JavaScript Objects
JavaScript Objects
Objects are variables too. But objects can contain many values. JavaScript objects are containers for named values called properties or methods.
JavaScript OOP
JavaScript is an Object Oriented Programming (OOP) language. A programming language can be called object-oriented if it provides four basic capabilities to developers
- Encapsulation − the capability to store related information, whether data or methods, together in an object.
- Aggregation − the capability to store one object inside another object.
- Inheritance − the capability of a class to rely upon another class (or number of classes) for some of its properties and methods.
- Polymorphism − the capability to write one function or method that works in a variety of different ways.
Objects are composed of attributes. If an attribute contains a function, it is considered to be a method of the object, otherwise the attribute is considered a property.
var car = {type:"Fiat", model:"500", color:"white"};
Object Definition
You define (and create) a JavaScript object with an object literal.
Spaces and line breaks are not important. An object definition can span multiple lines:
var person = {firstName:"John", lastName:"Doe", age:50, eyeColor:"blue"};
var person = {
firstName: "John",
lastName: "Doe",
age: 50,
eyeColor: "blue"
};
Accessing Object Properties
Object properties can be any of the three primitive data types, or any of the abstract data types, such as another object. Object properties are usually variables that are used internally in the object's methods, but can also be globally visible variables that are used throughout the page.You can access object properties in two ways:
objectName.propertyName
or
objectName["propertyName"]
person["lastName"];
Object Methods
Objects can also have methods. Methods are actions that can be performed on objects. Methods are stored in properties as function definitions.
Methods are the functions that let the object do something or let something be done to it. There is a small difference between a function and a method – at a function is a standalone unit of statements and a method is attached to an object and can be referenced by the this keyword.
Methods are useful for everything from displaying the contents of the object to the screen to performing complex mathematical operations on a group of local properties and parameters.
var person = {
firstName: "John",
lastName : "Doe",
id : 5566,
fullName : function() {
return this.firstName + " " + this.lastName;
}
};
User-Defined Objects
All user-defined objects and built-in objects are descendants of an object called Object.
The new Operator
The new operator is used to create an instance of an object. To create an object, the new operator is followed by the constructor method.
var employee = new Object();
var books = new Array("C++", "Perl", "Java");
var day = new Date("August 15, 1947");
The Object() Constructor
A constructor is a function that creates and initializes an object. JavaScript provides a special constructor function called Object() to build the object. The return value of the Object() constructor is assigned to a variable.
The variable contains a reference to the new object. The properties assigned to the object are not variables and are not defined with the var keyword.
<html>
<head>
<title>User-defined objects</title>
<script type = "text/javascript">
var book = new Object(); // Create the object
book.subject = "Perl"; // Assign properties to the object
book.author = "Mohtashim";
</script>
</head>
<body>
<script type = "text/javascript">
document.write("Book name is : " + book.subject + "");
document.write("Book author is : " + book.author + "");
</script>
</body>
</html>
The this Keyword
In a function definition, this refers to the "owner" of the function.
Accessing Object Methods
You access an object method with the following syntax: objectName.methodName()
name = person.fullName();
If you access a method without the parentheses, it will return the function definition:
name = person.fullName;
Do Not Declare Strings, Numbers, and Booleans as Objects!
When a JavaScript variable is declared with the keyword "new", the variable is created as an object: var carName;
var x = new String(); // Declares x as a String object
var y = new Number(); // Declares y as a Number object
var z = new Boolean(); // Declares z as a Boolean object
Avoid String, Number, and Boolean objects. They complicate your code and slow down execution speed.
