how to mock exported functions in module in Jest

jest-func-module-mock.md

• Good SO post: https://stackoverflow.com/questions/53174202/how-to-mock-a-function-in-jest

• Good Medium post: https://medium.com/@qjli/how-to-mock-specific-module-function-in-jest-715e39a391f4

• Good discussion here: jestjs/jest#936 (comment)

• Personal experience

• import the module as a whole. for instance, import * as Blah from '../blah';

• const mock = jest.spyOn(Blah, '<mocked_function_name>');

• mock.mockImplementationOnce( () => {});