CRUD stands for Create, Read, Update, Delete.
Read is different in that it causes no side effects, making it purely functional. It usually means that it won't trigger any events, as mutation observers aren't interested in it.
Update can also be the creation of a new thing + the destruction of an old thing, and being aware of that. Update is an illusion I guess?
Delete is not necessarily needed; like with git you can keep an index of the current HEAD + backwards pointers on new entries and never having to delete anything. Welcome to weird town.
Carve out a subset of available status codes, preferably place them behind a layer of abstraction for consistency.
200 OK - Everything worked as expected. 400 Bad Request - Often missing a required parameter. 401 Unauthorized - No valid API key provided. 402 Request Failed - Parameters were valid but request failed. 404 Not Found - The requested item doesn't exist. 500, 502, 503, 504 Server errors - something went wrong on the server's end.