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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.

  • Booleans can be objects (if defined with the new keyword)
  • Numbers can be objects (if defined with the new keyword)
  • Strings can be objects (if defined with the new keyword)
  • Dates are always objects
  • Maths are always objects
  • Regular expressions are always objects
  • Arrays are always objects
  • Functions are always objects
  • Objects are always objects

All JavaScript values, except primitives, are objects.

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:

  • string
  • number
  • boolean
  • null
  • undefined

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.

"Hello"string"Hello" is always "Hello"
3.14number3.14 is always 3.14
truebooleantrue is always true
falsebooleanfalse is always false
nullnull (object)null is always null
undefinedundefinedundefined is always undefined

Objects are Variables Containing Variables

JavaScript variables can contain single values:


var person = "John Doe";
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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).


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

  • Associative arrays in PHP
  • Dictionaries in Python
  • Hash tables in C
  • Hash maps in Java
  • Hashes in Ruby and Perl

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:

  • 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().

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:


var person = {firstName:"John", lastName:"Doe", age:50, eyeColor:"blue"};
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Spaces and line breaks are not important. An object definition can span multiple lines:


var person = {
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Using the JavaScript Keyword new

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


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


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
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Note: JavaScript variables are not mutable. Only JavaScript objects.