[proposal] expect(fn).toLog(str, level = 'log')

🚀 Feature Proposal

Proposal is to create “sibling” methods to .toThrow, .toThrowErrorMatchingSnapshot() & .toThrowErrorMatchingInlineSnapshot that would test if console[level] has been called during the fn invocation

Motivation

Make testing console.log/console.warn/console.error calls easier - testing it with jest.fn()/mocks is somewhat cumbersome.

Example

test('getItemProps logs a helpful error when no object is given', () => {
  expect(() =>
    render(
      <Downshift>
        {({getItemProps}) => (
          <div>
            <span {...getItemProps()} />
          </div>
        )}
      </Downshift>,
    ),
  ).toLogMatchingSnapshot('error')
})

Pitch

IMHO it’s as helpful as toThrow methods and could be included in the core.

[mattphillips]

Hey @Andarist I’m not sure I understand this as a matcher, its unclear that the matcher is actually watching the console to make assertions against it.

This behaviour feels closer to Jest’s jest.useFakeTimers(), perhaps a jest.useFakeConsole() which mocks all of the available methods would be more suitable?

That way you can use all of the existing matchers maybe something like:

test('getItemProps logs a helpful error when no object is given', () => {
  jest.useFakeConsole();
  render(
    <Downshift>
      {({getItemProps}) => (
        <div>
          <span {...getItemProps()} />
        </div>
      )}
    </Downshift>,
  );
  expect(console.error).toMatchSnapshot();
})

This is a little more verbose, but it is explicit over what you are doing.

An issue with wrapping the log code as a function that is ran by the matcher is that you may want to use the returned value (in this example the rendered component) in another assertions and now you would not be able to do that.

Let me know your thoughts? 😄

EDIT: cc/ @thymikee @SimenB @rickhanlonii I’d be curious to know your thoughts too 😉

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