Bookmarklet: See archive.com's most recent copy of the current page from the Wayback Machine

goWayback.js

// minified code for pasting into a bookmarklet:
javascript:(function(){var a="https://web.archive.org/web/";if(window.loadTimeData)location.href=a+window.loadTimeData.data_.summary.failedUrl;else document.location=a+document.location})()

// full, unminified code with comments

/**
 * Summary:
 *     Append the current location URL to a base Archive.org Wayback Machine URL and navigate there
 * 
 * Example:
 *     Hacker News is currently down for maintenance, so you want to check out the most recent saved copy of the HN front page
 *       1. Navigate to http://news.ycombinator.com; oh no, 404
 *       2. Ask for most recent archived version of HN
 *            desired_url = http://news.ycombinator.com
 *            archive_base_url = 
 *            new_url = archive_base_url + desired_url
 *                    = https://web.archive.org/web/http://news.ycombinator.com
 *       3. Archive.org redirects to a copy saved yesterday morning, yay content!
 */
(function(){
  // archive.org base URL
  // note: the the path segment following /web/ determines the wayback machine's search behavior; basically there are 4 cases:
  //   * URL immediately follows with no path segment between, .../web/<URL>: just take me to the most recent saved copy
  //   * bare asterisk before the URL, .../web/*/<URL>: show me a listing of every copy ever saved
  //   * partial date in yyyyMMddHHmmss format, followed by an asterisk: show me a all copies whose dates match the pattern; examples:
  //             .../web/201503*/<URL>     show me all saved copies of <URL> from March 2015
  //             .../web/20000101*/<URL>   show me all saved copies of <URL> from New Years Day 2000
  //   * complete date string with no asterisk: take me to the nearest copy fillowing the specified date; examples:
  //             .../web/20000101000000/<URL>    take me to the first copy on New Years Day 2000
  
  var archive = "https://web.archive.org/web/"; // take me straight to the most recent saved copy of the page
  //var archive = "https://web.archive.org/web/*/"; // show me all saved copies on a calendar
  //var archive = "https://web.archive.org/web/2016*/"; // show me the saved copies from 2016
  
  // works in Chrome even on the "Borked" page; not tested in Firefox or other browsers
  if (window.loadTimeData)
    location.href = archive+window.loadTimeData.data_.summary.failedUrl;
  // only works when the page has successfully loaded some content; won't work from browser error pages
  else
    document.location=archive+document.location;})()