JS Variables

Variables store data values in JavaScript. Understand how to declare and use variables effectively.

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Declaring Variables

In JavaScript, variables are declared using let, const, or (legacy) var.

let age = 25;
const siteName = "W4";
var oldStyle = "legacy";

let is used for variables that can change. It is block-scoped, meaning it only exists inside the block where it is defined.

let count = 0;
count = count + 1;

if (true) {
  let message = "Inside block";
  console.log(message);
}

const is used for variables that should not be reassigned. It must be initialized when declared.

const pi = 3.14159;
// pi = 3; // Error

Note: const prevents reassignment, but objects and arrays declared with const can still be modified internally.

const user = { name: "John" };
user.name = "Jane"; // Allowed

var is function-scoped and can lead to unexpected behavior. It is recommended to use let or const in modern JavaScript.

var x = 10;

if (true) {
  var x = 20;
}

console.log(x); // 20

Variables declared with let and const are limited to the block in which they are defined.

if (true) {
  let value = 100;
}
// console.log(value); // Error

Use const by default. Use let only when reassignment is required. Avoid var in modern code.

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