title: How do I update an Immutable array in a React/Redux environment?
body: <p>I have a React application that I'm using Redux to manage state.  I have the reducer working correctly, and I've successfully, passed the action and data I need. The problem is, up to this point, I've only worked with strings, so I've been using the <code>.set</code> method to update the state.</p>
  
  <p>Now, I need to update an array by a specific index number, but I'm not having any luck. </p>
  
  <p>Through my research, I've tried utilizing the <code>.update</code> method, but that's not working for me either. Another solution suggested mapping the state and modifying the index number, but that seemed a bit overkill.</p>
  
  <p>Here is what I have for my state object:</p>
  
  <pre><code>const initialState = fromJS({
      items: []
  });
  </code></pre>
  
  <p>Through a <code>componentWillMount</code> function, I populate the items array using an AJAX call successfully by calling <code>state.set('items', action.items);</code></p>
  
  <p>Now, I can't figure out how to update a single item in that array by an index number. I was able to get the index number using Immutable, but I don't know how to properly update the single array object. </p>
  
  <p>My reducer is setup like so:</p>
  
  <pre><code>function reducerName(state = initialState, action) {
      switch(action.type) {
          case TOGGLE_COMPLETED:
              let index = state.get('items').findIndex( (item) =&gt; { return item.id === action.reminder_id});
              return state
                  .setIn(['items', index, 'completed'], true)
      }
  }
  </code></pre>
  
  <p>Any ideas on how I can update a single index within the array instead of the entire array?</p>