phdcomics-keynav.user.js

// ==UserScript==
// @name PhD Comics Keyboard Navigation
// @namespace http://www.everythingtastesbetterwithchilli.com
// @description Use arrow keys to navigate through the phdcomics archive.
// @include http://*phdcomics.com/comics/*
// ==/UserScript==

function findHrefForParentOf(xpth) {
  var imgIterator = document.evaluate(
    xpth,
    document,
    null,
    XPathResult.UNORDERED_NODE_ITERATOR_TYPE,
    null);
  var img = imgIterator.iterateNext();
  return img ? img.parentNode.href : null;
}

var NEXTURL = findHrefForParentOf('//img[@src="images/next_button.gif"]');
var PREVURL = findHrefForParentOf('//img[@src="images/prev_button.gif"]');

function handleKeyEvent(e) {
  var evt = e || window.event;
  var code = evt.keyCode;
  // next: 'l' (76), 'j' (74), or right arrow key (39):
  if (code === 76 || code === 74 || code === 39) {
    if (NEXTURL) {
      window.location = NEXTURL;
    }
  }
  // previous: 'h' (72), 'k' (75), or left arrow key (37):
  else if (code === 72 || code === 75 || code === 37) {
    if (PREVURL) {
      window.location = PREVURL;
    }
  }
}

window.addEventListener('keydown', handleKeyEvent, true);