IE11

[xiaohutai]

Some code used will not work in IE11. I suggest using Babel in your workflow.

Spread Operator

-let options = {
-    icon: icon,
-    title: item.title
-};
+let options  = item;
+options.icon = icon;

Template Literals

Use String concatenation instead.

-marker.bindPopup(`
-    <h2>${item.title}</h2>
-    <p>${item.description}</p>
-    <a href='${item.link}'>Read more</a>
-`);
+let html = '<h2>' + item.title + '</h2>';
+html    += '<p>' + item.description + '</p>';
+html    += '<a href="' + item.link + '">Read more</a>';
+marker.bindPopup(html);

Or rather duplicate from a DOM element (note: this may be slower).
This is assuming you don't already use something like React or Vue.

Fetch

Use a polyfill: https://github.com/github/fetch and https://github.github.io/fetch/