reducer.js

import {
  SELECT_MIDI_CONTROLLER,
  SELECT_NUMBER_OF_KEYBOARD_OCTAVES,
  SET_WEB_MIDI_SUPPORTED,
} from './action-types';

export const initialState = {
  selectedDeviceName: 'APC Key 25',
  webMidiSupported: false,
};

function seePianoKeyApp(state = initialState, action) {
  switch (action.type) {
    case SELECT_MIDI_CONTROLLER:
      return {
        ...state,
        selectedDeviceName: action.selectedDeviceName
      };
    case SELECT_NUMBER_OF_KEYBOARD_OCTAVES:
      return {
        ...state,
        numberOfKeyboardOctaves: action.numberOfKeyboardOctaves
      }
    case SET_WEB_MIDI_SUPPORTED:
      return {
        ...state,
        webMidiSupported: action.webMidiSupported,
      };
    default:
      return state;
  }
}

export default seePianoKeyApp;

reducer.spec.js

import reducer, { initialState } from './reducer';
import * as types from './action-types';

describe('reducer', () => {
  it('should return initial state', () => {
    expect(reducer(undefined, {})).toEqual(initialState);
  });

  it('should handle SELECT_MIDI_CONTROLLER', () => {
    const fakeMidiController = 'fake-controller';

    expect(
      reducer(undefined, {
        type: types.SELECT_MIDI_CONTROLLER,
        selectedDeviceName: fakeMidiController,
      }),
    ).toEqual({
      ...initialState,
      selectedDeviceName: fakeMidiController,
    });
  });

  it('should handle SELECT_NUMBER_OF_KEYBOARD_OCTAVES', () => {
    const fakeNumberOfOctaves = 'fake-controller';

    expect(
      reducer(undefined, {
        type: types.SELECT_NUMBER_OF_KEYBOARD_OCTAVES,
        numberOfKeyboardOctaves: fakeNumberOfOctaves,
      }),
    ).toEqual({
      ...initialState,
      numberOfKeyboardOctaves: fakeNumberOfOctaves,
    });
  });

  it('should handle SET_WEB_MIDI_SUPPORTED', () => {
    expect(
      reducer(undefined, {
        type: types.SET_WEB_MIDI_SUPPORTED,
        webMidiSupported: true,
      }),
    ).toEqual({
      ...initialState,
      webMidiSupported: true,
    });
  });
});