title: Svelte execute function when any of group of variables changes
body: <p>In <a href="https://github.com/sveltejs/realworld/blob/master/src/routes/_components/ArticleList/index.svelte" rel="noreferrer">Svelte RealWord App</a> there is something like this:</p>
  <pre><code>$: query &amp;&amp; getData();
  </code></pre>
  <p>This calls REST API when page size or other query parameters change.</p>
  <p>I have similar situation for listing entities and do:</p>
  <pre><code> $: activePage &amp;&amp; sort &amp;&amp; pageSize &amp;&amp; getData();
  </code></pre>
  <p>This all works well (although the &amp;&amp; is a strange construct to say I want to execute <code>getData()</code> when <code>activePage</code>, <code>sort</code> or <code>pageSize</code> changes.</p>
  <p>With this approach problem arises when you want to also include variable which is evaluates to Falsy.</p>
  <p>Example, add <code>searchQuery</code> text:</p>
  <pre><code>let searchQuery = &quot;&quot;;
  $: searchQuery &amp;&amp; activePage &amp;&amp; sort &amp;&amp; pageSize &amp;&amp; getData();
  </code></pre>
  <p>Now reactivity does not work since <code>searchQuery</code> evaluates to <code>false</code>.</p>
  <p>We can do this:</p>
  <pre><code>$: activePage &amp;&amp; sort &amp;&amp; pageSize &amp;&amp; getData();
  $: searchQuery, getData();
  </code></pre>
  <p>But with this getData() gets called 2 times.</p>
  <p>Does anybody know of better approach for this?</p>