appsmithorg/appsmith · error · Error

Failed to load JS libraries

Error message

Failed to load JS libraries

What it means

Error thrown when failFastApiCalls reports that FETCH_JS_LIBRARIES_FAILED won. The eval worker cannot continue without the configured JS libraries (lodash, moment, dayjs, etc.) injected, so the saga aborts before spawning the eval worker request handler. This is a load-time gate, not a runtime eval failure.

Source

Thrown at app/client/src/sagas/EvaluationsSaga.ts:213

): any {
  const evalWorkerListenerChannel = yield call(setupWorkers, true);

  // Take the action from the appVi
  const jsLibrariesAction = yield take(initializeJSLibrariesChannel);
  const { applicationId, customJSLibraries } = jsLibrariesAction.payload;

  yield put(setIsFirstPageLoad());

  // Use failFastApiCalls to execute fetchJSLibraries
  const resultOfJSLibrariesCall: boolean = yield call(
    failFastApiCalls,
    [fetchJSLibraries(applicationId, customJSLibraries)],
    [ReduxActionTypes.FETCH_JS_LIBRARIES_SUCCESS],
    [ReduxActionErrorTypes.FETCH_JS_LIBRARIES_FAILED],
  );

  if (!resultOfJSLibrariesCall) {
    throw new Error("Failed to load JS libraries");
  }

  yield spawn(handleEvalWorkerRequestSaga, evalWorkerListenerChannel);
}

export function* updateDataTreeHandler(
  data: {
    evalTreeResponse: EvalTreeResponseData;
    unevalTree: UnEvalTree;
    requiresLogging: boolean;
    configTree: ConfigTree;
  },
  postEvalActions?: Array<AnyReduxAction>,
) {
  const { configTree, evalTreeResponse, requiresLogging, unevalTree } = data;
  const postEvalActionsToDispatch: Array<AnyReduxAction> =
    postEvalActions || [];

View on GitHub (pinned to 8cd9021c24)

Solutions

1. Open DevTools Network and look for the failing /libraries request or the worker's library script load (note the status code).
2. Remove recently added JS libraries from the app settings and re-add the correct version.
3. If behind a proxy, allow-list the JS library CDN domains.
4. Clear browser cache / hard reload so stale library metadata isn't served.

When it happens

Trigger: The /libraries endpoint returns an error, a library script fails to load inside the worker, the CDN for a pinned library version is unreachable, or a corporate proxy blocks the library URL.

Common situations: Air-gapped / proxied environments; library version deprecated upstream; first load after adding a new JS library that 404s; browser extension blocking third-party scripts.

Related errors

• The script at ${url} cannot be installed.
• ${extractExecutionErrorMessage(e)}
• Save page failed
• Unable to fetch newly created actions
• Failed importing template

AI-assisted analysis of appsmithorg/appsmith@8cd9021c24 (2026-08-12). Data as JSON: /api/errors/1cd47d984ba1a773. Report an issue: GitHub.