typing jest mocks in tests

Extract of a jest test:

jest.mock('../../js/comm/ajax');
import ajax from '../../js/comm/ajax';
describe('ConfigurationProvider', () => {
  it('calling the fetchConfig() should return a promise', () => {
    const expectedCfg = {x:'y'};
    ajax.mockReturnValueOnce(
      Promise.resolve({body:expectedCfg})
    );
    ...
  });
});

The original ajax module exported a function so jest will intercept and return a JestMockFn instead. Flow says property mockReturnValueOnce not found in statics of function. How do I let flow know of this? Should I cast the ajax symbol to the JestMockFn type? And how?

Thanks 😃

[jeffmo]

Yea, the wrapper-function is what i did in the flow-typed CLI:

https://github.com/flowtype/flow-typed/blob/master/cli/src/lib/__tests__/libDefs-test.js#L30-L32

I used any there, but it would probably be better to have the jest libdef export the interface of a mock and just have it cast to that:

import type {JestMockT} from "jest";
function _mock(mockFn) {
  return ((mockFn: any): JestMockT);
}

[aaronjensen]

Perhaps adding a utility function would be handy:

const mock = (mockObject: any): MockObject => mockObject

mock(ajax).mockReturnValueOnce(...)

You could even potentially make that function polymorphic and strongly type things like mockReturnValueOnce.