You call JSON.parse() (or res.json()) and get: SyntaxError: Unexpected token '<', "<!DOCTYPE...
You call JSON.parse() (or res.json()) and get:
SyntaxError: Unexpected token '<', "<!DOCTYPE "... is not valid JSON
Before you hunt for a missing comma — most of these errors aren't broken JSON at all.
"Unexpected token <" → you got HTML, not JSON
The #1 cause, and it has nothing to do with your JSON. The server returned an HTML page (a 404, a login redirect, a Cloudflare challenge) and your code parsed it as JSON.
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