What is hoisting in JavaScript?

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Hoisting is the JavaScript engine's behaviour of processing declarations before executing any code in a scope. During the creation phase of an execution context, variable and function declarations are registered in memory, which makes them appear as if they were "moved to the top" of their scope. Only the declaration is hoisted — the assignment stays where it is written. Function declarations Function declarations are hoisted in full, so the function is usable anywhere in its scope, even before the line that defines it. greet(); // "hi" function greet() { console.log("hi"); } var declarations var declarations are hoisted and initialised to undefined . The assignment happens when execution reaches it. console.log(x); // undefined var x = 5; console.log(x); // 5 let and const declarations let and const are also hoisted, but they remain uninitialised in the temporal dead zone until the declaration line executes. Accessing them early throws a ReferenceError rather than returning undefined . console.log(y); // ReferenceError let y = 5; Function expressions and arrow functions These follow the rules of the variable that holds them. A var -assigned function expression hoists as undefined ; a let or const one is in the TDZ. sayHi(); // TypeError: sayHi is not a function var sayHi = function () {}; sayBye(); // ReferenceError const sayBye = () => {}; Class declarations Classes are hoisted but stay in the TDZ, so they must be declared before use. They behave like let / const in this respect. Practical takeaway Hoisting explains why var and function declarations are forgiving about order, while let , const , and class are strict. The modern convention — declare variables before use — sidesteps hoisting quirks entirely. Know what hoists fully (function declarations), what hoists partially (var — undefined), and what hoists but stays in TDZ (let/const/class). The function expression vs declaration hoisting difference is the most tested detail.