Bài viết này hướng dẫn bạn cách thức định nghĩa, cài đặt một lớp trong ngôn ngữ lập trình JavaScript thông qua 3 cách khá cơ bản. Xem thêm Lập trình JavaScript hướng đối tượng dành cho người mới.

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Introduction

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.

1. 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 a class 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;
}

function getAppleInfo() {
return this.color + ' ' + this.type + ' apple';
}

To instantiate an object of the Apple class, set some properties and call methods you can do the following:

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

1.1. 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 same class, 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.

2. Using JSON
JSON stands for JavaScript Object Notation; it simply uses the short way of defining objects and arrays in JavaScript. To create an empty object using JSON you can do:

var o = {};
instead of the "normal" way:
var o = new Object();
For arrays you can do:
var a = [];
instead of:
var a = new Array();

So using the JSON you can define a class, while at the same time creating an instance (object) of that class. Such a class/object is also called "singleton" which means that you can have only one single instance of this class at any time, you cannot create more objects of the same class. Here's the same class described in the previous examples, but using JSON syntax this time:

var apple = {
type: "macintosh",
color: "red",
getInfo: function () {
return this.color + ' ' + this.type + ' apple';
}
}

In this case you don't need to (and cannot) create an instance of the class, it already exists. So you simply start using this instance.

apple.color = "reddish";
alert(apple.getInfo());

3. Singleton using a function
The third way presented in this article is a combination of the other two you already saw. You can use a function to define a singleton class. Here's the syntax:

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

So you see that this is very similar to 1.1. discussed above, but the way to use the object is exactly like in 2.

apple.color = "reddish";
alert(apple.getInfo());


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