Javascript Variables

When to Use let, const, in JavaScript

JavaScript let

let – Block Scope, Local Scope, and Redeclaration

let is used to declare variables that are block-scoped, meaning they are only available within the block ({}) they are declared in. It’s commonly used in loops, conditionals, and function blocks because the variable declared with let can be updated but not redeclared.

Block Scope:

if (true) {

let name = “John”;

console.log(name); // Works within the block

}

console.log(name); // Error: name is not defined

Local Scope: Variables declared inside a function using let are local to that function.

function greet() {

let greeting = “Hello!”;

console.log(greeting);

}

greet(); // Outputs: Hello!

console.log(greeting); // Error: greeting is not defined

Redeclaration: Variables declared with let cannot be redeclared within the same scope.

let age = 25;

let age = 30; // Error: Identifier ‘age’ has already been declared

JavaScript const

const – Constant Objects and Arrays

const is used for variables that should not be reassigned. While const values themselves cannot be changed, you can still modify the properties of objects or arrays declared with const.

Constant Objects

const person = { name: “John”, age: 30 };

person.age = 31; // This works, properties can change

person = { name: “Jane” }; // Error: Assignment to constant variable

Constant Arrays

const numbers = [1, 2, 3]; numbers.push(4); // This works, elements can be modified numbers = [5, 6, 7]; // Error: Assignment to constant variable

Difference Between var, let, and const

Feature	Var	let	const
Scope	Function/Global	Block	Block
Redeclaration	Allowed	Not Allowed	Not Allowed
Reassignment	Allowed	Allowed	Not Allowed
Hoisting	Yes (with undefined)	No (not initialized)	No (not initialized)

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Javascript Variables