"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.
Trace a wrong callback receiver, bind one stable listener, and verify start, stop and restart without duplicate or stale subscriptions.
Start with the recommended rules, fix what's automatic, turn noisy rules into warnings, and make CI fail only on new problems. The linter becomes useful on day one instead of ignored.
Trace a detached preview from its DOM node to an application Map, repair cleanup, and verify repeated open-and-close cycles with heap snapshots.
Your HTML file opens blank, shows as text, or downloads instead of rendering. Check the extension, the file:// open path, the doctype and a missing closing tag before you rewrite the page.
Trace a reproducible colour and spacing conflict from the rendered result to the winning rule, then verify the repair in the source file.
Reproduce a fixed-width overflow on a narrow screen, locate the offending element and verify a repair without clipping the page.
Measure contrast by number, run axe, and walk the keyboard focus path with a script to catch what the tool cannot see.
Install the Node types, turn them on in tsconfig.json, and switch require to import. Also covers the other causes of the same error: a typo, a missing import and a missing lib.
Read an error in three passes and match the eight most common phrases to their causes, using real tsc output. After this, a wall of errors is a list you can work through.
Let a declared return type find a branch with no return, an arrow function with braces, and an async function typed wrong. After this, a function cannot quietly give back undefined.