Introduction to JavaScript

Undefined

What is Undefined

Undefined is also a data type in JavaScript just like the null. When a variable is created or declared without assigning any value to it, then the value of the variable will be set as undefined by default.

let age;

console.log(age);


              12
            
let age;
console.log(age);

Task

Swipe to start coding

Declare a variable numberOfItems and assign the value of undefined to the variable and show the value of the variable.

Solution

let numberOfItems = undefined;

console.log(numberOfItems)

So, undefined variables can also be declared using the undefined keyword.

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Section 3. Chapter 31

Introduction to JavaScript

1. Introduction

Writing our first code on JavaScript.

Writing our first code

Output Using Variables 1/2

Output Using Variables 2/2

2. JavaScript syntax

The syntax of the JavaScript programming language is the set of rules that defines how a JavaScript program will be written and interpreted (by both the runtime system and by human readers).

JavaScript Syntax

JavaScript Variables

JavaScript Operators

Identifiers in JavaScript

Naming Identifiers

Reserved keywords in JavaScript

Naming Convention in JavaScript. Case Styles

Variables Naming Convention

Boolean Naming Convention

Constant Naming Convention

Function & Classes Naming Convention

Multiline Strings

Multiline Strings. Concatenating

Commenting in JavaScript

Commenting in JavaScript. Multi-line Comments

3. Data Types and Variables

JavaScript is dynamically typed (also called loosely typed) scripting language. That is, in javascript variables can receive different data types over time. Datatypes are basically typed of data that can be used and manipulated in a program.