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JavaScript Objects

In JavaScript, objects are king. If you understand objects, you understand JavaScript.

In JavaScript, almost "everything" is an object.

JavaScript Primitives

A primitive value is a value that has no properties or methods.
A primitive data type is data that has a primitive value.
JavaScript defines 5 types of primitive data types:

Primitive values are immutable (they are hardcoded and therefore cannot be changed).

if x = 3.14, you can change the value of x. But you cannot change the value of 3.14.

Value Type Comment
"Hello" string "Hello" is always "Hello"
3.14 number 3.14 is always 3.14
true boolean true is always true
false boolean false is always false
null null (object) null is always null
undefined undefined undefined is always undefined

Objects are Variables

JavaScript variables can contain single values:

Example

<!DOCTYPE html>
<html>
<body>

<h2>JavaScript Variables</h2>

<p id="demo"></p>

<script>
// Create aand display a variable:
var person = "John Doe";
document.getElementById("demo").innerHTML = person;
</script>

</body>
</html>

output

JavaScript Variables

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).

Example

<!DOCTYPE html>
<html>
<body>

<p>Creating a JavaScript Object.</p>

<p id="demo"></p>

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

document.getElementById("demo").innerHTML =
person.firstName + " " + person.lastName;
</script>

</body>
</html>

output

Creating a JavaScript Object.
John Doe

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.

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:

Example

<!DOCTYPE html>
<html>
<body>

<p>Creating a JavaScript Object.</p>

<p id="demo"></p>

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

document.getElementById("demo").innerHTML =
person.firstName + " is " + person.age + " years old.";
</script>

</body>
</html>
Spaces and line breaks are not important. An object definition can span multiple lines: Example

output

Creating a JavaScript Object.
John is 50 years old.

Spaces and line breaks are not important. An object definition can span multiple lines:

Example

<!DOCTYPE html>
<html>
<body>

<p>Creating a JavaScript Object.</p>

<p id="demo"<>/p>

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

document.getElementById("demo").innerHTML =
person.firstName + " is " + person.age + " years old.";
</script>

</body>
</html>

output

Creating a JavaScript Object.
John is 50 years old.

Using the JavaScript Keyword new

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

Example

<!DOCTYPE html>
<html>
<body>

<p id="demo"></p>

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

document.getElementById("demo").innerHTML =
person.firstName + " is " + person.age + " years old.";
</script>

</body>
</html>

output

John is 50 years old.

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).

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

Example

<!DOCTYPE html>
<html>
<body>

<p>JavaScript objects are mutable.</p>
<p>Any changes to a copy of an object will also change the original.</p>

<p id="demo"></p>

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

var x = person;
x.age = 10;

document.getElementById("demo").innerHTML =
person.firstName + " is " + person.age + " years old.";
</script>

</body>
</html>

output

JavaScript objects are mutable.
Any changes to a copy of an object will also change the original.
John is 10 years old.