new·The score now tells you which way it movedA brain's exam only ever grows: its own material writes questions, and so does every question a real caller asked and did not get answered. The score is a percentage over that growing set, so a brain that learned more could post a smaller number — and this week three did. One of them answered two MORE questions than the week before and showed eighteen points less. Printed as a single percentage, that reads as decline to a reader and as punishment to anyone who contributes material.all news →
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explore / Web & frontend / mozg

React

Answer questions about React as documented on react.dev today: components and JSX, state and the rules of hooks, effects and when not to use them, the reference API with each hook's exact signature and rules, and the compiler errors and warnings as they are worded.

2,604 notes across 4 brains · updated 2026-08-09 · trained 78%

since last sitting: +67 newly passed — this brain is learning · the verified changelog →

use this brain
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$ claude mcp add --transport http mozg \
https://mozg.sh/mcp --header "Authorization: Bearer …"
> use mozg/react — …
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Ask it things like

answers 57/72 on its latest exam · anti-bluff 10/19
What is the difference between storing a value in state vs in a ref?Advanced Hooks✓
If I fetch data in useEffect and the component unmounts before the fetch completes, what might happen?Effects Antipatterns & Traps✓
What does calling useState([]) return?Hooks: Core & State✓
Can I call useState inside an if statement?Hooks: Core & State✓
What is the signature of useReducer?Hooks: Core & State✓
What is the purpose of the dependency array in useEffect?Hooks: Effects & Synchronization✓
What is the cleanup function in useEffect and when is it called?Hooks: Effects & Synchronization✓
What happens if I pass an empty dependency array [] to useEffect?Hooks: Effects & Synchronization✓

What is inside

asking this brain searches all 3
React · API referenceAnswer questions about the React API exactly as specified: every hook, component, directive and API in react and react-dom with its signature, parameters, return value and usage rules.1769 notes73%React · Errors and warningsAnswer questions about a specific React error or warning message: what the message says verbatim, what causes it and how to fix it.45 notes93%React · LearnAnswer questions from the React learn docs: describing the UI, adding interactivity with state, managing state with reducers and context, escape hatches — refs, effects and their lifecycle — and installation and setup for every framework.790 notes86%

What it can answer

72 coverage checks
78%
Answers 67 of the exam's 91 questionsIt answers 67 questions, 67 more than last time, out of an exam that grew by 91.Nothing it used to answer has stopped working.
✓Advanced Hooks11 / 11
▲JSX & Component Basics10 / 12
✓Hooks: Effects & Synchronization9 / 9
▲Hooks: Core & State7 / 10
▲Performance & Optimization6 / 10
▲Refs & DOM6 / 7
▲Effects Antipatterns & Traps5 / 7
▲Errors, Compiler & Debugging3 / 6
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Attacks survived

re-run weekly · 2026-08-09
✓gate-battery12/12 known hostile payloads caught by the ingest gate
✓injection-corpusevery active note scanned — no steering language
✓secret-corpusevery active note scanned — no keys or credentials

heuristic scans against known attack classes — measured and dated, not a promise of unpoisonability

Inside

    Licence

    CC BY-NC-SA 4.0

    Use it, copy it, build on it, with credit. Selling it is not allowed.