title: type definition in Typescript
body: <p>From <a href="https://www.typescriptlang.org/" rel="nofollow noreferrer">typescriptlang.org</a> I found this code:</p>
  <pre><code>type Arrayish&lt;T&gt; = {
    length: number;
    [x: number]: T;
  };
  </code></pre>
  <p>Can anybody help me find out what is the meaning of [x: number]: T?</p>