Skip to content

02 - Variables and Keywords in JavaScript.md

Latest commit

 

History

History
263 lines (168 loc) · 4.16 KB

02 - Variables and Keywords in JavaScript.md

File metadata and controls

263 lines (168 loc) · 4.16 KB
Raw

Variables and Keywords in JavaScript

Introduction

JavaScript is a Dynamically Typed Language.

This means:

  • The programmer does not need to specify the data type while declaring a variable.
  • The datatype is assigned automatically at runtime based on the value stored in the variable.

Example:

let age = 25;          // Number
let name = "Basha";    // String
let status = true;     // Boolean

Keywords Used to Declare Variables

JavaScript provides three keywords for declaring variables:

  1. var
  2. let
  3. const

1. var Keyword

The var keyword is used to declare variables in JavaScript.

Features of var

  • Function Scoped
  • Can be re-declared
  • Can be re-assigned
  • Does not support block scope
  • Older way of declaring variables

Example

{
    var firstName = "Apple";
    console.log(firstName);

    var firstName = "Banana";
    console.log(firstName);
}

console.log(firstName);

Output

Apple
Banana
Banana

Explanation

  • var allows re-declaration.
  • Variables declared with var inside a block are accessible outside the block.
  • Therefore, console.log(firstName) works outside the braces.

2. let Keyword

The let keyword is used to declare block-scoped variables.

Features of let

  • Block Scoped
  • Cannot be re-declared in the same scope
  • Can be re-assigned
  • Introduced in ES6 (ECMAScript 2015)

Example

{
    let age = 26;

    console.log(age);

    age = 30;

    console.log(age);
}

// console.log(age); // Error

Output

26
30
ReferenceError: age is not defined

Explanation

  • let variables exist only inside the block.
  • They can be reassigned.
  • They cannot be accessed outside the block.

3. const Keyword

The const keyword is used to declare constants.

Features of const

  • Block Scoped
  • Must be initialized while declaration
  • Cannot be re-assigned
  • Cannot be re-declared

Example

{
    const isMarried = true;

    console.log(isMarried);

    // isMarried = false; // Error
}

// console.log(isMarried); // Error

Output

true

TypeError: Assignment to constant variable.
ReferenceError: isMarried is not defined

Explanation

  • A const variable cannot be reassigned.
  • It is accessible only inside the block.
  • Attempting to change its value causes an error.

HTML File

Variables.html

<!DOCTYPE html>
<html lang="en">

<head>
    <title>Variables in JavaScript</title>

    <script src="demo.js"></script>
</head>

<body>

</body>

</html>

JavaScript File

demo.js

// var keyword

{
    var firstName = "Apple";

    console.log(firstName);

    var firstName = "Banana";

    console.log(firstName);
}

console.log(firstName);


// let keyword

{
    let age = 26;

    console.log(age);

    age = 30;

    console.log(age);
}

// console.log(age);


// const keyword

{
    const isMarried = true;

    console.log(isMarried);

    // isMarried = false;
}

// console.log(isMarried);

Difference Between var, let and const

Feature var let const
Scope Function Scope Block Scope Block Scope
Re-declare Yes No No
Re-assign Yes Yes No
Hoisting Yes Yes (TDZ) Yes (TDZ)
Introduced In ES5 ES6 ES6

Important Notes

  1. JavaScript is a Dynamically Typed Language.
  2. var is function scoped.
  3. let is block scoped.
  4. const is block scoped and immutable.
  5. Modern JavaScript development prefers let and const.
  6. Use const by default and let only when reassignment is required.

Summary

JavaScript variables are declared using var, let, and const.

  • var → Old way, function scoped.
  • let → Block scoped and re-assignable.
  • const → Block scoped and cannot be re-assigned.

For modern JavaScript development, let and const are recommended because they provide better scope management and help avoid bugs.