JavaScript Objects
In JavaScript, objects are king. If you understand objects, you understand JavaScript.
In JavaScript, almost "everything" is an object.
In JavaScript, all values, except primitive values, are objects.
Primitive values are: strings ("John Doe"), numbers (3.14), true, false, null, and undefined.
Objects are Variables Containing Variables
JavaScript variables can contain single values:
Objects are variables too. But objects can contain many values.
The values are written as name : value pairs (name and value separated by a colon).
A JavaScript object is a collection of named values
Object Properties
The named values, in JavaScript objects, are called properties.
| Property | Value |
|---|---|
| firstName | John |
| lastName | Doe |
| age | 50 |
| eyeColor | blue |
Objects written as name value pairs are similar to:
Object Methods
Methods are actions that can be performed on objects.
Object properties can be both primitive values, other objects, and functions.
An object method is an object property containing a function definition.
| Property | Value |
|---|---|
| firstName | John |
| lastName | Doe |
| age | 50 |
| eyeColor | blue |
| fullName | function() {return this.firstName + " " + this.lastName;} |
JavaScript objects are containers for named values, called properties and methods.
You will learn more about methods in the next chapters.
Creating a JavaScript Object
With JavaScript, you can define and create your own objects.
There are different ways to create new objects:
In ECMAScript 5, an object can also be created with the function Object.create().
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:
Spaces and line breaks are not important. An object definition can span multiple lines:
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";
Try it Yourself
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).
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, 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.
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
JavaScript Objects are Mutable
Objects are mutable: They are addressed by reference, not by value.
If person is an object, the following statement will not create a copy of person:
var x = person; // This will not create a copy of person.
The object x is not a copy of person. It is person. Both x and person is the same object.
Any changes to x will also change person, because x and person 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
Try it Yourself
Note: JavaScript variables are not mutable. Only JavaScript objects.