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420+ challenges
730+ quiz chapters
60+ projects
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React Hooks · Question 1 of 3

What does this log on the first two renders?

const [n, setN] = useState(0);
useEffect(() => { setN(1); }, []);
console.log(n);

challenges / flatten-arrayEasy

1function flatten(arr) {
2  return arr.reduce((acc, item) =>
3    Array.isArray(item)
4      ? acc.concat(flatten(item))
5      : acc.concat(item), []);
6}

flatten([1, [2, 3]]) → [1, 2, 3]
flatten([1, [2, [3, [4]]]]) → [1, 2, 3, 4]
flatten([]) → []
+15 hidden tests run on submit

5 / 5 visible tests passedSubmit

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Give us the date and your stacks. Each day you get a short plan, a mock interview when you are close, and a score per stack that says what is still weak.

Your interview

in 7 days

Today

Quiz · React Hooks, Hard10 q
Challenge · debounceMedium
Review · mistakes due6 cards

Mock interview · React

45 min

84/ 100

3 of 3 accepted. Challenge 2 re-sorts on every call: cache the sorted keys. Edge case missed on challenge 3: empty input.

Readiness

ready at 75

React
82
TypeScript
64
Vue
58
Angular
71

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Get better at the stack you already use.

50 guided paths, real projects built against their own tests, quizzes in four tiers, and a deck that brings back what you got wrong.

Guided path

React Fundamentals

week 2 / 6

Rendering & reconciliation
State beyond useState · you are here
Effects & data fetching

Project

mini-redux

step 4 / 9

createStore6 / 6
subscribe4 / 4
combineReducers5 / 5
middleware2 / 5

Review your mistakes

6 cards due today

Missed 3 days ago

Which hook keeps a value across renders without re-rendering?

Right again, it comes back in 3, 7, 16, then 35 days. Wrong, tomorrow.

AI mentor

It reads your code, not just the result.

After each submission: the complexity, the edge case you missed, and a more idiomatic version written from your code.

1function firstUnique(s) {
2  for (let i = 0; i < s.length; i++)
3    if (s.indexOf(s[i]) === s.lastIndexOf(s[i]))
4      return s[i];
5  return null;
6}

Lines 2-3 · complexity

indexOf and lastIndexOf scan the string inside the loop: O(n²). Count characters once in a Map: O(n).

Line 5 · edge case

The tests expect "" for an empty string, not null. Hidden test 7 failed on this.

Idiomatic version

const counts = new Map();
for (const c of s) counts.set(c, (counts.get(c) ?? 0) + 1);
return [...s].find((c) => counts.get(c) === 1) ?? "";

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What is free, and what does the 7-day Pro trial include?

Free covers challenges, learning paths, the first quiz series of every topic and 10 mistake reviews a day. Finish onboarding and Pro opens for 7 days: every quiz series, full mock interviews and the AI mentor. No credit card, and you drop back to Free on its own.

Do I need to install anything?

No. The editor, the tests and the projects all run in the browser.

Which stacks and languages are covered?

JavaScript, TypeScript, React, Vue, Angular, Node.js, Python and SQL, plus NestJS, Next.js and system design at onboarding. Node.js is covered by quizzes. Pick your stacks and the catalogue reorders around them.

How does the AI mentor work?

After a submission it reads your code and explains the complexity, the edge cases you missed and a more idiomatic version. Free accounts get 2 reviews, Pro is unlimited.

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