Add a variable to a javascript callback

Question

I am using a library where I must define a callback function, and the library will execute this function upon a certain event:

// initialize the callback for the library.
// `lib` is the main variable for the library and is defined globally
function initializations() {
    var extra_var = 'pass me into the callback';
    var libprops = {
        libcallback: function(settings) { do stuff }
    };
    lib.reconfigure(libprops);
}

The library then runs the callback like so (I have no control over this):

var settings = 'xyz';
libprops.libcallback(settings);

So clearly, one of the variables input to my defined callback will be the settings variable. However I also want to pass in a variable of my own:

function mycallback(settings, extra_var) {
    // do stuff involving settings
    // do stuff involving extra_var
}

How can I define libprops.libcallback within initializations() so that extra_var is passed in, but with function mycallback defined elsewhere? Ie like so:

function mycallback(settings, extra_var) {
    // do stuff involving settings
    // do stuff involving extra_var
}

Is this possible? The reason I want to define mycallback() outside of initializations() is that mycallback() is quite large and its messy to have it defined inside initializations().

It seems like a closure would solve the issue, but I'm not quite sure how to construct it. Here is a preliminary attempt:

function initializations() {
    var extra_var = 'pass me into the callback';
    var libprops = {
        libcallback: (function(settings) {
            mycallback(settings, extra_var)
        })(extra_var)
    };
    lib.reconfigure(libprops);
}

you already have a closure, therefore you don't need to pass in extra_var anywhere, it's available to your anonymous callback function already, as extra_var — Jaromanda X
– Jaromanda X, Commented Aug 29, 2016 at 8:29
Thanks @JaromandaX, I realize that, but I want to define mycallback() outside of initializations(). I have updated the question to include this important requirement. Sorry to change the game. It was obvious to me, but I realized it was not originally included in the question :P — mulllhausen
– mulllhausen, Commented Aug 29, 2016 at 8:33
just: libcallback: function(settings) {..}, remove the function call with extra_var — Can Nguyen
– Can Nguyen, Commented Aug 29, 2016 at 8:34
your attempt will set libcallback to undefined, because your IIFE is returning undefined - however, you'll have more issues because mycallback will be called immediately rather than as a callback — Jaromanda X
– Jaromanda X, Commented Aug 29, 2016 at 8:38

Jaromanda X · Accepted Answer · 2016-08-29 08:32:49Z

1

you already have a closure, therefore you don't need to pass in extra_var anywhere, it's available to your anonymous callback function already, as extra_var

function initializations() {
    var extra_var = 'pass me into the callback';
    var libprops = {
        libcallback: function(settings) {
            // do stuff 
            // you can do stuff with extra_var here too
        }
    };
    lib.reconfigure(libprops);
}

as per extra information in comment, just call your myfunction in the anonymous callback, passing in extra_vars as the second parameter (or first if you want, doesn't matter, it's your function)

function initializations() {
    var extra_var = 'pass me into the callback';
    var libprops = {
        libcallback: function(settings) {
            mycallback(settings, extra_var);
        }
    };
    lib.reconfigure(libprops);
}

there's probably another way using .bind - however, no need for such gymnastics in such a simple scenario

answered Aug 29, 2016 at 8:32

Jaromanda X

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5 Comments

mulllhausen Over a year ago

Looks promising! I will test and accept if it works. Many thanks!

mulllhausen Over a year ago

how can i access lib from within mycallback()? If its an anonymous function inline with the object then its just this, but this does not refer to lib within mycallback()

mulllhausen Over a year ago

ah I found i can pass this to mycallback() as a third parameter from the anonymous function during the definition like so: libcallback: function(settings) { mycallback(settings, extra_var, this);}

Jaromanda X Over a year ago

how can i access lib from within mycallback() - I know you found an answer, but I didn't think there was an issue with accessing lib as it was not treated in any way special in the question - I automatically assumed it'd just be "in scope" at that point (within initializations functions as it is in your code)

mulllhausen Over a year ago

I know I did originally said lib was defined globally. This was true at the time I wrote the question, but I have since changed that requirement. No worries about not addressing that - I didn't expect you to read my mind haha

Alberto Chiesa · Accepted Answer · 2016-08-29 08:31:53Z

0

Yes, a closure will do, but you don't need to call the function "now". You can create a function that references your local extra_var and pass it around:

function initializations() {
    var extra_var = 'pass me into the callback';
    var libprops = {
        libcallback: function(settings) {
            // you can access both settings passed in as a parameter
            // and extra_var, referenced locally.
            var settingsLocal = settings,
              another = extra_var;
        }
    };
    lib.reconfigure(libprops);
}

If extra_var has a fixed value like in the example, you're done. Elsewhere, you need to elaborate on your request.

answered Aug 29, 2016 at 8:31

Alberto Chiesa

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