Stop grinding random algorithms.
Practice the exact JS, TS, React, Vue, Angular, Node.js, Python and SQL patterns that appear in frontend & fullstack interviews — with real hidden tests and AI feedback.
1// Flatten a nested array to any depth2function flatten(arr) {3 return arr.reduce((acc, item) =>4 Array.isArray(item)5 ? acc.concat(flatten(item))6 : acc.concat(item),7 []);8}
✓ 5 / 5 visible tests passed
Submit to unlock hidden tests
How it works
Three steps. The same flow as a real technical screening.
Choose from 400+ challenges across JS, TS, React, Vue, Angular, Node.js, Python and SQL — sorted by difficulty and topic. No Java. No C++.
Code in the browser — no setup, no installs. The same editor flow as real take-home tests, with syntax highlighting and keyboard shortcuts.
Submit against public AND hidden test cases. Per-test pass/fail, error messages, and an AI mentor that explains exactly what went wrong.
Features
Every feature is designed around the actual frontend & fullstack interview experience.
Every challenge includes visible examples AND secret tests revealed only after submission — exactly how companies filter candidates.
✓ flatten([1,[2,3]]) → [1,2,3] ✓ flatten([]) → [] ● +15 hidden edge cases (submit to reveal)
After each submission get a line-by-line breakdown — time complexity, missed edge cases, and idiomatic alternatives written for your code.
// Your solution: O(n²) — quadratic // Optimal: O(n) — use a Map // Missed edge case: empty string input
Timer on. Multiple challenges. A final score and an AI-generated performance report — like a real technical interview, pressure and all.
⏱ 00:42:17 remaining ✓ Challenge 1 ACCEPTED (12 min) ◌ Challenge 2 in progress...
Your Interview Readiness Score updates after every session — broken down by language so you know precisely what to focus on next.
Interview Readiness 82 / 100 JavaScript ████████░░ 81% TypeScript █████░░░░░ 54% React ███████░░░ 71%
By the numbers
400+
Coding challenges
70+
Quiz chapters
8
Languages & Frameworks
< 3s
Avg. feedback time
Join 10,000+ developers preparing for their next interview
“Went from 0 to offer in 6 weeks. The hidden tests are brutal — in the best way.”
“The mock interview timer made me realize I was way too slow. Fixed that before the real thing.”
“Finally a platform that focuses on real JS/TS patterns, not linked lists and binary trees.”
You read the whole page.
Start your first challenge in 30 seconds. No setup. No credit card.
No credit card required · Cancel anytime