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JavaScript Object Constructors


Using an Object Constructor

The examples from the previous chapters are limited in many situations. They only create single objects.

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, eye) {
    this.firstName = first;
    this.lastName = last;
    this.age = age;
    this.eyeColor = eye;
}
var myFather = new Person("John", "Doe", 50, "blue");
var myMother = new Person("Sally", "Rally", 48, "green");
Try it yourself »

The above function (Person) is an object constructor.

It is considered good practice to name constructor functions with an upper-case first letter.

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");


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.


Built-in JavaScript Constructors

JavaScript has built-in constructors for native objects:

Example

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

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


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 object
var x2 = "";            // new primitive string
var x3 = 0;             // new primitive number
var x4 = false;         // new primitive boolean
var x5 = [];            // new array object
var x6 = /()/           // new regexp object
var x7 = function(){};  // new function object
Try it Yourself »

String Objects

Normally, strings are created as primitives: var firstName = "John"

But strings can also be created as objects using the new keyword: var firstName = new String("John")

Learn why strings should not be created as object in the chapter JS Strings.


Number Objects

Normally, numbers are created as primitives: var x = 123

But numbers can also be created as objects using the new keyword: var x = new Number(123)

Learn why numbers should not be created as object in the chapter JS Numbers.


Boolean Objects

Normally, booleans are created as primitives: var x = false

But booleans can also be created as objects using the new keyword: var x = new Boolean(false)

Learn why booleans should not be created as object in the chapter JS Booleans.