What is hoisting in JavaScript?

The engine does not physically move your code. Before executing a scope, it records the declarations it contains. What each declaration is initialised to depends on the kind of declaration.

console.log(hoistedVar); // undefined
var hoistedVar = 1;

sayHi(); // "hi"
function sayHi() {
  console.log("hi");
}

console.log(notYet); // ReferenceError
let notYet = 2;

What each declaration gets

• var: hoisted and initialised to undefined
• function declaration: hoisted and fully callable before its line
• let and const: hoisted but uninitialised, so accessing them throws during the temporal dead zone
• class: behaves like let; accessing it before the declaration throws

Function expressions follow the variable rules, not the function-declaration rules. A var holding a function expression is undefined until the assignment runs.

sayHi(); // TypeError: sayHi is not a function
var sayHi = function () {
  console.log("hi");
};

Common mistakes

• Saying everything is initialised to undefined. Function declarations are not.
• Saying let and const are not hoisted. They are hoisted but blocked by the temporal dead zone.
• Relying on hoisting in production code. It makes control flow harder to read.