ExpressJS - Jest & Unit Testing 

Without the parenthesis, everything inside the curly braces, { }, would need to be statements. The parenthesis wrapping the curly braces indicates the return value is an object. It's a shorthand for doing this: () => { return { ... } }

When you call next() inside the middleware function, remember that "next" function is a parameter passed to the middleware. So you would pass in a mocked function, e.g: const mockedNext = jest.fn(); Then you would assert that mockedNext was called, e.g: expect(mockedNext).toHaveBeenCalled() or expect(mockedNext).toHaveBeenCalledWith(...);

It's the same exact way, only difference is you will have TypeScript complaining about fields missing. If your user model is simple, you can just fill out all the fields. If it is complex and has a lot relations which end up with an object full of aggregated data (fields that map to objects), then you can either manually mock those objects too, or just mock the fields you need for the mock User and cast the object to a User manually to avoid TypeScript errors.

@@ansonthedev Thanks a lot Anson. Keep up the good work :). I've been following your NestJs playlist and it helped me clear the coding round of one of the interviews I appeared for.