What are properties and methods in javascript

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.

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

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

A JavaScript object is a collection of named values

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

Example

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()

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

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

Example

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»;

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»;

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:

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

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JavaScript Objects

A car has properties like weight and color, and methods like start and stop:

All cars have the same properties, but the property values differ from car to car.

All cars have the same methods, but the methods are performed at different times.

JavaScript Objects

You have already learned that JavaScript variables are containers for data values.

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

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

This code assigns many values (Fiat, 500, white) to a variable named car:

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

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

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

Object Definition

You define (and create) a JavaScript object with an object literal:

Example

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

Example

Object Properties

The name:values pairs in JavaScript objects are called properties:

Property	Property Value
firstName	John
lastName	Doe
age	50
eyeColor	blue

Accessing Object Properties

You can access object properties in two ways:

Example1

Example2

JavaScript objects are containers for named values called properties.

Object Methods

Objects can also have methods.

Methods are actions that can be performed on objects.

Methods are stored in properties as function definitions.

Property	Property Value
firstName	John
lastName	Doe
age	50
eyeColor	blue
fullName	function()

A method is a function stored as a property.

Example

const person = <
firstName: «John»,
lastName : «Doe»,
id : 5566,
fullName : function() <
return this.firstName + » » + this.lastName;
>
>;

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

I.E. this.firstName means the firstName property of this.

I.E. this.firstName means the firstName property of person.

What is this?

In JavaScript, the this keyword refers to an object.

Which object depends on how this is being invoked (used or called).

The this keyword refers to different objects depending on how it is used:

In an object method, this refers to the object.

Alone, this refers to the global object.

In a function, this refers to the global object.

In a function, in strict mode, this is undefined .

In an event, this refers to the element that received the event.

Methods like call() , apply() , and bind() can refer this to any object.

Note

See Also:

The this Keyword

In a function definition, this refers to the «owner» of the function.

In the example above, this is the person object that «owns» the fullName function.

In other words, this.firstName means the firstName property of this object.

Accessing Object Methods

You access an object method with the following syntax:

Example

If you access a method without the () parentheses, it will return the function definition:

Example

Do Not Declare Strings, Numbers, and Booleans as Objects!

When a JavaScript variable is declared with the keyword » new «, the variable is created as an object:

x = new String(); // Declares x as a String object
y = new Number(); // Declares y as a Number object
z = new Boolean(); // Declares z as a Boolean object

Avoid String , Number , and Boolean objects. They complicate your code and slow down execution speed.

You will learn more about objects later in this tutorial.

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