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JavaScript Objects

In real life, a car is an object

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";

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"};

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"};

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"
};

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;

Example2

person["lastName"];

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();

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

Example

name = person.fullName;

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();

Start the Exercise

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