What is the difference between const, let, and var?

var, let, and const are three ways to declare variables in JavaScript, differing in scope and reassignability.

• var declares a variable with function or global scope. Variables can be reassigned and redeclared. It is subject to hoisting.

• let has block scope (e.g., inside {}). The variable can be reassigned but not redeclared in the same scope. It is not subject to hoisting.

• const also has block scope, but the variable must be initialized immediately and cannot be reassigned. However, if it is an object or array, its contents can be modified.

Example:

var a = 1;
let b = 2;
const c = 3;

b = 4; // OK
// c = 5; // Error

if (true) {
  var a = 10; // will overwrite the outer a
  let b = 20; // new variable within the block
}
console.log(a); // 10
console.log(b); // 4