Retrieve variable from javascript object method callback

Question

In my code I need to call an object method, retrieve the data from its callback, and pass it to another method or function.

someObject.getSomeData({option1:'value1', option2:'value2'},
    function(data) {
        doAwesomeStuff(data);
    }
);

However, the callback does not recognize any functions/objects/variables outside its scope.

What I've tried to do right now is wrap everything around a function.

var myData = '';
(function(myData) {
    someObject.getSomeData({option1:'value1', option2:'value2'},
        function(data) {
            myData = data;
        }
    );
});
doAwesomeStuff(myData);

However that doesn't work either.

Anybody know how to properly accomplish this?

What variables are you referring to by saying "outside its scope"? — pimvdb
– pimvdb, Commented Mar 6, 2012 at 13:56
> However, the callback does not recognize any > functions/objects/variables outside its scope. That sounds a bit weird... where is doAwesomeStuff() defined? If it's within the scope in which your call is placed (or into a broader one) it must be reachable. — mamoo
– mamoo, Commented Mar 6, 2012 at 13:57
it got defined and executed actually, but this was in nodejs and it was doing some data processing, and while I had a console.out statement to verify it got executed, it seems it got lost among other output. Seems like I hurried posting the question, I did that in school, I figured it out as soon as I got home and had a fresh look at it. thanks everyone for your help, I'll flag it for deletion or something :) — Felix Mc
– Felix Mc, Commented Mar 6, 2012 at 18:36

T.J. Crowder · Accepted Answer · 2012-03-06 14:04:01Z

3

You haven't really given us enough to go on there, but this statement:

However, the callback does not recognize any functions/objects/variables outside its scope.

...is incorrect. A function has access to everything in scope where it's defined, so for instance:

var a = 10;
function foo(b) {
    bar(5);
    function bar(c) {
        alert(a + b + c);
    }
}
foo(12); // alerts "27"

Note how bar had access not only to c, but also to b (from the call to foo) and a (from the outermost scope shown).

So in your example, the anonymous function you're passing in as the callback has access to everything that's in scope where it's defined; doAwesomeStuff having been defined elsewhere presumably has access to different information, so you'll have to have the callback pass it any data it needs.

So I'm guessing your code looks something like this:

function doAwesomeStuff(data) {
    // ...
}

function doSomethingNifty() {
    var a = 10,
        b = 20;
    someObject.getSomeData({option1:'value1', option2:'value2'},
        function(data) {
            doAwesomeStuff(data);
        }
    );
}

...and you want doAwesomeStuff to have access to a and b from the call to doSomethingNifty. If so, your only options are to pass them into it as arguments (probably best) or export them to variables some scope that doSomethingNifty and doAwesomeStuff share (probably not ideal, too much like globals).

edited Mar 6, 2012 at 14:04

answered Mar 6, 2012 at 13:58

T.J. Crowder

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1 Comment

Felix Mc Over a year ago

you're right, but it was something different, thanks for feedback! see my comment above on my own question for more details

Community · Accepted Answer · 2017-05-23 11:55:53Z

1

You can bind required variables to the function passed into the async method.

Also, this SO question has a good treatment of the topic.

Your second version is not going to work at all, since you are trying to immediately access the data that are not yet available (not until the callback has been invoked.)

edited May 23, 2017 at 11:55

CommunityBot

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answered Mar 6, 2012 at 14:00

Alexander Pavlov

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1 Comment

Felix Mc Over a year ago

you're right about the callback, I was able to figure it out :) my function was poorly defined, see my comment on my question above

toto2 · Accepted Answer · 2012-03-06 15:23:20Z

1

Your first method:

someObject.getSomeData({option1:'value1', option2:'value2'},
    function(data) {
        doAwesomeStuff(data);
    }
);

looks fine. Please provide more details on what is not working.

One problem could be that getSomeData() does not actually call the callback function.

doAwesomeStuff() can modify many different variables from the received data. The variables which can be accessed by doAwesomeStuff() are those that were available to it (in its scope) where it was created..

answered Mar 6, 2012 at 15:23

toto2

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1 Comment

Felix Mc Over a year ago

you're right, but it was something different, thanks for feedback! see my comment above on my own question for more details

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