/**
 * Shows a DOM element by setting style.display to block.
 * No effect if element does not exist
 */
function on(targetId) {
  var target = $(targetId);
  if (target && target.style.display == "none") {
    target.style.display = "block";
  }
}

/**
 * Hides a DOM element by setting style.display to none.
 * No effect if element does not exist
 */
function off(targetId) {
  var target = $(targetId);
  if (target && target.style.display != "none") {
    target.style.display = "none";
  }
}

/**
 * Toggles a DOM element's visibility by setting style.display to block or none.
 * No effect if element does not exist
 */
function onoff(targetId) {
  var target = $(targetId);
  if (target) {
    if (target.style.display == "none")
      target.style.display = "block";
    else
      target.style.display = "none";
  }
}