#JavaScript

Unexpected token '<' is not valid JSON — 5 Reasons Your API Returned HTML Instead

fetch throws SyntaxError: Unexpected token '<' when the response body is HTML, not JSON. Covers the 5 root causes: SPA history fallback returning index.html, auth 302 redirects, gateway 502 error pages, wrong baseURL, and missing res.ok checks — with curl diagnostics.

How to Fix 'Cannot read properties of undefined (reading map)'

A complete debugging guide for the most common React runtime error: why undefined.map throws, handling async data correctly, optional chaining and default guards, empty-state rendering, and using our JSON tools to inspect the real payload.