Using Jest, how can I check that an argument to a mocked function is a function?

Question

I'm trying this:

expect(AP.require).toBeCalledWith('messages', () => {}) 

where AP.require is a mocked function that should receive a string and a function as the second argument.

Test fails with the message:

Expected mock function to have been called with:   [Function anonymous] as argument 2, but it was called with [Function anonymous] 

Answer 1

To assert any function, you can you use expect.any(constructor):

So with your example it would be like this:

expect(AP.require).toBeCalledWith('messages', expect.any(Function)) 

Answer 2

The problem is that a function is an object and comparing objects in JavaScript will fail if they are not the same instance

() => 'test' !== () => 'test' 

To solve this you can use mock.calls to check the parameters seperataly

const call = AP.require.mock.calls[0] // will give you the first call to the mock expect(call[0]).toBe('message') expect(typeof call[1]).toBe('function')