WTF, JavaScript?!
Prove you actually know it.

316 challenges that will make you question everything you think you know about this language. 118 hands-on coding problems judged by a merciless sandbox — the rest are brutal multiple choice and drag-and-drop mind games. Every challenge ships a hint. Using it is between you and your ego.

Gigachad-tier JavaScript

casuals welcome — egos enter at their own risk

📅 Today's daily →🎮 Play a game Browse problems Start exam Leaderboard

What you get inside

Solve a coding problem and the engine is drawn from your own execution — call stack, Web APIs, both queues, with your callbacks on them. Here it is running the program everyone gets wrong the first time.

Event loopSix beats. Watch where B goes, and how long it waits.

JS engine

Memory heapobjects & closures live here

Beat 0 of 15 The program is about to run. Nothing has touched the stack yet.

Topics

Play in your terminal

npm install -g wtfjavascript   # or: bun add -g wtfjavascript
wtfjs            # full styled quiz in your terminal
wtfjs --topic "Proxy & Reflect"
npx wtfjavascript              # or: bunx wtfjavascript (zero-install)

wtfjs — answering challenges from the command line

$ wtfjs

wtf(js) wtf, JavaScript?! — prove you actually know it

Your name: ada

1/8 Hard The == algorithm, step by step · Type Coercion

│ console.log(null == undefined, null == 0);

A. true false B. true true C. false false

Answer (A-D, 'h' hint, 's' skip): a

✔ Accepted (1 try) — null only pairs with undefined

2/8 Medium Swap two values without a temp variable

[Enter] open $EDITOR · 'r' run only · 'h' hint · 's' skip: ⏎

… your own editor opens — write, save, close …

✔ Accepted (1 try) Score so far: 200

▌

Read the books behind the pain

You Don't Know JS Yet: Get Started — book cover

You Don't Know JS Yet: Scope & Closures — book cover

You Don't Know JS Yet — Kyle Simpson

The book series that goes deeper than any challenge here can. Scope, closures, this, coercion — explained properly, from the spec up. The entire series is free to read on GitHub, with print editions available from the same page.

Read the series →

Acknowledgments

Every challenge here stands on work that others gave away for free. Lydia and Kyle spent years writing the questions and books that teach this language to anyone willing to learn — hours of unpaid, careful work done purely to help the community. If these challenges taught you something, it's because they taught us first. Thank you. Go star their work.

Lydia Hallie @lydiahallie

javascript-questions, advanced-web-dev-quiz & the JavaScript patterns collection — the quizzes and patterns behind dozens of these challenges

Kyle Simpson @getify

You Don't Know JS — the book series behind our scope, this & coercion deep-cuts

Built by

Jeffrey Marvin Forones @aiscenblue

Developer — wtf(js)

WTF, JavaScript?!Prove you actually know it.