What causes "Attempting to import a mock without a factory" in jestjs/jest?

Use `jest.unstable_mockModule('x', () => ({ ... }))` with an explicit factory function returning the mock namespace.

jestjs/jest · error · Error

Attempting to import a mock without a factory

Error message

Attempting to import a mock without a factory

What it means

Thrown by EsmLoader.importMock when an ESM import resolves to a mocked module but no factory was registered for it via `jest.unstable_mockModule`. The async ESM mocking path requires a factory to synthesize the mock module; without one Jest cannot construct it.

Source

Thrown at packages/jest-runtime/src/internals/EsmLoader.ts:1324

  private async importMock<T = unknown>(
    moduleName: string,
    moduleID: string,
    context: VMContext,
  ): Promise<T> {
    if (this.registries.hasModuleMock(moduleID)) {
      return this.registries.getModuleMock(moduleID) as T;
    }

    const factory = this.mockState.getEsmFactory(moduleID);
    if (factory) {
      const invokedFactory = (await factory()) as Record<string, unknown>;
      const module = syntheticFromExports(moduleName, context, invokedFactory);
      this.registries.setModuleMock(moduleID, module);
      return evaluateSyntheticModule(module) as T;
    }

    throw new Error('Attempting to import a mock without a factory');
  }

  private async importWasmModule(
    source: BufferSource,
    identifier: string,
    context: VMContext,
  ): Promise<SyntheticModule> {
    // Use async `WebAssembly.compile` (rather than the sync constructor used
    // by the v24.9+ sync core) to avoid blocking the event loop on large wasm
    // modules in the legacy async path.
    const wasmModule = await WebAssembly.compile(source);
    const moduleLookup: Record<string, VMModule> = {};
    for (const {module} of WebAssembly.Module.imports(wasmModule)) {
      if (moduleLookup[module] === undefined) {
        const resolvedModule = await this.resolveModule<VMModule>(
          module,
          identifier,
          context,

View on GitHub (pinned to f49721c78e)

Solutions

Use `jest.unstable_mockModule('x', () => ({ ... }))` with an explicit factory function returning the mock namespace.
Do not call the CJS `jest.mock('x')` (without factory) for modules imported as ESM; ESM automocking is unsupported.
Ensure the factory is registered before the import: call `unstable_mockModule` in setupFilesAfterEach or at the top of the test before the dynamic `import()`.
If you need to unmock, call `jest.unstable_unmockModule('x')` rather than leaving a stub without a factory.

Example fix

// before
jest.mock('my-esm-module'); // CJS automock, no factory
import('my-esm-module'); // throws

// after
jest.unstable_mockModule('my-esm-module', () => ({
  doThing: () => 'mocked',
}));
await import('my-esm-module');

Defensive patterns

Strategy: validation

Validate before calling

// Before importing an ESM module you intend to mock, ensure a factory is registered
function mockEsm(name, factory) {
  if (typeof factory !== 'function') {
    throw new Error(`unstable_mockModule('${name}') needs a factory function`);
  }
  jest.unstable_mockModule(name, factory);
}

Prevention

Always pass a factory to `jest.unstable_mockModule` — never use bare `jest.mock` for ESM.
Register the mock factory before the dynamic `import()` of the target module.
Do not mix CJS `jest.mock` automocking with ESM imports of the same module.

When it happens

Trigger: Calling `jest.mock('x')` (CJS-style, no factory) on a module that is later imported as ESM, or calling `jest.unstable_mockModule('x')` without a factory argument. The ESM loader looks up `mockState.getEsmFactory(moduleID)`, finds nothing, and throws.

Common situations: Mixing CJS `jest.mock` automocking with ESM imports (automocking is not supported for ESM — you must provide an explicit factory); forgetting the second argument to `unstable_mockModule`; a setup file calling `jest.mock` for a module that the test imports via ESM.

Related errors

AI-assisted analysis of jestjs/jest@f49721c78e (2026-08-03). Data as JSON: /data/errors/24ee3ebb283de168.json. Report an issue: GitHub.