Programming tutorials for people who want code that actually runs, not code that only looks right. Pick a language, start at the beginning, and don't skip the basics. They are where most problems come from.
fetch only rejects when there's no response. A 404 or 500 resolves normally, so check response.ok, add a timeout, and keep the status on the error so the caller can decide what to tell the user.
"Unexpected token in JSON" means JSON.parse got text that isn't JSON. Read the exact message, log the raw text, and match it to one of five causes. Most of the time it's an HTML error or login page.
Replace a data-or-error object with a union tagged by success, then add a generic Outcome and an unwrap helper. After this, the compiler knows which property exists in each case.
Replace scattered casts with one assertion function and one typed fetch helper. After this, a 404 or a changed payload fails with a clear message instead of carrying a wrong type.
Receive a nested payload as unknown and narrow it with two small helpers until every property is checked. After this, you can write a parser that rejects wrong data and returns a real type.
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