badReducer.js

function brokenReducer(state = initialState, action) {
  switch(action.type) {
    case 'INCREMENT':
      // NO! BAD: this is changing state!
      state.count++;
      return state;

    case 'DECREMENT':
      // NO! BAD: this is changing state too!
      state.count--;
      return state;

    default:
      // this is fine.
      return state;
  }
}