elements scroll by jk shortcut

gistfile1.js

function registerJumpElements(elements) {

  var KEYCODE_J = 74;
  var KEYCODE_K = 75;

  var targetElements = elements;
  var currentIndex = -1;

  var scollElement = function(event) {

     if (event.keyCode != KEYCODE_J && event.keyCode != KEYCODE_K) {
       return;
     }

     // 次の位置を計算
     var nextIndex =
             currentIndex + ((event.keyCode == KEYCODE_J) ? 1 : -1);
 
     if (nextIndex < 0) {
       nextIndex = 0;
     } else if (nextIndex >= targetElements.length) {
       nextIndex = targetElements.length - 1;
     }

     // 要素が表示されるようにスクロール
     targetElements[nextIndex].scrollIntoView();
     currentIndex = nextIndex;
   }

  if (document.addEventListener) {
    document.addEventListener(
      'keydown', scollElement, false);
  } else {
    // IE8など
    document.attachEvent(
      'onkeydown', scollElement);
  }
}