Modifying callback function's argument(s) before calling them

Question

I have a custom object that implements a function that'll be executed later. Here's how someone would call it:

customObject.onSomething(function(e) {
    // do something with e
    console.log('foobar');
});

Here's how onSomething is getting created:

var CustomObject = function() {
    this.onSomething = function(callback) {
        // If the user passes in parameter(s), how can I modify them before calling?
        callback.apply(this);
    }
}

How can I modify the argument(s) the user passed in before performing apply or call on the function?

I want to set e to something that'll be meaningful to the user when the user is calling the function. — Sai
– Sai, Commented Oct 13, 2012 at 20:39

Avish · Accepted Answer · 2012-10-13 20:44:07Z

8

apply takes a second parameter which is a list of arguments to pass to the function. call does the same, except it passes its own argument-list (everything after the first parameter which is used as this).

So, if you know which parameters you expect, you can just add them to the invoking function as the second parameter to apply (or as a list of parameters to call):

this.onSomething = function(arg1, arg2) {
   // reverse the first and second arguments
   callback.apply(this, [arg2, arg1]);
   // equivalent:
   callback.call(this, arg2, arg1);
};

If you don't know what kind of arguments to expect, but you still want to do something with them, you can do so with the builtin arguments pseudo-array which holds the arguments given to the current function (even when you don't declare them explicitly).

You can use this to invoke the callback with the same arguments given to the invoking function, or some transformation of them; e.g.:

this.onSomething = function() {
    // call callback with the same arguments we got
    callback.apply(this, arguments);

    // or, make some changes
    var newArgs = ["extra argument", arguments[1], arguments[0]];
    callback.apply(this, newArgs);
};

edited Oct 13, 2012 at 20:44

answered Oct 13, 2012 at 20:27

Avish

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2 Comments

Sai Over a year ago

This is what I was looking for. Thanks! Let me ask you this: What if the user sends in an n number of args (unknown). How can I dynamically pass those back into apply?

Avish Over a year ago

Well, arguments will hold n items which are the arguments given to the invoking function. If you need to pass these unchanged, call apply(this, arguments). If you want to change them, make a new array from the one you got (you can inspect arguments, check for its length, copy its elements etc.) and pass that to apply. I edited my answer to reflect this.

Kelvin · Accepted Answer · 2012-10-13 20:27:23Z

1

Sounds like what you're asking for is fairly simple, see below:

var CustomObject = function() {
    this.onSomething = function(callback, param1, param2) {
        param1 += 4;
        param2 = 'Something about ' + param2 + ' is different...';
        callback.apply(this, [param1, param2]);
    }
}

answered Oct 13, 2012 at 20:27

Kelvin

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