title: how to tell typescript the value exist?
body: <p>I have the following code</p>
  <pre><code>   options: { url?: string } = {};
     if (!Checker.present(this.options.url)) {
        throw new Error('Options must have a url');
     }
     new CustomUrl(this.options)
  </code></pre>
  <p>but I get an error cause <code>CustomUrl</code> <strong>require</strong> an url, and my interface is optional.</p>
  <p>My checked function looks like this</p>
  <pre><code>  static present(value: any) {
      return value !== undefined &amp;&amp; value !== null;
    }
  </code></pre>
  <p>but how do I tell typescript that I checked, and that value is present ? (counting that I can't use the <code>!</code> in my code</p>