Define a JavaScript class

JavaScript is a very flexible object-oriented language when it comes to syntax. In this article you can find three ways of defining and instantiating an object. Even if you have already picked your favorite way of doing it, it helps to know some alternatives in order to read other people's code.

It's important to note that there are no classes in JavaScript. Functions can be used to somewhat simulate classes, but in general JavaScript is a class-less language. Everything is an object. And when it comes to inheritance, objects inherit from objects, not classes from classes as in the "class"-ical languages.

Using a function

This is probably one of the most common ways. You define a normal JavaScript function and then create an object by using the new keyword. To define properties and methods for an object created using function(), you use the this keyword, as seen in the following example.

function Apple (type) {
this.type = type;
this.color = "red";
this.getInfo = getAppleInfo;
}
// anti-pattern! keep reading...
function getAppleInfo() {
return this.color + ' ' + this.type + ' apple';
}

To instantiate an object using the Apple constructor function, set some properties and call methods you can do the following:

var apple = new Apple('macintosh');
apple.color = "reddish";
alert(apple.getInfo());

Methods defined internally

In the example above you see that the method getInfo() of the Apple "class" was defined in a separate function getAppleInfo(). While this works fine, it has one drawback – you may end up defining a lot of these functions and they are all in the "global namespece". This means you may have naming conflicts if you (or another library you are using) decide to create another function with the same name. The way to prevent pollution of the global namespace, you can define your methods within the constructor function, like this:

function Apple (type) {
this.type = type;
this.color = "red";
this.getInfo = function() {
return this.color + ' ' + this.type + ' apple';
};
}

Using this syntax changes nothing in the way you instantiate the object and use its properties and methods.
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