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JavaScript Variables (var, let, const)

Variables are used to store data in JavaScript. Think of a variable as a container where you put a value and use it later in your code. JavaScript allows you to create variables using three keywords:

  • var
  • let
  • const

Each one behaves differently, so it’s important to understand how they work.

1. var – The Old Way

var was the original way to declare variables in JavaScript.
However, it has some issues, so modern JavaScript recommends using let or const instead

Features of var
  • Can be redeclared.
  • Can be updated.
  • Has function scope (NOT block scope).
  • Gets hoisted (moves to top of scope automatically).
Example
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var name = "John";
console.log(name); // John

var name = "David"; // Redeclaration allowed
console.log(name); // David
Why you should avoid var
  • It can cause bugs because of hoisting.
  • Variables leak outside block {} — not safe.

2. let – The Modern, Safe Variable

let is the recommended way to declare variables that can change later.

Features of let
  • Cannot be redeclared in the same scope.
  • Can be updated.
  • Has block scope (safer).
  • Hoisted but not initialized.
Example
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let age = 25;
age = 26; // allowed
console.log(age); // 26

let age = 30; 
// ❌ Error: cannot redeclare 'age'
Block Scope Example
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if (true) {
    let x = 10;
}
console.log(x); // ❌ Error: x is not defined

3. const – For Values That Should Not Change

const is used for variables that should stay constant (unchanged).

Features of const
  • Must be assigned a value immediately.
  • Cannot be updated.
  • Cannot be redeclared.
  • Has block scope.
Example
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const country = "India";
console.log(country); // India

country = "USA"; 
// ❌ Error: Assignment to constant variable

Important Note About const and Objects/Arrays

The variable itself cannot change, but the content inside an object or array can change.

Example with Array
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const numbers = [1, 2, 3];
numbers.push(4); // allowed
console.log(numbers); // [1, 2, 3, 4]
Example with Object
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const user = { name: "learn2kode" };
user.name = "Suraj"; // allowed
console.log(user.name); // Suraj

Quick Comparison Table

Feature var let const
Redeclaration ✔ Allowed ✖ Not allowed ✖ Not allowed
Updating Value ✔ Allowed ✔ Allowed ✖ Not allowed
Scope Type Function scope Block scope Block scope
Hoisting Yes (initialized as undefined) Yes (not initialized) Yes (not initialized)
Recommended? ❌ No ✔ Yes ✔ Yes (for fixed values)

Which One Should You Use?

Use this rule:

  • const → Default choice Use for values that do not need to change.
  • let → Use when the value needs to change.
  • var → Avoid using it in modern JavaScript.
Simple Example to Remember All Three
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var city = "Chennai"; // old way
let score = 10;       // value will change
const pi = 3.14;      // value stays constant

score = 20;          // ✔ allowed
pi = 22;             // ❌ error
JavaScript Syntax
JavaScript Data Types