JavaScript Variables

Last Updated : 15 Jan, 2026

Variables in JavaScript are used to store data values. They can be declared in different ways depending on how the value should behave.

  • Variables can be declared using var, let, or const.
  • JavaScript is dynamically typed, so types are decided at runtime.
  • You don’t need to specify a data type when creating a variable.
JavaScript 
// Old style
var a = 10    

// Prferred for non-const
let b = 20;    

// Preferred for const (cannot be changed)
const c = 30;  

console.log(a);
console.log(b);
console.log(c);

Declaring Variables in JavaScript

  • Before ES6 (2015): Variables were declared only with var, which is function-scoped and global-scoped, causing issues like hoisting and global pollution.
  • ES6 Introduction:let and const were introduced to provide safer alternatives for declaring variables.
  • Scope: let and const are block-scoped (limited to { } block) or global-scoped, reducing errors compared to var.

1. var keyword

var is a keyword in JavaScript used to declare variables and it is Function-scoped and hoisted, allowing redeclaration but can lead to unexpected bugs.

JavaScript 
var a = "Hello Geeks";
var b = 10;
console.log(a);
console.log(b);

2. let keyword

let is a keyword in JavaScript used to declare variables and it is Block-scoped and not hoisted to the top, suitable for mutable variables

JavaScript 
let a = 12
let b = "gfg";
console.log(a);
console.log(b);

3. const keyword

const is a keyword in JavaScript used to declare variables and it is Block-scoped, immutable bindings that can't be reassigned, though objects can still be mutated.

JavaScript 
const a = 5
let b = "gfg";
console.log(a);
console.log(b);

Rules for Naming Variables

When naming variables in JavaScript, follow these rules

  • Variable names must begin with a letter, underscore (_), or dollar sign ($).
  • Subsequent characters can be letters, numbers, underscores, or dollar signs.
  • Variable names are case-sensitive (e.g., age and Age are different variables).
  • Reserved keywords (like function, class, return, etc.) cannot be used as variable names.
JavaScript 
let userName = "Suman";  // Valid
let $price = 100;         // Valid
let _temp = 0;            // Valid
let 123name = "Ajay";    // Invalid
let function = "gfg"; // Invalid

Interesting Facts about Variables in JavaScript

1. let or const are preferred over var: Initially, all the variables in JavaScript were written using the var keyword but in ES6 the keywords let and const were introduced. The main issue with var is, scoping.

JavaScript 
if (true) {
    var x = 10;
    let y = 20;
}

console.log(x);  // 10 (var is function-scoped)
console.log(y);  // Error (let is block-scoped)


2. var is function scoped: Can be accessed outside block if within the function.

JavaScript 
if (true) {
  var x = 10; 
}

// Accessible outside the block
// because we are in same function
console.log(x);

3. let and const are block scoped : Cannot be accessed outside block even if inside the same function

JavaScript 
if (true) {
  let y = 20;
  const z = 30;
}
console.log(y, z); // ReferenceError

^

4. var can be redeclared in the same scope, but let and const cannot be

JavaScript 
var x = 10;
var x = 20; // Allowed

let y = 30;
let y = 40; // SyntaxError

const z = 50;
const z = 60; // SyntaxError

5. We can change elements of array or objects even if declared as const.

JavaScript 
const ob = { a: 10 };
ob.a = 20; // Allowed

const arr = [10, 20, 30]
arr[2] = 40
console.log(arr)  // Allowed

/* TypeError in the below lines
obj = { b: 30 }; 
arr = [50, 100] */
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