document.onkeydown = function(e) {
  var key;
  
  if ( document.all ) { key = event.keyCode; }
  else                { key = e.which;       }

  // http://www.scottklarr.com/topic/126/how-to-create-ctrl-key-shortcuts-in-javascript/
  // 72 = H, 74 = J, 82 = R, 83 = S
  if ( key == 72 && previous && previous != '' ) { window.location = previous; }
  if ( key == 74 && next && next != '')          { window.location = next;     }
  if ( key == 82 && random && random != '' )     { window.location = random;   }
  if ( key == 83 )                               { switchCensoring();          }
}