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Hoisting

Declarations are moved to the top of their scope before execution.

1. Explanation

JavaScript hoists var declarations (initialised to undefined) and function declarations (fully) to the top of their scope. let and const are hoisted but not initialised — accessing them before the declaration throws a ReferenceError (the Temporal Dead Zone).

2. Example

Try it yourself

console.log(a); // undefined (var hoisted)
var a = 5;

hello();        // works — function declarations hoist fully
function hello() { console.log("hi"); }

// console.log(b); // ❌ ReferenceError (TDZ)
let b = 10;

Console

Click Run to see output.

3. Expected output

output

undefined
hi

4. Common mistakes

✗Relying on var hoisting for cleaner-looking code — it's confusing, prefer let/const
✗Calling a function expression before it's defined
✗Forgetting that class declarations are also in the TDZ

5. Practice questions

Log a let variable before it's declared and observe the error
Define a function expression and try calling it before
Explain in comments why a particular snippet logs undefined

→ Try more practice problems