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JavaScript Basics

JavaScript Variables

Last Updated : 29 Jan, 2025

Variables in JavaScript can be declared using var, let, or const. JavaScript is dynamically typed, so variable types are determined at runtime without explicit type definitions.

JavaScript

var a = 10     // Old style let b = 20;    // Prferred for non-const const c = 30;  // Preferred for const (cannot be changed)  console.log(a); console.log(b); console.log(c);

Output

10 20 30

Declaring Variables in JavaScript

1. JavaScript 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. JavaScript 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. JavaScript 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

Variable Shadowing in JavaScript

Variable shadowing occurs when a variable declared within a certain scope (e.g., a function or block) has the same name as a variable in an outer scope. The inner variable overrides the outer variable within its scope.

JavaScript

let n = 10; // Global scope  function gfg() {     let n = 20;  // Shadows the global 'n' inside this function     console.log(n);  // Output: 20 }  gfg(); console.log(n);  // Output: 10 (global 'n' remains unchanged)

Output

20 10

The inner n shadows the outer n in its scope.
The outer n is still accessible outside the function.

To read more about this follow the Article- Variable Shadowing in JavaScript

Variable Scope in JavaScript

Scope determines the accessibility of variables in your code. JavaScript supports the following types of scope

1. Global Scope

Variables declared outside any function or block are globally scoped. While var, let, and const can all have global scope when declared outside a function, their behavior differs:

var is added to the window object in browsers.
let and const do not attach to the window object, making them safer for modern usage.

JavaScript

var globalVar = "I am global"; let globalLet = "I am also global"; const globalConst = "I am global too";

2. Function Scope

Variables declared inside a function are accessible only within that function. This applies to var, let, and const:

JavaScript

function test() {     var localVar = "I am local";     let localLet = "I am also local";     const localConst = "I am local too"; } console.log(localVar); // Error: not defined

3. Block Scope

Variables declared with let or const inside a block (e.g., inside {}) are block-scoped, meaning they cannot be accessed outside the block. var, however, is not block-scoped and will leak outside the block.

JavaScript

{     let blockVar = "I am block-scoped";     const blockConst = "I am block-scoped too"; } console.log(blockVar); // Error: not defined

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.

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);

Output

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

Output:

Hangup (SIGHUP)
/home/guest/sandbox/Solution.js:5
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

Output

SyntaxError: Identifier 'y' has already been declared

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] */

Output

[ 10, 20, 40 ]

When to Use var, let, or const

We declare variables using const if the value should not be changed
We should use let if we want mutable value or we can not use const
We use var only if we support old browser.

To learn more about the scope of variables refer to this article Understanding variable scopes in JavaScript

Comparison of properties of let, var, and const keywords in JavaScript:

Property	var	let	const
Scope	Function scoped	Block scoped	Block scoped
Updation	Mutable	Mutable	Immutable
Redeclaration	Can be redeclared	Cannot be redeclared	Cannot be redeclared
Hoisting	Hoisted at top	Hoisted at top	Hoisted at top
Origins	Pre ES2015	ES2015(ES6)	ES2015(ES6)
Support	Supported in the old version of Browser	Not supported in the old version of the Browser	Not supported in the old version of the Browser

JavaScript Basics

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