Differences between JavaScript variables created using let, var or const

var is the old way of creating variables, while let and const are the modern, safer ways.

• let: can be reassigned.
• const: cannot be reassigned.
• var: works differently and often leads to bugs.

Differences in behavior

Scope

• var: function-scoped.
• let & const: block-scoped.

Hoisting

• var: hoisted, initialized as undefined.
• let & const: hoisted but in the Temporal Dead Zone (TDZ) → error if accessed early.

Re-declaration & Re-assignment

• var: can redeclare & reassign.
• let: can reassign but not redeclare.
• const: neither redeclare nor reassign (but object/array contents are mutable).

Global binding

• var attaches to window in browsers.
• let & const do not.

// Scope
if (true) {
  var a = 1; // function-scoped
  let b = 2; // block-scoped
  const c = 3; // block-scoped
}
console.log(a); // 1
// console.log(b); // Error
// console.log(c); // Error

// Hoisting
console.log(x); // undefined
var x = 10;
// console.log(y); // ReferenceError (TDZ)
let y = 20;

// Re-declaration & Re-assignment
var m = 1; var m = 2; // allowed
let n = 3; // let n = 4; // Error
const p = 5; // p = 6; // Error

// const with objects
const obj = { name: "John" };
obj.name = "Mike"; // allowed (mutating object property)