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Modifying Objects

Creating your own objects is just part of the fun in ECMAScript. How would you like to modify the

behavior of existing objects? This is completely possible in ECMAScript, so dream up whatever methods

you’d like for a

String

Array

Number

, or any other object, because the possibilities are endless.

Remember the

prototype

object from an earlier section in this chapter? You already know that each

constructor has a

prototype

property that can be used to define methods. What you don’t already

know is that each of the native objects in ECMAScript also has a

prototype

property that can be used

in the exactly same way.

Creating a new method

You can define a new method for any existing class by using its

prototype

property, just as you would

with your own classes. For instance, remember the

toString()

method of

Number

that outputs a hexa-

decimal string if you pass in 16 as an argument? Wouldn’t it be nicer to have a

toHexString()

method

to handle the process? It’s simple to create it:

Number.prototype.toHexString = function () {

return this.toString(16);

};

In this context, the

this

keyword points to the instance of

Number

and so has full access to all the

Number

methods. With this code, it is possible to do this:

var iNum = 15;

alert(iNum.toHexString()); //outputs “F”

Because the number 15 is equal to hexadecimal F, the alert displays

“F”

. And remember the discussion

about using an

Array

as a queue? The only thing missing was properly named methods. You can add

enqueue()

and

dequeue()

Array

and just have them call the existing

push()

and

shift()

methods

respectively:

Array.prototype.enqueue = function(vItem) {

this.push(vItem);

};

Array.prototype.dequeue = function() {

return this.shift();

};

You can also, of course, add methods that don’t rely on existing methods at all. For example, say that

you want to determine the position of a particular item in an array. You have no native method to do

such a thing. You can easily create a method that does this:

Array.prototype.indexOf = function (vItem) {

for (var i=0; i < this.length; i++) {

if (vItem == this[i]) {

return i;

Object Basics

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