alteration to the original code. – Ben Feb 28 '16 at 04:29
I see, this might be a personal preference thing in the end – Ben Feb 28 '16 at 04:30
FWIW, you have some typos there, `callack` vs `callback`. – Felix Kling Feb 28 '16 at 04:31
@FelixKling nice call XD – Ben Feb 28 '16 at 04:33
One more reason to keep it simple :P – Felix Kling Feb 28 '16 at 04:34
@Ben please remove the return from `return callback(title);`. In a more strongly typed language the callback would be a void function and returning a value would be a compile time error. In JavaScript you return a value to a caller that isn't doing anything with it. That is confusing and will lead people to misunderstand your code. Confusing people who read your code is not a good coding style. – Frank van Puffelen Feb 28 '16 at 08:20

Return a value from nested function in Javascript

1 Answers1