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

var is function-scoped, hoisted with undefined initialization, can be redeclared and reassigned. let is block-scoped, hoisted but not initialized (temporal dead zone), cannot be redeclared, can be reassigned. const is block-scoped, must be initialized at declaration, cannot be reassigned (but object properties can change). Best practices: use const by default, let when reassignment needed, avoid var. let and const were introduced in ES6 to fix var's scoping issues and provide clearer intent.