Demo: 5 puzzles to start
A set of 7 challenges of different types and levels — from a warm-up to hard (L4) tasks. Solve them right away, no account needed.
The whole site works just like this demo: you write code or dig around the browser, and we check the result on the spot. The only difference — with a free account you earn points and join the leaderboard. The core is and stays free, and you never pay with your data.
The site already has 211 puzzles – all free. You can solve 7 right away in the demo, no login.
- Easy Coding goal Unlock access 20 pts
- Warm-up Coding function Is it even? 10 pts
- Medium Analysis predict Truthiness of '0': !!'0' 30 pts
- Warm-up DevTools recon Flag in a comment 10 pts
- Easy DevTools tamper The role cookie 20 pts
- Hard DevTools tamper Forged role token (Base64) 40 pts
- Hard Analysis predict Async Order 40 pts
Challenge types - several ways to play
SolveJS is not one format. Challenges split into three categories and twelve types - each trains a different skill. Here is the full set.
Coding
Write functions, fix bugs, unlock a goal. Code runs in an isolated Web Worker.
- Write a function Write a function - build a solution from scratch. Trains clean coding and algorithms.
- Find and fix the bug Find and fix the bug - the starter contains a defect. Trains reading other people's code and debugging.
- Modify the code Unlock the goal - change locked code so it returns the expected value. Trains reasoning about conditions.
- Shortest code (golf) Shortest code (golf) - solve it, but length counts. Fit within the character limit and beat the shortest-solution record. Trains conciseness and language tricks.
- Program the output Program the output - the reverse of "Predict the result": write code that prints exactly the given string to the console. Trains control over what reaches the output.
Analysis
Predict console output and decode strings. The JS quirks everyone trips on.
- Predict the output Predict the console output - the most shareable type, built on JS quirks. Trains deep understanding of the language.
- Decode Decode the string - decipher data (Base64, ROT13, Caesar cipher). Trains recognising encodings.
- Write a regular expression Write a regular expression - match some strings, reject others. Checked server-side with catastrophic-backtracking (ReDoS) protection. Trains practical regex.
- Shortest regex (golf) Shortest regex (golf) - hit every "match" and no "skip" using as few characters as possible. Trains regex and minimalism.
- Spot the vulnerability Spot the vulnerability - read a code snippet and identify what it is vulnerable to (XSS, injection, leak). Multiple choice, checked server-side. Trains an eye for flaws.
DevTools
Find the flag in DevTools and tamper GET/POST requests. Codewars and JSchallenger do not have this.
- Find the flag Find the flag - locate a hidden value in DevTools (HTML comment, header, localStorage, XHR). Trains recon.
- Tamper the request Tamper the request - change GET, POST, a cookie or a header so the server returns the flag. Trains real web techniques.
Done with the demo? Unlock the rest for free
Create a free account to solve all 211 challenges, earn points and climb the weekly and global leaderboards. No fees, and we never sell your data.