title: TypeScript allows a property to be set but not accessed (index signature)
body: <p>This interface is an example from the TypeScript docs.<br>
  The compiler shows an error for <code>height</code>, even though it allows me to set it?</p>
  
  <pre><code>// Interface taken from https://www.typescriptlang.org/docs/handbook/interfaces.html
  interface SquareConfig {
      color?: string;
      width?: number;
      [propName: string]: any;
  }
  
  const sq = &lt;SquareConfig&gt;{
      color: 'red',
      width: 7890,
      height: 888
  }
  
  console.log(sq.height); // Property 'height' does not exist on type 'SquareConfig'
  </code></pre>