What is the difference between var and const?

var is declared globally or at the function level, can be overridden and reassigned. const is block-scoped, cannot be reassigned, but for objects and arrays, their contents can be modified.

Characteristic	var	const
Scope	Function or global	Block ({})
Reassignment	Yes	No
Redeclaration	Yes	No in the same scope
Initialization	Optional	Mandatory at declaration
Hoisting	Declaration and initialization (with undefined) are hoisted	Hoisted but without initialization (TDZ)

// var
var x = 10;
console.log(x); // 10
x = 20;
console.log(x); // 20
var x = 30; // Redeclaration is possible
console.log(x); // 30

function testVar() {
  var y = 5;
  console.log(y); // 5
}
testVar();
// console.log(y); // ReferenceError: y is not defined

if (true) {
  var z = 100;
}
console.log(z); // 100 (var does not have block scope)

// const
const a = 10;
console.log(a); // 10
// a = 20; // TypeError: Assignment to constant variable.

const obj = { name: 'Alice' };
console.log(obj.name); // Alice
obj.name = 'Bob'; // The contents of a const object can be changed
console.log(obj.name); // Bob
// obj = { name: 'Charlie' }; // TypeError: Assignment to constant variable.

if (true) {
  const b = 200;
  console.log(b); // 200
}
// console.log(b); // ReferenceError: b is not defined (const has block scope)

// Hoisting
console.log(hoistedVar); // undefined
var hoistedVar = 1;

// console.log(hoistedConst); // ReferenceError: Cannot access 'hoistedConst' before initialization (TDZ)
const hoistedConst = 2;