jestjs/jest · error · TypeError

Cannot define a method called ${name}

Error message

Cannot define a method called ${name}

What it means

Error "Cannot define a method called ${name}" thrown in jestjs/jest.

Source

Thrown at packages/jest-worker/src/index.ts:148

        workerSchedulingPolicy: this._options.workerSchedulingPolicy,
      },
    );

    this._bindExposedWorkerMethods(workerPath, this._options);
  }

  private _bindExposedWorkerMethods(
    workerPath: string,
    options: WorkerFarmOptions,
  ): void {
    for (const name of getExposedMethods(workerPath, options)) {
      if (name.startsWith('_')) {
        continue;
      }

      // eslint-disable-next-line no-prototype-builtins
      if (this.constructor.prototype.hasOwnProperty(name)) {
        throw new TypeError(`Cannot define a method called ${name}`);
      }

      // @ts-expect-error: dynamic extension of the class instance is expected.
      this[name] = this._callFunctionWithArgs.bind(this, name);
    }
  }

  private _callFunctionWithArgs(
    method: string,
    ...args: Array<unknown>
  ): Promise<unknown> {
    if (this._ending) {
      throw new Error('Farm is ended, no more calls can be done to it');
    }

    return this._farm.doWork(method, ...args);
  }

View on GitHub (pinned to f49721c78e)

Solutions

1. Do not expose a method whose name collides with reserved/internal names on the jest-worker farm. Rename the method you are trying to define on the worker to a non-conflicting name.

When it happens

Trigger: Thrown at packages/jest-worker/src/index.ts:148 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/a77a1883728be099.json. Report an issue: GitHub.