Looking up a field that does not exist returns 400, not 404
GET …/fields/GetByInternalNameOrTitle('X') for a field that is not there answers HTTP 400 (an SPException "column does not exist"), not 404 — an idempotent "create the column if missing" check that waits for 404 therefore throws on exactly the case it was written for; check existence with …/fields?$filter=InternalName eq 'X' and an empty value array.
GET …/fields/GetByInternalNameOrTitle('X') na pole, které neexistuje, vrátí HTTP 400 (SPException „sloupec neexistuje“), ne 404 – idempotentní kontrola „založ, když chybí“ čekající na 404 tak spadne právě v případě, pro který vznikla; existenci zjišťuj přes …/fields?$filter=InternalName eq 'X' a prázdné pole value.
Symptom
A setup script meant to add a column only when missing stops with field check HTTP 400 on its very first run, against a library that has never had the column.
Cause
Lists and files do answer 404 for a missing path (GetList('…'), GetFolderByServerRelativePath), which makes 404 the natural expectation. Field lookups by name are different: SharePoint raises an argument-style exception, which the REST layer maps to 400.
Fix
const r = await fetch(list + "/fields?$select=Id&$filter=InternalName eq 'MeetingDate'", { headers });
if (!r.ok) throw new Error('field check HTTP ' + r.status); // a real failure
const exists = (await r.json()).value.length > 0;A 400 on the direct lookup cannot be told apart from a genuinely malformed request, so treating "400 = missing" is not a safe shortcut.