JavaScript Variables: var, let, and const

Explained

In JavaScript, variables are used to store data values. There are three main ways to declare variables:


1. var – Function-scoped, older way

  • Can be re-declared and updated.
  • Function scoped, not block scoped.
  • Gets hoisted (moved to the top of its scope), but not initialized.

Example:

var message =
"Hello";
console.log(message); // Output: Hello
var message =
"Hi again";
console.log(message); // Output: Hi again

2. let – Block-scoped, modern way

  • Can be updated but not re-declared in the same scope.
  • Introduced in ES6 (ES2015).
  • Recommended for reassignable values.

Example:

CopyEdit
let name =
"Alice";
console.log(name); // Output: Alice
name =
"Bob"; //  allowed
console.log(name); // Output: Bob
// let name =
"Charlie";  Error: Identifier 'name' has already been
declared

3. const – Block-scoped, for constants

  • Cannot be updated or re-declared.
  • Must be initialized at the time of declaration.
  • Best for constant values (like config values or fixed settings).

Example:

const PI = 3.14;
console.log(PI); // Output: 3.14
// PI = 3.14159;  Error: Assignment to constant variable
// const PI = 3.14159;  Error: Identifier 'PI' has already been
declared