Greasemonkey: links con teclado

// ==UserScript==
// @name Keylinks
// @namespace programar.cloud
// @match *://*/*
// @grant none
// @require https://code.jquery.com/jquery-2.2.4.min.js
// ==/UserScript==

console.log('Initializing keyliks.');

console.log('Declaring isInViewport plugin.');
$.fn.isInViewport = function() {
  var elementTop = $(this).offset().top;
  var elementBottom = elementTop + $(this).outerHeight();

  var viewportTop = $(window).scrollTop();
  var viewportBottom = viewportTop + $(window).height();

  return elementBottom > viewportTop && elementTop < viewportBottom;
};

console.log('Creating tip template.');
let $elem=$('<a>')
.addClass('keylink')
.css({
  //position: 'absolute',
  display : 'inline-block',
  //width: '16px',
  //height: '16px',
  zIndex: 9999,
  left: '-16px',
  top : '0px',
  backgroundColor : 'yellow',
  color : 'black',
  fontSize: '60%'
});


$(document).keyup(function(evt) {
  console.log(evt.which, evt.shiftKey);
  if (evt.which !== 70 /*f*/ || evt.shiftKey === false || evt.ctrlKey === false) return;

  let $oldLinks = $('.keylink');
  if ($oldLinks.length > 0) {
    $oldLinks.detach();
  } else {
    let $links = $('a')
          .filter(function(){return $(this).isInViewport()})
          .prepend(idx => $elem.clone().text('l'+idx));
  }

});