Separate missing, invalid and numeric form input, then compare normalized quantities without treating an empty field as zero.
for...of gives you the values of an array. for...in gives you the keys of an object, as strings. Mix them up and you get indexes instead of items, or worse.
Trace a const reassignment inside an input handler, preserve the original list, and verify that filtering and clearing the query both work.
Backticks and ${} replace most string gluing with +. You get readable text, values inside it, and multi-line strings without \n everywhere.
"5" + 1 is "51" because + joins text before it adds numbers. Convert values on purpose with Number(), String() and parseInt(), check for NaN, and stop letting JavaScript guess for you.
"Uncaught TypeError: Cannot read properties of undefined" means you used a value that wasn't there. Read the message, find the line, check what is actually undefined, then fix the cause instead of hiding it.
Getters compute a value when you read it, so totals never go stale. Setters check a value when you assign it, so a negative price throws instead of saving. With private fields, nobody can go around them.
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.
Replace long lists of positional arguments with one options object, destructured in the parameter list with defaults. Add = {} so a call with no argument doesn't crash, and know that defaults only replace undefined.
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.
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.