🐞 Error Root-Cause Finder
Paste an error message or stack trace and get the most likely root cause (not just the symptom), fixes ranked by likelihood, and how to prevent it.
- Root cause, not the symptom. It reads the whole stack — including async/framework frames — to find the line that actually broke, not the one that threw.
- Fixes ranked by likelihood. Concrete steps ordered most- to least-probable, each with how to apply it and what to check to disambiguate.
- No guessed internals. When library behaviour is uncertain it says "verify" instead of inventing an API that doesn't exist.
See the quality — a real example
Sample only · no credits usedInput — the error you paste
TypeError: Cannot read properties of undefined (reading 'map')
at ProductList (ProductList.jsx:14:30)
at renderWithHooks (react-dom.development.js:16305:18)
at mountIndeterminateComponent (react-dom.development.js:20074:13) Context: React 18, products comes from a useEffect fetch; crashes on first paint.
Output — what it means & how to fix
Root cause
On the first render products is still undefined — the useEffect fetch hasn't resolved yet — so products.map(...) at ProductList.jsx:14 runs on undefined. The crash is a render-timing bug, not bad API data.
Fixes (most likely first)
- Initialise state to an array:
useState([])so.mapis always safe. - Guard the render:
{(products ?? []).map(...)}or return a loading state until data arrives. - If you expect data and still get undefined, log the fetch — a non-200 / wrong shape may be leaving state unset.
Prevention: initialise list state to [] never undefined, and add an explicit loading branch before mapping.
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Root cause