Variables are one of the most important building blocks of JavaScript. They store data that you can use and modify throughout your program.
In this guide, you’ll learn what variables are, how they work, and when to use var, let, and const.
⭐ What Are Variables in JavaScript?
A variable is a container that holds a value.
You can store numbers, text, objects, arrays, and more.
Example
let name = "Sagar";
console.log(name); // Output: Sagar
⭐ Types of Variables in JavaScript
JavaScript provides three keywords to declare variables:
✔ var
✔ let
✔ const
Each works differently.
⭐ 1. var – Function Scoped (Old Way)
var was the original way to declare variables in JavaScript.
✔ Function-scoped
✔ Allows redeclaration
✔ Should be avoided in modern JavaScript
Example
var x = 5;
var x = 10; // Allowed
console.log(x); // Output: 10
Problem:
var ignores block scope.
if (true) {
var a = 100;
}
console.log(a); // Works — NOT GOOD!
⭐ 2. let – Block Scoped (Modern Way)
Introduced in ES6, let is the preferred way to declare variables.
✔ Block-scoped
✔ Cannot be redeclared in the same block
✔ Can be updated
Example
let count = 1;
count = 2; // Allowed
console.log(count);
Block scoping:
if (true) {
let y = 50;
}
console.log(y); // Error: y is not defined
⭐ 3. const – Block Scoped & Constant
const is used when the value should NOT change.
✔ Block-scoped
✔ Must be initialized
✔ Cannot be reassigned
Example
const PI = 3.14;
PI = 3.15; // ❌ Error: Assignment not allowed
Important:const prevents reassignment, but not mutation of objects.
const user = {name: "Sam"};
user.name = "John"; // ✔ Allowed
⭐ Variable Naming Rules
You can use:
- letters
- digits
_underscore$dollar sign
You cannot:
- start with a number
- use reserved keywords (like
let,class,return)
Valid:
let userName;
let $price;
let _totalAmount;
Invalid:
let 1name; // ❌
let let; // ❌
⭐ Dynamic Typing in JavaScript
JavaScript is dynamically typed, meaning variable types can change at runtime.
Example
let data = 10; // number
data = "Hello"; // now a string
⭐ Which One Should You Use?
| Keyword | Scope | Redeclare | Reassign | Best Use |
|---|---|---|---|---|
| var | function | yes | yes | Avoid using |
| let | block | no | yes | General variables |
| const | block | no | no | Constants & objects |
Recommended:
✔ Use const by default
✔ Use let when you need to change the value
✔ Avoid var
⭐ Conclusion
Variables allow JavaScript programs to store and manipulate data. Understanding the differences between var, let, and const helps you write clean, modern, and bug-free code.
📌 Citations
🔗 View other articles about Javascript:
https://savanka.com/category/learn/js/
🔗 External Javascript Documentation: