How to test API request failures with Redux Saga?

Mark’s answer is correct. Middleware executes those instructions. But this makes your life easier: in the test, you can provide whatever you want as the argument to next(), and the generator function will receive it as a result of yield. This is exactly what saga middleware does (except that it actually fires up a request instead of giving you a fake response).

To make yield get an arbitrary value, pass it to next(). To make it “receive” an error, pass it to throw(). In your example:

it('should return a LOGIN_FAIL action', () => {
  const action = {
    payload: {
      name: 'toto',
      password: '123456'
    }
  };
  const generator = login(action);

  // Check that Saga asks to call the API
  expect(
    generator.next().value
  ).to.be.eql(
    call(api.login, action)
  );

  // Note that *no actual request was made*!
  // We are just checking that the sequence of effects matches our expectations.

  // Check that Saga reacts correctly to the failure
  expect(
    generator.throw({
      error: 'user not found'
    }).value
  ).to.be.eql(
    put({
      type: 'LOGIN_FAIL',
      payload: { error: 'user not found' }
    })
  );
});