JavaScript Objects

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JavaScript Objects


Real Life Objects, Properties, and Methods
In real life, a car is an object.
A car has properties like weight and color, and methods like start and stop:




Object


Properties


Methods







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


car.start()
car.drive()
car.brake()
car.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:


let car = "Fiat";

Try it Yourself »

Objects are variables too. But objects can contain many
values.
This code assigns many values (Fiat, 500, white) to a
variable
named car:


const car = {type:"Fiat", model:"500", color:"white"};

Try it Yourself »

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

const 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:

Example

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

Try it Yourself »


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:


objectName.propertyName

or


objectName["propertyName"]



Example1

person.lastName;

Try it Yourself »


Example2

person["lastName"];

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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() {return this.firstName + " " + this.lastName;}



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
this is not a variable. It is a keyword. You cannot change the value of this.


See Also:
The JavaScript this Tutorial


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.
Learn more about this in The JavaScript this Tutorial.

Accessing Object Methods
You access an object method with the following syntax:


objectName.methodName()


Example

name = person.fullName();
Try it Yourself »

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

Example

name = person.fullName;
Try it Yourself »


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.



Test Yourself With Exercises

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


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

alert();



Submit Answer »
Start the Exercise














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Reference: https://www.w3schools.com/js/js_objects.asp