popup.js

// Pure JS:
document.addEventListener('DOMContentLoaded', function() {
    document.getElementById("myInput").addEventListener("keyup", handler);
  });
  
  // The handler also must go in a .js file
  function handler() {
       var input, filter, ul, li, a, i, txtValue;
      input = document.getElementById("myInput");
      filter = input.value.toUpperCase();
      ul = document.getElementById("myUL");
      li = ul.getElementsByTagName("li");
      for (i = 0; i < li.length; i++) {
          a = li[i].getElementsByTagName("a")[0];
          txtValue = a.textContent || a.innerText;
          if (txtValue.toUpperCase().indexOf(filter) > -1) {
              li[i].style.display = "";
              
          } else {
              li[i].style.display = "none";
          }
      }
  }