Extend Date prototype with a interface Typescript
<p>I want to add a getWeekNumber function to the Date prototype in javascript / typescript. I want to do it with an interface becease otherwise I get a error that he does not know the method getWeekNumber().</p>

<p>First I tried with a standart Date interface like this:</p>

<pre><code>interface Date {
    getWeekNumber(): number;
}
</code></pre>

<p>This had the resolve that all the methods of the Date are not call able anymore. </p>

<p>I want to know of there is a way to extend the Date with an interface.</p>