Stop checking error messages with includes(). Extend Error, set name, add the fields the handler needs, catch with instanceof, and wrap low-level errors with cause so the real reason isn't lost.
Your date moves a day because one part of the code uses UTC and another uses local time. Find which step does it, keep calendar days as YYYY-MM-DD strings, and name the time zone when you format.
The file was loaded as a classic script or CommonJS, where import isn't allowed. The fix in the browser is type="module". In Node it's "type": "module" or .mjs, and recent npm init writes "type": "commonjs" for you.
"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.
Add Vitest to a project you already have and get real tests passing: a pure function with it.each, then an async function that calls fetch, tested with a fake instead of the network.