jestjs/jest · error · TypeError

"messageParent" can only be used inside a worker

Error message

"messageParent" can only be used inside a worker

What it means

Error ""messageParent" can only be used inside a worker" thrown in jestjs/jest.

Source

Thrown at packages/jest-worker/src/workers/messageParent.ts:47

    }
  } else if (typeof parentProcess.send === 'function') {
    try {
      parentProcess.send([PARENT_MESSAGE_CUSTOM, message]);
    } catch (error) {
      if (
        isError(error) &&
        // if .send is a function, it's a serialization issue
        !error.message.includes('.send is not a function')
      ) {
        // Apply specific serialization only in error cases
        // to avoid affecting performance in regular cases.
        parentProcess.send([PARENT_MESSAGE_CUSTOM, packMessage(message)]);
      } else {
        throw error;
      }
    }
  } else {
    throw new TypeError('"messageParent" can only be used inside a worker');
  }
}

View on GitHub (pinned to f49721c78e)

Solutions

1. messageParent can only be called from code running inside a jest-worker child process (a file passed as workerPath). Do not call it from the parent/main process; use it only inside the worker module.

When it happens

Trigger: Thrown at packages/jest-worker/src/workers/messageParent.ts:47 when the library encounters an invalid state.

Common situations: See trigger scenarios.

AI-assisted analysis of jestjs/jest@f49721c78e (2026-08-03). Data as JSON: /data/errors/82a47a6270f75944.json. Report an issue: GitHub.