What is hoisting in JavaScript?

Q: What is hoisting in JavaScript?

**Target Audience:** Engineers preparing for JavaScript fundamentals interview questions. **Difficulty:** Easy > **How to read this doc:** Concepts are explained in plain language first, then tagged with 📌 Interview term: . Every result below was actually run on Node.js v24.19.0 — the printed output is real, captured output, not illustrative sample text. ## 1. Why This Even Matters — A Story First Think of a scope like a conference room with name tags laid out before the meeting starts. Every attendee name that will be used in this room is already on the table the moment the room opens -- that is hoisting, the engine already knows every declared name in this scope before running a single line. But whether that name can actually be used yet depends on the tag type: a var name tag behaves as if that seat is already occupied, silently answering undefined if addressed early; a let, const, or class name tag sits face down -- it exists, but touching it before its own declarat

The problem

Question presented to candidate: "What is hoisting in JavaScript, and how does it actually behave differently for var, let, const, function declarations, and class declarations?"

What a strong answer should cover:

Hoisting means the engine registers every declared name in a scope before executing any code in that scope -- the name exists from the top, even though source reads top to bottom.
var declarations are hoisted AND initialized to undefined immediately, so reading a var before its declaration line returns undefined rather than throwing.
let, const, and class declarations are hoisted but left uninitialized in the temporal dead zone (TDZ) -- reading any of them before their own declaration line throws a ReferenceError, not undefined.
function declarations are hoisted completely, including their body, so they are callable even before the line they are written on; a function EXPRESSION assigned to a var only hoists the var itself (as undefined), never the assignment.
Real motivation to state clearly: hoisting is not "moving code around" -- the engine does a scope-analysis pass first, and each declaration form reacts differently to being read before its own line actually runs.

Clarifying questions expected:

"Do you want me to also cover class hoisting and the TDZ specifically, or keep this to var versus let and const?"

Code / implementation expected: Yes -- a short, runnable snippet demonstrating each hoisting behavior side by side.

hoisting
scope
variables
functions

Try it yourself

Sketch your approach before you read the solution. That is what the interview tests.

Stuck? Get a hint.

A short nudge toward the approach. It will not give the answer away.

Solution

Target Audience: Engineers preparing for JavaScript fundamentals interview questions. Difficulty: Easy

How to read this doc: Concepts are explained in plain language first, then tagged with 📌 Interview term:. Every result below was actually run on Node.js v24.19.0 — the printed output is real, captured output, not illustrative sample text.

1. Why This Even Matters — A Story First

Think of a scope like a conference room with name tags laid out before the meeting starts. Every attendee name that will be used in this room is already on the table the moment the room opens -- that is hoisting, the engine already knows every declared name in this scope before running a single line. But whether that name can actually be used yet depends on the tag type: a var name tag behaves as if that seat is already occupied, silently answering undefined if addressed early; a let, const,

The solution is hidden so you can try first

8 min read · 8 sections

Run the code

Edit the code and run it here. Each console line with an expected value becomes a test.

JavaScriptHoisting across var, let, function declaration, function expression, and class (run directly)