Note: many of the contents of this page are taken from w3school website.

JavaScript Objects

Object Oriented Programming

JavaScript is an Object Oriented Programming (OOP) language. An OOP language allows you to define your own objects and make your own variable types.

However, creating your own objects will be explained later, in the Advanced JavaScript section. We will start by looking at the built-in JavaScript objects, and how they are used. The next pages will explain each built-in JavaScript object in detail.

Note that an object is just a special kind of data. An object has properties and methods.

Properties

Properties are the values associated with an object.

In the following example we are using the length property of the String object to return the number of characters in a string:

<script type="text/javascript">
  var txt="Hello World!"
  document.write(txt.length)
</script>

The output of the code above will be:

12

Methods

Methods are the actions that can be performed on objects.

In the following example we are using the toUpperCase() method of the String object to display a text in uppercase letters:

<script type="text/javascript">
  var str="Hello world!"
  document.write(str.toUpperCase())
</script>

The output of the code above will be:

HELLO WORLD!

Creating New Objects

JavaScript Objects

Earlier we have seen that JavaScript has several built-in objects, like String, Date, Array, and more. In addition to these built-in objects, you can also create your own.

An object is just a special kind of data, with a collection of properties and methods.

Let's illustrate with an example: A person is an object. Properties are the values associated with the object. The persons' properties include name, height, weight, age, skin tone, eye color, etc. All persons have these properties, but the values of those properties will differ from person to person. Objects also have methods. Methods are the actions that can be performed on objects. The persons' methods could be eat(), sleep(), work(), play(), etc.

Properties

The syntax for accessing a property of an object is:

objName.propName

You can add properties to an object by simply giving it a value. Assume that the personObj already exists - you can give it properties named firstname, lastname, age, and eyecolor as follows:

personObj.firstname="John"
personObj.lastname="Doe"
personObj.age=30
personObj.eyecolor="blue"
document.write(personObj.firstname)

The code above will generate the following output:

John

Methods

An object can also contain methods.

You can call a method with the following syntax:

objName.methodName()

Note: Parameters required for the method can be passed between the parentheses.

To call a method called sleep() for the personObj:

personObj.sleep()

Creating Your Own Objects

There are different ways to create a new object:

1. Create a direct instance of an object

The following code creates an instance of an object and adds four properties to it:

personObj=new Object()
personObj.firstname="John"
personObj.lastname="Doe"
personObj.age=50
personObj.eyecolor="blue"

Adding a method to the personObj is also simple. The following code adds a method called eat() to the personObj:

personObj.eat=eat

2. Create a template of an object

The template defines the structure of an object:

function person(firstname,lastname,age,eyecolor)
{
  this.firstname=firstname
  this.lastname=lastname
  this.age=age
  this.eyecolor=eyecolor
}

Notice that the template is just a function. Inside the function you need to assign things to this.propertyName. The reason for all the "this" stuff is that you're going to have more than one person at a time (which person you're dealing with must be clear). That's what "this" is: the instance of the object at hand.

Once you have the template, you can create new instances of the object, like this:

myFather=new person("John","Doe",50,"blue")
myMother=new person("Sally","Rally",48,"green")

You can also add some methods to the person object. This is also done inside the template:

function person(firstname,lastname,age,eyecolor)  
{
  this.firstname=firstname
  this.lastname=lastname
  this.age=age
  this.eyecolor=eyecolor
  this.newlastname=newlastname
}

Note that methods are just functions attached to objects. Then we will have to write the newlastname() function:

function newlastname(new_lastname)
{
  this.lastname=new_lastname
}

The newlastname() function defines the person's new last name and assigns that to the person. JavaScript knows which person you're talking about by using "this.". So, now you can write: myMother.newlastname("Doe").