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javascriptprototypejsprototypejavascript-objects
Dariush Jafari
Dariush Jafari

Reputation: 5453

Set a prototype for a predefined object in javascript

How can I manipulate the prototype of a predefined object (for example an Array) so that it does something when creating instances of that object?

Simply I want to alert('an array was created!') whenever an Array is instantiated.

Upvotes: 0

Views: 141

Answers (3)

Christian Jørgensen
Christian Jørgensen

Reputation: 337

I would suggest the you just create an array namespace for it:

array = {};
array.create = function() {

    alert("Created an array");     
    return [];

}

So whenever you create an array you use: array.create(); .

You should not, and in this case can not, change native functionality. You have to be in charge of every array creation.

Upvotes: 1

zzzzBov
zzzzBov

Reputation: 179256

You can set a new method on an array by adding it to the Array.prototype object:

Array.prototype.fizz = function () {
    alert('works');
};

var arr = [];
arr.fizz();

However, this only allows you to create new methods, this does not allow you to extend existing methods*, nor does it allow you to override the Array constructor.

Be careful adding new methods to existing types. This can adversely affect the entire scripting environment, and cause unexpected behavior in some libraries. Although some might call it "bad practice", it's quite common to use polyfills for cross-browser compatibility, such as by creating Array.prototype.indexOf.

There is no such thing as a "newed" array, the word is "instantiated":

var a1, a2;
a1 = []; //a1 was instantiated with a new Array
a2 = new Array(); //a2 was also instantiated with a new Array

There is no cross-browser means of overriding the Array constructor.

* it's possible to wrap an existing method in a function so that, when called, the existing method performs its original functionality, in addition to the new functionality. Although this might be referred to as extending an existing method, it is in fact creating a new method.

Upvotes: 3

gen_Eric
gen_Eric

Reputation: 227310

You can try to override Array with your own function. It seems to work when doing new Array, but not when doing [].

(function() {
    var _ac = Array;
    Array = function() {
        alert('an array was newed!');
        return _ac.apply(this, arguments);
    };
}());

DEMO: http://jsfiddle.net/DAg9A/

Upvotes: 2

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