JavaScript Objects

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Object is the main data type in JavaScript. "Everything" in JavaScript is an Object.

In addition, JavaScript allows you to define your own objects.

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Everything is an Object

In JavaScript almost everything is an object.

Even primitive data types (except null and undefined) can be treated as objects.

• Booleans can be objects (or primitive data treated as objects)
• Numbers can be objects (or primitive data treated as objects)
• Strings are also objects (or primitive data treated as objects)
• Dates are always objects
• Maths and Regular Expressions are always objects
• Arrays are always objects
• Even functions are always objects

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JavaScript Objects

A JavaScript object is a complex variable, with properties and methods.

The content of an object is composite: It can contain multiple property types (both primitive values and other objects).   

In JavaScript, all values except primitive values are objects.

Primitive values are: strings ("John Doe"), numbers (3.14), true, false, null, and undefined.  

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Object Properties

Properties are the values associated with an object.

A JavaScript object is an unordered collection of properties.

Each property has a name and a value.

The syntax for accessing the property of an object is:

objectName.property

or

objectName[expression]

You will learn more about properties in the next chapter.

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Object Methods

Methods are the actions that can be performed on objects.

You can call a method with the following syntax:

objectName.methodName()

You will learn more about methods in the next chapter.

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Creating a JavaScript Object

With JavaScript, you can define and create your own objects.

There are different ways to create new objects:

• Define and create a single object, using an object literal.
• Define and create a single object, with the keyword new.
• Define an object constructor, and then create objects of the constructed type.

	In ECMAScript 5, an object can also be created with the function Object.create().

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Using an Object Literal

This is the easiest way to create a JavaScript Object.

Using an object literal, you both define and create an object in one statement.

An object literal is a list of name:value pairs (like age:50) inside curly braces {}.

The following example creates a new JavaScript object with four properties:

Example

var person = {firstName:"John", lastName:"Doe", age:50, eyeColor:"blue"};

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Using the JavaScript Keyword new

The following example also creates a new JavaScript object with four properties:

Example

var person = new Object();
person.firstName = "John";
person.lastName = "Doe";
person.age = 50;
person.eyeColor = "blue";

	The two examples above do exactly the same. There is no need to use new Object(). For simplicity, readability and execution speed, use the first one (the object literal method).

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Using an Object Constructor

The examples above are limited in many situations. They only create a single object.

Sometimes we like to have an "object type" that can be used to create many objects of one type.

The standard way to create an "object type" is to use an object constructor function:

Example

function person(first, last, age, eyecolor) {
    this.firstName = first;
    this.lastName = last;
    this.age = age;
    this.eyeColor = eyecolor;
}
var myFather = new person("John", "Doe", 50, "blue");
var myMother = new person("Sally", "Rally", 48, "green");

The above function (person) is an object constructor.

Once you have an object constructor, you can create new objects of the same type:

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

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The this Keyword

In JavaScript, the thing called this, is the object that "owns" the JavaScript code.

The value of this, when used in a function, is the object that "owns" the function.

The value of this, when used in an object, is the object itself.

The this keyword in an object constructor does not have a value. It is only a substitute for the new object.

The value of this will become the new object when the constructor is used to create an object.

	Note that this is not a variable. It is a keyword. You cannot change the value of this.

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Built-in JavaScript Constructors

JavaScript has built-in constructors for native objects:

Example

var x1 = new Object();    // A new Object objectvar x2 = new String();    // A new String objectvar x3 = new Number();    // A new Number objectvar x4 = new Boolean()    // A new Boolean objectvar x5 = new Array();     // A new Array objectvar x6 = new RegExp();    // A new RegExp objectvar x7 = new Function();  // A new Function objectvar x8 = new Date();      // A new Date object

The Math() object is not in the list above. Math is a global object. The keyword new cannot be used on global objects.

	Did You Know?

As you can see, JavaScript has object versions of the primitive data types String, Number, and Boolean.

There is no reason to create complex objects. Primitive values execute much faster.

And there is no reason to use new Array(). Use array literals instead: []

And there is no reason to use new RegExp(). Use pattern literals instead: /()/

And there is no reason to use new Function(). Use function expressions instead: function () {}.

And there is no reason to use new Object(). Use object literals instead: {}

Example

var x1 = {};            // new objectvar x2 = "";            // new primitive stringvar x3 = 0;             // new primitive numbervar x4 = false;         // new primitive booleanvar x5 = [];            // new array objectvar x6 = /()/           // new regexp objectvar x7 = function(){};  // new function object

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JavaScript Objects are Mutable

Object are mutable: They are addressed by reference, not by value.

If y is an object, the following statement will not create a copy of y:

var x = y;  // This will not create a copy of y.

The object x is not a copy of y. It is y. Both x and y points to the same object.

Any changes to y will also change x, because x and y are the same object.

Example

var person = {firstName:"John", lastName:"Doe", age:50, eyeColor:"blue"}
var x = person
x.age = 10;  // This will change both x.age and person.age