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<!DOCTYPE html>
<html>
<title>Sort a HTML List Alphabetically</title>
<body>

<p>Click the button to sort the list ascending (A to Z):</p>
<p>Click again to sort the list descending (Z to A):</p>

<button onclick="sortListDir()">Sort</button>

<ul id="id01">
  <li>Oslo</li>
  <li>Stockholm</li>
  <li>Helsinki</li>
  <li>Berlin</li>
  <li>Rome</li>
  <li>Madrid</li>
</ul>

<script>
function sortListDir() {
  var list, i, switching, b, shouldSwitch, dir, switchcount = 0;
  list = document.getElementById("id01");
  switching = true;
  // Set the sorting direction to ascending:
  dir = "asc"; 
  // Make a loop that will continue until no switching has been done:
  while (switching) {
    // start by saying: no switching is done:
    switching = false;
    b = list.getElementsByTagName("LI");
    // Loop through all list-items:
    for (i = 0; i < (b.length - 1); i++) {
      //start by saying there should be no switching:
      shouldSwitch = false;
      /* check if the next item should switch place with the current item,
      based on the sorting direction (asc or desc): */
      if (dir == "asc") {
        if (b[i].innerHTML.toLowerCase() > b[i + 1].innerHTML.toLowerCase()) {
          /* if next item is alphabetically lower than current item,
          mark as a switch and break the loop: */
          shouldSwitch = true;
          break;
        }
      } else if (dir == "desc") {
        if (b[i].innerHTML.toLowerCase() < b[i + 1].innerHTML.toLowerCase()) {
          /* if next item is alphabetically higher than current item,
          mark as a switch and break the loop: */
          shouldSwitch= true;
          break;
        }
      }
    }
    if (shouldSwitch) {
      /* If a switch has been marked, make the switch
      and mark that a switch has been done: */
      b[i].parentNode.insertBefore(b[i + 1], b[i]);
      switching = true;
      // Each time a switch is done, increase switchcount by 1:
      switchcount ++;
    } else {
      /* If no switching has been done AND the direction is "asc",
      set the direction to "desc" and run the while loop again. */
      if (switchcount == 0 && dir == "asc") {
        dir = "desc";
        switching = true;
      }
    }
  }
}
</script>
</body>
</html>