title: Dynamic key in immutability update helper for array
body: <p><a href="https://facebook.github.io/react/docs/update.html">https://facebook.github.io/react/docs/update.html</a></p>
  
  <p>I have an update function that receives an index along with the event so as to alter the value for that specific item in the array.</p>
  
  <p>How do I get <code>index</code> to be evaluated instead of treated as a set key in this scenario? Is it even possible?</p>
  
  <pre><code>updateValue: function(index, e) {
    var items = React.addons.update(this.state.items, {
     index: {
       amount: {$set: e.target.value}
     }
    });
    this.setState({
      items: items
    })
  }
  </code></pre>
  
  <p>Right now this obviously doesn't work because it's attempting to update <code>this.state.items['index']['amount']</code> which is not set, when I want to modify <code>this.state.items[1]['amount']</code> for an index of 1.</p>