Chapter 2 — Hoisting

📖 Definition

Hoisting is JavaScript's behavior of moving declarations (not initializations) to the top of their containing scope during the compilation phase, before code executes.

🔍 Explanation

Before any code runs, the JS engine scans the code and registers all declarations:

Temporal Dead Zone (TDZ) — the period between entering the scope and the actual let/const/class declaration line. Accessing the variable in that window throws ReferenceError.

💻 Code Example — var Hoisting

console.log(a); // undefined  (declaration hoisted, value not)
var a = 10;
console.log(a); // 10

// Equivalent to:
// var a;
// console.log(a);
// a = 10;
// console.log(a);

💻 Code Example — let/const and the TDZ

console.log(b); // ❌ ReferenceError: Cannot access 'b' before initialization
let b = 20;

console.log(c); // ❌ ReferenceError
const c = 30;

💻 Code Example — Function Declaration vs Expression

// Function DECLARATION — fully hoisted
sayHi();                       // "Hi!"
function sayHi() { console.log("Hi!"); }

// Function EXPRESSION assigned to var — only var is hoisted
sayBye();                      // ❌ TypeError: sayBye is not a function
var sayBye = function () { console.log("Bye!"); };

// Function EXPRESSION assigned to let — TDZ
sayHello();                    // ❌ ReferenceError
let sayHello = function () { console.log("Hello!"); };

💻 Code Example — Arrow Functions

// Arrow functions are NOT hoisted like declarations
greet();                       // ❌ ReferenceError
const greet = () => console.log("Hello");

💻 Code Example — Hoisting Order

function test() {
  console.log(x);              // [Function: x]
  var x = 10;
  function x() {}              // function declarations win the initial value
  console.log(x);              // 10
}
test();

🌍 Real-World Impact

• Always declare with const/let to avoid var hoisting surprises.
• Place function declarations near the top of files for readability — hoisting masks bad ordering.
• Linters (ESLint no-use-before-define) catch most hoisting bugs.

🎯 Likely Interview Questions

1. What is hoisting?
2. What is the Temporal Dead Zone?
3. Are arrow functions hoisted? — No. They follow variable hoisting rules.
4. What's the difference between function declarations and function expressions in terms of hoisting?
5. Output prediction:

   var a = 1;
   function foo() {
     console.log(a);
     var a = 2;
   }
   foo(); // undefined — local `a` is hoisted and shadows the outer

← Closures | Index | Next: Scope →