no-untyped-mock-factory.md

Disallow using jest.mock() factories without an explicit type parameter (no-untyped-mock-factory)

🔧 This rule is automatically fixable by the --fix CLI option.

By default, jest.mock and jest.doMock allow any type to be returned by a mock factory. A generic type parameter can be used to enforce that the factory returns an object with the same shape as the original module, or some other strict type. Requiring a type makes it easier to use TypeScript to catch changes needed in test mocks when the source module changes.

Warning

This rule expects to be run on TypeScript files only. If you are using a codebase that has a mix of JavaScript and TypeScript tests, you can use overrides to apply this rule to just your TypeScript test files.

Rule details

This rule triggers a warning if mock() or doMock() is used without a generic type parameter or return type.

The following patterns are considered errors:

jest.mock('../moduleName', () => {
  return jest.fn(() => 42);
});

jest.mock('./module', () => ({
  ...jest.requireActual('./module'),
  foo: jest.fn(),
}));

jest.mock('random-num', () => {
  return jest.fn(() => 42);
});

The following patterns are not considered errors:

// Uses typeof import()
jest.mock<typeof import('../moduleName')>('../moduleName', () => {
  return jest.fn(() => 42);
});

jest.mock<typeof import('./module')>('./module', () => ({
  ...jest.requireActual('./module'),
  foo: jest.fn(),
}));

// Uses custom type
jest.mock<() => number>('random-num', () => {
  return jest.fn(() => 42);
});

// No factory
jest.mock('random-num');

// Virtual mock
jest.mock(
  '../moduleName',
  () => {
    return jest.fn(() => 42);
  },
  { virtual: true },
);