title: Type 'undefined' cannot be used as index type
body: <p>I am following WintellectNow React with TypeScript Tutorial. In the fifth part Sorting and filtering the author creates an interface with optional properties like below :</p>
  <pre><code>interface IWidgetToolState {
     filterCol?: WidgetTableCols;
     filterValue?: string;
     sortCol?: WidgetTableCols;
  }
  </code></pre>
  <p>There is an enum called WidgetTableCols as below :</p>
  <pre><code>enum WidgetTableCols {
      None, Name, Color, Size, Quantity, Price,
  }
  </code></pre>
  <p>In a function the author gets the value of enum like this :</p>
  <pre><code>const fName: string = 
  WidgetTableCols[this.state.sortCol].toLocaleLowerCase();
  </code></pre>
  <p>Here I am getting Type <code>undefined</code> cannot be used as an index type. If I remove <code>?</code> from the interface it works but later the author creates another function that only sets one of the state values and TypeScript says not all of the state properties are set.</p>
  <p>Can anybody let me know how to solve this issue?</p>