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JS Tutorial

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JS Objects

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JS Functions

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JS Classes

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JS HTML DOM

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JS Examples

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JS References

JavaScript Objects HTML DOM Objects


JavaScript Objects


Real Life Objects

In real life, objects are things like: houses, cars, people, animals, or any other subjects.

Here is a car object example:

Car Object Properties Methods

car.name = Fiat

car.model = 500

car.weight = 850kg

car.color = white

car.start()

car.drive()

car.brake()

car.stop()

Object Properties

A real life car has properties like weight and color:

car.name = Fiat, car.model = 500, car.weight = 850kg, car.color = white.

Car objects have the same properties, but the values differ from car to car.


Object Methods

A real life car has methods like start and stop:

car.start(), car.drive(), car.brake(), car.stop().

Car objects have the same methods, but the methods are performed at different times.


JavaScript Variables

JavaScript variables are containers for data values.

This code assigns a simple value (Fiat) to a variable named car:

Example

let car = "Fiat";
Try it Yourself »

JavaScript Objects

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

This code assigns many values (Fiat, 500, white) to an object named car:

Example

const car = {type:"Fiat", model:"500", color:"white"};
Try it Yourself »

Note:

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

Learn more about using const with objects in the chapter: JS Const.


JavaScript Object Definition

How to Define a JavaScript Object

  • Using an Object Literal
  • Using the new Keyword
  • Using an Object Constructor

JavaScript Object Literal

An object literal is a list of name:value pairs inside curly braces {}.

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

Note:

name:value pairs are also called key:value pairs.

object literals are also called object initializers.


Creating a JavaScript Object

These examples create a JavaScript object with 4 properties:

Examples

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

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

// Create an Object
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:

// Create an Object
const person = {};

// Add Properties
person.firstName = "John";
person.lastName = "Doe";
person.age = 50;
person.eyeColor = "blue";
Try it Yourself »

Using the new Keyword

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

Example

// Create an Object
const person = new Object();

// Add Properties
person.firstName = "John";
person.lastName = "Doe";
person.age = 50;
person.eyeColor = "blue";
Try it Yourself »

Note:

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.



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

Accessing Object Properties

You can access object properties in two ways:

objectName.propertyName
objectName["propertyName"]

Examples

person.lastName;
Try it Yourself »
person["lastName"];
Try it Yourself »

JavaScript Object Methods

Methods are actions that can be performed on objects.

Methods are function definitions stored as property values.

Property Property Value
firstName John
lastName Doe
age 50
eyeColor blue
fullName function() {return this.firstName + " " + this.lastName;}

Example

const person = {
  firstName: "John",
  lastName : "Doe",
  id       : 5566,
  fullName : function() {
    return this.firstName + " " + this.lastName;
  }
};
Try it Yourself »

In the example above, this refers to the person object:

this.firstName means the firstName property of person.

this.lastName means the lastName property of person.


In JavaScript, Objects are King.

If you Understand Objects, you Understand JavaScript.

Objects are containers for Properties and Methods.

Properties are named Values.

Methods are Functions stored as Properties.

Properties can be primitive values, functions, or even other objects.

In JavaScript, almost "everything" is an object.

  • Objects are objects
  • Maths are objects
  • Functions are objects
  • Dates are objects
  • Arrays are objects
  • Maps are objects
  • Sets are 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:

  • 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

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;

The object x is not a copy of person. The object x is person.

The object x and the object person share the same memory address.

Any changes to x will also change person:

Example

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

// Create a copy
const x = person;

// Change Age in both
x.age = 10;
Try it Yourself »

Note:

You will learn a lot more about objects in the following chapters.


Test Yourself With Exercises

Exercise:

Alert "John" by extracting information from the person object.

const person = {
  firstName: "John",
  lastName: "Doe"
};

alert();

Start the Exercise


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