feat(ui): Throw error on non-json api responses (#57634)

static/app/api.tsx

@@ -488,18 +488,18 @@ export class Client {
  // GET requests may not have a body
  const body = method !== 'GET' ? data : undefined;
 
- const headers = new Headers(this.headers);
+ const requestHeaders = new Headers(this.headers);
 
  // Do not set the X-CSRFToken header when making a request outside of the
  // current domain. Because we use subdomains we loosely compare origins
  if (!csrfSafeMethod(method) && isSimilarOrigin(fullUrl, window.location.origin)) {
- headers.set('X-CSRFToken', getCsrfToken());
+ requestHeaders.set('X-CSRFToken', getCsrfToken());
  }
 
  const fetchRequest = fetch(fullUrl, {
  method,
  body,
- headers,
+ headers: requestHeaders,
  credentials: this.credentials,
  signal: aborter?.signal,
  });
@@ -535,6 +535,8 @@ export class Client {
 
  const responseContentType = response.headers.get('content-type');
  const isResponseJSON = responseContentType?.includes('json');
+ const wasExpectingJson =
+ requestHeaders.get('Content-Type') === 'application/json';
 
  const isStatus3XX = status >= 300 && status < 400;
  if (status !== 204 && !isStatus3XX) {
@@ -550,6 +552,16 @@ export class Client {
  // this should be an error.
  ok = false;
  errorReason = 'JSON parse error';
+ } else if (
+ // Empty responses from POST 201 requests are valid
+ responseText?.length > 0 &&
+ wasExpectingJson &&
+ error instanceof SyntaxError
+ ) {
+ // Was expecting json but was returned something else. Possibly HTML.
+ // Ideally this would not be a 200, but we should reject the promise
+ ok = false;
+ errorReason = 'JSON parse error. Possibly returned HTML';
  }
  }
  }