JavaScript Object Constructors
Example
function Person(first, last, age, eye) {
this.firstName = first;
this.lastName = last;
this.age = age;
this.eyeColor = eye;
}
Try
it yourself »It is considered good practice to name constructor functions with an upper-case first letter.
Object Types (Blueprints) (Classes)
The examples from the previous chapters are limited. They only create single objects.
Sometimes we need a "blueprint" for creating many objects of the same "type".
The way to create an "object type", is to use an object constructor function.
In the example above, function Person() is an object constructor function.
Objects of the same type are created by calling the constructor function with the new keyword:
var myFather = new Person("John", "Doe", 50, "blue");
var myMother = new Person("Sally", "Rally", 48, "green");
Try
it yourself »
The this Keyword
In JavaScript, the thing called this is the object that "owns" the code.
The value of this, when used in an object, is the object itself.
In a constructor function this does not have a value.
It is a substitute for the new object. The value of this will become the new object when
a new object is created.
Note that this is not a variable.
It is a keyword. You cannot change the value of this.
Adding a Property to an Object
Adding a new property to an existing object is easy:
The property will be added to myFather. Not to myMother. (Not to any other person objects).
Adding a Method to an Object
Adding a new method to an existing object is easy:
Example
myFather.name = function () {
return this.firstName + " " + this.lastName;
};
Try it Yourself »
The method will be added to myFather. Not to myMother. (Not to any other person objects).
Adding a Property to a Constructor
You cannot add a new property to an object constructor the same way you add a new property to an existing object:
To add a new property to a constructor, you must add it to the constructor function:
Example
function Person(first, last, age, eyecolor) {
this.firstName = first;
this.lastName = last;
this.age = age;
this.eyeColor = eyecolor;
this.nationality = "English";
}
Try it Yourself »
This way object properties can have default values.
Adding a Method to a Constructor
Your constructor function can also define methods:
Example
function Person(first, last, age, eyecolor) {
this.firstName = first;
this.lastName = last;
this.age = age;
this.eyeColor = eyecolor;
this.name = function() {return this.firstName + " " + this.lastName;};
}
Try it Yourself »
You cannot add a new method to an object constructor the same way you add a new method to an existing object.
Adding methods to an object must be done inside the constructor function:
Example
function Person(firstName, lastName, age, eyeColor) {
this.firstName = firstName;
this.lastName = lastName;
this.age = age;
this.eyeColor = eyeColor;
this.changeName = function (name) {
this.lastName = name;
};
}The changeName() function assigns the value of name to the person's lastName property.
JavaScript knows which person you are talking about by "substituting" this with myMother.
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 above, JavaScript has object versions of the primitive
data types String, Number, and Boolean. But there is no reason to create complex objects. Primitive values
are much faster.
ALSO:
Use object literals {} instead of new Object().
Use string literals "" instead of new String().
Use number literals 12345 instead of new Number().
Use boolean literals true / false instead of new Boolean().
Use array literals [] instead of new Array().
Use pattern literals /()/ instead of new RegExp().
Use function expressions () {} instead of new Function().
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.
