reducer.js

function processLoadingActions(loads, fails, successes, state, action) { 
  if (loads.indexOf(action.type) !== -1) {
    return { ...state, isLoading: true };
  }
  if (fails.indexOf(action.type) !== -1) {
    return { ...state, isLoading: false, error: action.error };
  }
  if (successes.indexOf(action.type) !== -1) {
    return { ...state, isLoading: false, payload: action.payload };
  }
  return state; 
}

// ... REDUCER!
function reducer(state, action) {
  state = processLoadingActions(
    ['action_load'],
    ['action_fail'],
    ['action_success'],
    state,
    action
  );
  switch (action.type) { 
    case 'action_something':
    return { ...state, data: action.payload };
    default: return state;
  }
}

reducer2.js

const defaultState = {
  isLoading: false,
  error: null,
  payload: null,
};

function loagingReducer(state=defaultState, action) {
  if (action.type.lastIndexOf('_REQUEST') !== -1) {
      return {
        ...state,
        isLoading: true,
      };
  }
  if (action.type.lastIndexOf('_SUCCESS') !== -1) {
      return {
        ...state,
        isLoading: false,
        payload: action.payload,
      };
  }
  if (action.type.lastIndexOf('_FAIL') !== -1) {
      return {
        ...state,
        isLoading: false,
        error: action.error,
      };
  }
  return state;
}