title: Object assign with dynamic variable
body: <p>I have this <code>initialState</code> in my Redux store:</p>
  
  <pre><code>const initialState = {
    isFetching : false,
    active     : {}
  }
  </code></pre>
  
  <p>Where <code>active</code> is an object.</p>
  
  <p>Now I have an action that <em>should</em> append or add a property to <code>active</code>’s <code>data</code> property, like so:</p>
  
  <pre><code>[DASHBOARD_TEMPLATE_DATA_RECEIVE]: (state, action) =&gt; {
      return Object.assign({}, state, {
        isFetching   : false,
        active       : Object.assign({}, active, {data[action.key]: action.data})
      })
  }
  </code></pre>
  
  <p>As you can see, <code>data[action.key]</code> is not permitted. How do I do it?</p>