Spawing 'threads'

      function animate(id, delay) {
        var elem = document.getElementById(id);
        var x = 0;
        while (true) {
          elem.style.left = x++;
          x %= 400;
          hold(delay);
        }
      }

      spawn(function() { animate('red_box', 10); });
      spawn(function() { animate('blue_box', 30); });

• spawn(f) returns immediately, and runs function f in background