What is hoisting?

Hoisting describes the fact that declarations are processed before any code runs. During compilation the engine registers every declaration in its scope, so a name exists from the start of the block even though the line declaring it comes later. Nothing physically moves — what differs is whether the binding is initialised yet.

var is registered and initialised to undefined. let, const and class are registered but left uninitialised: the stretch between the top of the scope and the declaration is the temporal dead zone, and touching the name there throws.

console.log(a);   // undefined — declared, not yet assigned
var a = 1;

console.log(b);   // ReferenceError: Cannot access 'b' before initialization
let b = 2;

Function declarations are the exception: both the name and the body are ready before execution, which is why you can call one above its definition. A function expression is only as hoisted as the variable holding it:

greet();          // works
function greet() { return "hi"; }

speak();          // TypeError: speak is not a function
var speak = function () {};

Why it matters

The TDZ is a feature, not an annoyance — it turns a silent undefined into a loud error at the exact line of the mistake. Declaring variables with const by default, close to where they are used, means hoisting rarely surprises you.