title: Why .filter predicate returns boolean and not any?
body: <p>So i have the following code  </p>
  
  <pre><code>let someValue = Observable.of([{'id': 1}, {'id': 2}, {'id': 3}, undefined, undefined]);
  </code></pre>
  
  <p>so now, if i want to filter undefined values, i either have to</p>
  
  <pre><code>someValue.filter(val =&gt; !!val) OR
  someValue.filter(val =&gt; val !== undefined)
  </code></pre>
  
  <p>in TS, the .filter method returns typeof Any, so a filter method would work like  </p>
  
  <pre><code>let someValue = [{'id': 1}, {'id': 2}, {'id': 3}, undefined, undefined];
  someValue.filter(val =&gt; val)
  </code></pre>
  
  <p>and that would filter any objects that are undefined, so i was wondering why .filter in RxJS predicate is boolean instead of any?</p>