title: How to Update specific slice of redux state in React
body: <p>I seem to have hit a snag when updating state using redux and react-redux. When I update an individual slice of state, all of the others get removed. I know the answer to this will be simple but I can't figure it out and haven't found anything else online.</p>
  
  <p>So to clarify, here's my reducer:</p>
  
  <pre><code>const initialState = {
          selectedLevel: null,
          selectedVenue: null,
          selectedUnitNumber: null,
          selectedUnitName: null,
          selectedYear: null
  }
  
  export default (state = initialState, action) =&gt; {
      console.log('reducer: ', action);
      switch (action.type){
          case 'CHOOSE_LEVEL':
              return action.payload;
          case 'CHOOSE_VENUE':
              return action.payload;
          case 'CHOOSE_UNIT':
              return action.payload;
          case 'SHOW_COURSES':
              return action.payload;
      }
  
      return state;
  }
  </code></pre>
  
  <p>And my combine reducer:</p>
  
  <pre><code>export default combineReducers({
      workshopSelection: WorkshopSelectReducer
  });
  </code></pre>
  
  <p>So my initial state looks like this:</p>
  
  <pre><code>workshopSelection: {
      selectedLevel: null,
      selectedVenue: null,
      selectedUnitNumber: null,
      selectedUnitName: null,
      selectedYear: null
  }
  </code></pre>
  
  <p>But when I use one of my action creators, for example:</p>
  
  <pre><code>export function chooseVenue(venue){
      return {
          type: 'CHOOSE_VENUE',
          payload: {
              selectedVenue: venue
          }
      }
  }
  </code></pre>
  
  <p>I end up with state looking like this:</p>
  
  <pre><code>workshopSelection: {
      selectedVenue: 'London',
  }
  </code></pre>
  
  <p>All of the rest of the state within this object that wasn't affected by this action creator has been completely wiped out. Instead, I just want all other entries to stay as they are with their original values - <code>null</code> in this example, or whatever other value has been assigned to them.</p>
  
  <p>Hope that all makes sense.</p>
  
  <p>Cheers!</p>