Tutorials References Exercises Videos Menu
Create Website Get Certified Upgrade

JS Tutorial

JS HOME JS Introduction JS Where To JS Output JS Statements JS Syntax JS Comments JS Variables JS Let JS Const JS Operators JS Arithmetic JS Assignment JS Data Types JS Functions JS Objects JS Events JS Strings JS String Methods JS String Search JS String Templates JS Numbers JS BigInt JS Number Methods JS Number Properties JS Arrays JS Array Methods JS Array Sort JS Array Iteration JS Array Const JS Dates JS Date Formats JS Date Get Methods JS Date Set Methods JS Math JS Random JS Booleans JS Comparisons JS If Else JS Switch JS Loop For JS Loop For In JS Loop For Of JS Loop While JS Break JS Iterables JS Sets JS Maps JS Typeof JS Type Conversion JS Bitwise JS RegExp JS Precedence JS Errors JS Scope JS Hoisting JS Strict Mode JS this Keyword JS Arrow Function JS Classes JS Modules JS JSON JS Debugging JS Style Guide JS Best Practices JS Mistakes JS Performance JS Reserved Words

JS Versions

JS Versions JS 2009 (ES5) JS 2015 (ES6) JS 2016 JS 2017 JS 2018 JS 2019 JS 2020 JS 2021 JS IE / Edge JS History

JS Objects

Object Definitions Object Properties Object Methods Object Display Object Accessors Object Constructors Object Prototypes Object Iterables Object Sets Object Maps Object Reference

JS Functions

Function Definitions Function Parameters Function Invocation Function Call Function Apply Function Bind Function Closures

JS Classes

Class Intro Class Inheritance Class Static

JS Async

JS Callbacks JS Asynchronous JS Promises JS Async/Await

JS HTML DOM

DOM Intro DOM Methods DOM Document DOM Elements DOM HTML DOM Forms DOM CSS DOM Animations DOM Events DOM Event Listener DOM Navigation DOM Nodes DOM Collections DOM Node Lists

JS Browser BOM

JS Window JS Screen JS Location JS History JS Navigator JS Popup Alert JS Timing JS Cookies

JS Web APIs

Web API Intro Web Forms API Web History API Web Storage API Web Worker API Web Fetch API Web Geolocation API

JS AJAX

AJAX Intro AJAX XMLHttp AJAX Request AJAX Response AJAX XML File AJAX PHP AJAX ASP AJAX Database AJAX Applications AJAX Examples

JS JSON

JSON Intro JSON Syntax JSON vs XML JSON Data Types JSON Parse JSON Stringify JSON Objects JSON Arrays JSON Server JSON PHP JSON HTML JSON JSONP

JS vs jQuery

jQuery Selectors jQuery HTML jQuery CSS jQuery DOM

JS Graphics

JS Graphics JS Canvas JS Plotly JS Chart.js JS Google Chart JS D3.js

JS Examples

JS Examples JS HTML DOM JS HTML Input JS HTML Objects JS HTML Events JS Browser JS Editor JS Exercises JS Quiz JS Certificate

JS References

JavaScript Objects HTML DOM Objects


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.

3.14 is a primitive value

A primitive data type is data that has a primitive value.

JavaScript defines 7 types of primitive data types:

Examples

  • string
  • number
  • boolean
  • null
  • undefined
  • symbol
  • bigint

Immutable

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

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

ValueTypeComment
"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

JavaScript variables can contain single values:

Example

let person = "John Doe";
Try it Yourself »

JavaScript variables can also contain many values.

Objects are variables too. But objects can contain many values.

Object values are written as name : value pairs (name and value separated by a colon).

Example

let person = {firstName:"John", lastName:"Doe", age:50, eyeColor:"blue"};
Try it Yourself »

A JavaScript object is a collection of named values

It is a common practice to declare objects with the const keyword.

Example

const person = {firstName:"John", lastName:"Doe", age:50, eyeColor:"blue"};
Try it Yourself »


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:

  • Create a single object, using an object literal.
  • Create a single object, with the keyword new.
  • Define an object constructor, and then create objects of the constructed type.
  • Create an object using 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

const person = {firstName:"John", lastName:"Doe", age:50, eyeColor:"blue"};
Try it Yourself »

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

Example

const person = {
  firstName: "John",
  lastName: "Doe",
  age: 50,
  eyeColor: "blue"
};
Try it Yourself »

This example creates an empty JavaScript object, and then adds 4 properties:

Example

const person = {};
person.firstName = "John";
person.lastName = "Doe";
person.age = 50;
person.eyeColor = "blue";
Try it Yourself »

Using the JavaScript Keyword new

The following example create a new JavaScript object using new Object(), and then adds 4 properties:

Example

const person = new Object();
person.firstName = "John";
person.lastName = "Doe";
person.age = 50;
person.eyeColor = "blue";
Try it Yourself »

The examples above do exactly the same.

But there is no need to use new Object().

For readability, simplicity and execution speed, use 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:

const x = person;  // 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

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

const x = person;
x.age = 10;      // Will change both x.age and person.age
Try it Yourself »