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3.3 JS 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";

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.

Object Definition

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

Example 1: JS Objects

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 2: more JS Objects

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

objectName["propertyName"]

Example 3: JS Objects part 4

person.lastName;

Example 4: JS Objects part 5

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.

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.
You will learn more about this in a later chapter.

Accessing Object Methods

You access an object method with the following syntax:

objectName.methodName()

Example 5: JS Objects part 6

name = person.fullName();

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

Example 6: JS Objects part 7

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.

JavaScript Fundamentals course home
Module 3. Data Types, Functions, Objects and Events

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