<script src="https://code.jquery.com/jquery-1.10.2.js"></script>
<div id="data"></div>
<script>
  function makeComputerID() {
    var text = "";
    var possible = "abcdefghijklmnopqrstuvwxyz";

    for (var i = 0; i < 12; i++)
      text += possible.charAt(Math.floor(Math.random() * possible.length));
    return text;
  }
  
  var id = localStorage.getItem('computer-id');
  $("#data").html("Computer ID: " + id);
  if (id == null || id.length != 12) {
    id = makeComputerID();
    localStorage.setItem('computer-id', id);
    $("#data").append("<br>ID set to: " + id);
  }
</script>

Began life as a copy of #1013944