Update

public/assets/js/mastodon.js

@@ -1,3 +1,6 @@
+let query = window.location.pathname;
+
+if(query == "/mastodon") {
     let tplUrl = 'json/mastodon-tpl.html';
     let response = await fetch(tplUrl);
     var tpl = "";
@@ -16,7 +19,7 @@ try {
 
     } catch(error) {
         // DOMException: The user aborted a request.
-    console.log('Error: ', error)
+        console.log('Error: ', error);
         log.innerHTML = "<p>1) Ой всё сломалось :(</p>";
     }
 
@@ -104,3 +107,4 @@ async function openInNewTab(url) {
     }
 
     fetchData();
+}

public/index.html

@@ -287,6 +287,8 @@
 
     <script src="assets/js/script.js"></script>
 
+    <script src="assets/js/mastodon.js" type="module" async="async"></script>
+
     <script>
 
     </script>

public/mastodon.tpl.html

@@ -2,5 +2,4 @@
         <link rel="stylesheet" href="assets/css/mastodon.css" media="all">
 
         <div id="log"></div>
-        <script src="assets/js/mastodon.js" type="module" async="async"></script>
       </div><!-- /.container -->