title: Specifying an array's length in its type signature
body: <p>How do I specify an <code>Array'</code>s length in its type signature in TypeScript?</p>
  <pre><code>declare const a: Array&lt;any&gt;[20]; // wishful syntax
  ...
  foo(a[10]); // fine
  foo(a[100]); // type error: index is out of bounds
  </code></pre>