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

JavaScript has three ways to declare variables: var, let, and const. They differ in scope, hoisting behavior, and mutability.

var

• Function-scoped (or globally scoped if declared outside a function)
• Hoisted to the top of its scope and initialized as undefined
• Can be redeclared and reassigned

let

• Block-scoped (limited to the {} block it is declared in)
• Hoisted but NOT initialized — accessing it before declaration throws a ReferenceError (Temporal Dead Zone)
• Cannot be redeclared in the same scope, but can be reassigned

const

• Block-scoped, same as let
• Cannot be redeclared or reassigned — the binding is constant
• Objects and arrays declared with const are still mutable (their contents can change)

Rule of thumb: prefer const by default, use let when you need to reassign, avoid var.