Hoisting

Variable Hoisting with var

Variables declared with var are hoisted to the top of their scope. Only the declaration is hoisted, not the assignment.

console.log(message); // undefined (not an error)
var message = "Hello!";
console.log(message); // Hello!

The interpreter sees the code above as:

var message;
console.log(message); // undefined
message = "Hello!";
console.log(message); // Hello!

Hoisting of let and const

let and const are hoisted but not initialized. Accessing them before declaration throws a ReferenceError due to the temporal dead zone.

// console.log(myLet); // ReferenceError
let myLet = 5;
console.log(myLet); // 5

// console.log(myConst); // ReferenceError
const myConst = 10;
console.log(myConst); // 10

Function Declaration Hoisting

Function declarations are fully hoisted — both the name and the body. You can call them before they appear in the code.

console.log(add(2, 3)); // 5

function add(a, b) {
  return a + b;
}

Function Expressions Are Not Hoisted

Function expressions assigned to variables follow the hoisting rules of the variable keyword used.

// console.log(subtract(5, 2)); // TypeError or ReferenceError
const subtract = function(a, b) {
  return a - b;
};
console.log(subtract(5, 2)); // 3