Explain hoisting

// Usage
// Hoisting: declarations are moved to the top of their scope
// during the compile phase. Only the declaration is hoisted,
// not the initialisation.

// var — hoisted and initialised to undefined
console.log(x) // undefined (no error)
var x = 5
console.log(x) // 5

// let/const — hoisted but NOT initialised (Temporal Dead Zone)
console.log(y) // ReferenceError: Cannot access 'y' before init
let y = 10

// Function declarations — fully hoisted (name + body)
greet() // "Hello!" — works before the declaration
function greet() { console.log('Hello!') }

// Function expressions — only the variable is hoisted
sayHi() // TypeError: sayHi is not a function
var sayHi = function() { console.log('Hi!') }

// Key rule: prefer const/let and declare before use.