moveto succeeds and your code reports a failure — action endpoints return an empty body

Symptom

A file move completes in SharePoint, and the dialog says:

Not moved: Failed to execute 'json' on 'Response': Unexpected end of JSON input

The user tries again — on a file that is no longer at the source path. Removing a member from a SharePoint group goes the same way: the member is gone, the panel shows an error and does not refresh, and the admin clicks a row that no longer exists.

Cause

async function post(url: string, body?: unknown): Promise<any> {
  const r = await client.post(url, cfg, { body: JSON.stringify(body) });
  if (!r.ok) throw new Error(`HTTP ${r.status}`);
  return r.json();            // ← throws on an empty body, after a successful operation
}

…/getfilebyserverrelativeurl('…')/moveto(newurl='…',flags=0) and …/sitegroups(<id>)/users/removebyid(<id>) return nothing to parse. The helper was written for calls that return an entity and then reused for calls that do not.

Fix

Two helpers, chosen by whether the caller reads the result:

/** Actions: success is the status code. */
async function postVoid(url: string, body?: unknown): Promise<void> {
  const r = await client.post(url, cfg, body === undefined ? {} : { body: JSON.stringify(body) });
  if (!r.ok) throw new Error(`HTTP ${r.status}: ${(await r.text()).slice(0, 300)}`);
}

/** Calls that return an entity you actually use. */
async function postJson<T>(url: string, body?: unknown): Promise<T> {
  const r = await client.post(url, cfg, body === undefined ? {} : { body: JSON.stringify(body) });
  if (!r.ok) throw new Error(`HTTP ${r.status}: ${(await r.text()).slice(0, 300)}`);
  const text = await r.text();
  return (text ? JSON.parse(text) : undefined) as T;
}

Notes

• A failure after the operation is worse than a failure before it. It invites a retry of something that already happened. For every write, ask what the user sees when only the parsing of the response fails.
• Use the status-only helper for every action you call for its effect rather than its data — copyto, checkin, addroleassignment, removeroleassignment and breakroleinheritance included — and parse a body only where you read something from it. Not every action is empty: recycle() on a list item returns the GUID of the new recycle-bin item (REST DELETE on an item is permanent).
• The retry is not harmless either: the source is gone, so a second moveto answers 404 and reads like a real error. Batch file move: group by file, not by record shows how a rerun can adopt a file that is already at the destination.