title: TypeScript: augmenting built-in types
body: <p>how does one augment one of the 'built-in' types? eg Array?</p>
  
  <p>In JS, I'd do something like</p>
  
  <pre><code>Array.prototype.shuffle = function () { ... };
  </code></pre>
  
  <p>what's the equivalent in TypeScript?</p>