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 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?

• javascript
• parameters
• callback
• dom-events

Share Improve this question Follow edited May 9, 2020 at 11:06 Brian Tompsett - 汤莱恩 5,8937272 gold badges6161 silver badges133133 bronze badges asked Oct 13, 2012 at 20:22 SaiSai 71333 gold badges1010 silver badges2727 bronze badges 2

• 1 How would you want to modify it? – David G Commented Oct 13, 2012 at 20:25
• I want to set e to something that'll be meaningful to the user when the user is calling the function. – Sai Commented Oct 13, 2012 at 20:39

Add a ment  | 

2 Answers 2

Sorted by: Reset to default 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);
};

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]);
    }
}