What are the differences between var, let, and const in JavaScript?

var vs let vs const

Scope

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

Hoisting

console.log(x); // undefined (var hoisted, not initialized)
var x = 5;

console.log(y); // ReferenceError (Temporal Dead Zone)
let y = 5;

Re-declaration

var a = 1; var a = 2; // ✅ OK
let b = 1; let b = 2; // ❌ SyntaxError
const c = 1; const c = 2; // ❌ SyntaxError

Re-assignment

var a = 1; a = 2;   // ✅ OK
let b = 1; b = 2;   // ✅ OK
const c = 1; c = 2; // ❌ TypeError

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Comparison

Feature	var	let	const
Scope	Function	Block	Block
Hoisting	✅ (undefined)	✅ (TDZ)	✅ (TDZ)
Re-declare	✅	❌	❌
Re-assign	✅	✅	❌

Best practice: Default to const, use let when reassignment is needed, avoid var.