What is a Variable in JavaScript

What is a Variable in JavaScript?

A variable in JavaScript serves as a storage unit for data values. It allows you to save, update, and reuse information in your code. Variables are fundamental to programming, enabling dynamic behavior by holding values that can change during execution.

Declaring Variables in JavaScript

JavaScript provides three main ways to declare variables:

var – Old way (not recommended for modern code).
let – Declares variables that can be updated or reassigned.
const – Used for variables that should not be reassigned (constant values).

Syntax

let name = "John";    // Defines a variable using 'let'
const age = 25;       // Declares a constant with 'const'
var city = "Paris";   // Old way, avoid if possible

How Variables Work

A variable is declared with a name (identifier) and can store values such as numbers, strings, objects, or functions.
The value of the variable can be updated (if declared with let or var).

Example:

let score = 10;           // Declare and initialize
score = 20;               // Update value
const pi = 3.14;          // Declare constant (cannot change)

Why Use Variables?

Store and Manage Data – Hold information that can be used and manipulated later.
Reusability – Avoid repeating the same values by referencing variables.
Dynamic Behavior – Allow your program to change and respond to user input or calculations.
Readable Code – Use meaningful names to make the code clearer and easier to understand.

When to Use Variables?

Store User Input

let userName = prompt("Enter your name:");
alert("Hello, " + userName);

Perform Calculations

let price = 100;
let tax = 0.2;
let total = price + (price * tax);
console.log(total);

Track State or Changes

let clicks = 0;
function incrementClick() {
    clicks += 1;
    console.log(clicks);
}

Variable Naming Rules (Identifiers)

Must start with a letter, $, or _.
Cannot start with a number.
Cannot use reserved JavaScript keywords (like let, const, function).
Case-sensitive – myVar and myvar are different.

Clear Naming – Choose meaningful names to enhance readability.

let userAge = 30;     // Good
let x = 30;           // Avoid single-letter names (unless temporary)

Examples

Basic Variable Declaration and Update

let message = "Welcome beginners!";  // Declare and assign
console.log(message);      // Output: Welcome beginners!

message = "Hiii!";        // Update value
console.log(message);      // Output: Hiii!

Using const

const birthYear = 2000;
console.log(birthYear);

// birthYear = 1999;  // Error! Cannot reassign a 'const' variable

Difference Between `var`, `let`, and `const`

Feature	`var`	`let`	`const`
Scope	Function-scoped	Block-scoped	Block-scoped
Re-declaration	Allowed	Not allowed	Not allowed
Reassignment	Allowed	Allowed	Not allowed
Hoisting	Yes (initialized as `undefined`)	Yes (but not initialized)	Yes (but not initialized)

Best Practices for Variables

Use const by default, and let only when the value needs to change.
Avoid using var to prevent scope-related bugs.
Choose descriptive and meaningful variable names.
Keep variables scoped as narrowly as possible to avoid conflicts.

What is a Variable in JavaScript