What is the difference between var, let and const?

The three keywords differ in scope, hoisting and reassignment.

• var is function-scoped, hoisted and initialized with undefined, and can be redeclared.
• let is block-scoped. It is hoisted but sits in the temporal dead zone until its declaration, so reading it earlier throws a ReferenceError.
• const is block-scoped like let, but the binding cannot be reassigned. An object held by a const can still be mutated.

for (var i = 0; i < 3; i++) setTimeout(() => console.log(i))
// 3 3 3
for (let j = 0; j < 3; j++) setTimeout(() => console.log(j))
// 0 1 2

In modern code, default to const and use let only when a value must change.