How do let, const and var differ?

let lets you assign a different value to the variable later. const prevents reassigning that variable. Both declarations have block scope, e.g. the inside of an if. The older var has function scope and different rules for access before its declaration, which makes unexpected behaviour easier.

const doesn't make the whole object immutable. With const user = {name: 'Ala'} I can change user.name, although I can't assign a completely different object to user. Day to day I choose const when I don't need reassignment and let when I do.