Hack 94. Compare Book Prices

Add competitors' prices to online book retailers.

When the Web was new, pundits and trade magazines hyped the possibility of agents that followed you around online and fetched the lowest prices, or recommended similar sites, or made your coffee, or something. The hype died down quickly, but many of the best ideas have eventually resurfaced in one form or another. It turns out that most people don't actually care about some bureaucrat's idea of "similar sites," but everybody cares about finding the lowest prices.

This hack adds a floating window to online bookstores, such as Amazon. com, that shows you the price for the same book on other sites. Really.

11.6.1. The Code

This user script runs on several online book stores:

• Amazon.com

• BarnesAndNoble.com

• Powells.com

• Half.com

• Buy.com

The script parses the ISBN a globally unique identifier for the book you're viewing and uses it to fetch pricing information from other sites. Sites like Amazon.com provide an official web services API for getting this information; for other sites, the script relies on tried-and-true techniques of screen scraping.

By default, this script adds associate IDs to the links it constructs, which gives the script's author a small referral fee. If you dislike this, or you have your own associate ID, you can change the first few lines of code to define your own IDs or remove them altogether.

Save the following user script as bookburro.user.js:

 // ==UserScript== // @name Book Burro - Remixing the bookstore // @namespace http://overstimulate.com/userscripts/ // @description Find the cheapest books // @include http://amazon.com/* // @include http://www.amazon.com/* // @include http://www.powells.com/* // @include http://half.ebay.com/* // @include http://buy.com/* // @include http://www.buy.com/* // @include http://search.barnesandnoble.com/* // @include http://barnesandnoble.com/* // @include http://www.barnesandnoble.com/* // @exclude // ==/UserScript== // based on code by Jesse Andrews and Britt Selvitelle  // and included here with their gracious permission // Change these as desired var amazon_associate_code = 'anotherjesse-20'; var amazon_dev_key = '0XYJJ825QSB9Q7F2XN02'; var bn_associate_code = '41456445'; var half_associate_code = '1698206-1932276'; function checkISBN( isbn ) { try {        isbn=isbn.toLowerCase().replace(/-/g,'').replace(/ /g,''); if (isbn.length != 10) return false; var checksum = 0; for (var i=0; i<9; i++) { if (isbn[i] == 'x') { checksum += 10 * (i+1); } else { checksum += isbn[i] * (i+1); } } checksum = checksum % 11; if (checksum == 10) checksum = 'x'; if (isbn[9] == checksum) return isbn; else return false; } catch (e) { return false; }  } function dom_createLink(url, txt, title) {  var a = document.createElement("a"); a.setAttribute("href", url); a.setAttribute("style", "color: #00a; text-decoration: none; " + "font-weight: bold"); if (title) a.setAttribute("title", title); a.appendChild(document.createTextNode(txt)); return a; } function add_site(url, title, loc_id ) { var a = dom_createLink( url, title, title + ' Search'); var b = document.createElement("b"); b.innerHTML = 'fetching'; b.setAttribute("id", loc_id); var tr = document.createElement("tr"); var td_left = document.createElement("td"); var td_right = document.createElement("td"); td_left.appendChild(a); td_right.appendChild(b); td_right.setAttribute("align", "right"); tr.appendChild(td_left); tr.appendChild(td_right); return tr; } function str2xml(strXML) {  //create a DOMParser var objDOMParser = new DOMParser(); //create new document from string var objDoc = objDOMParser.parseFromString(strXML, "text/xml"); return objDoc; } function int2money( cents ) {  var money = "$" if (cents< 100) {  money = money + '0.'; } else {  money = money + Math.floor(cents/100) + '.'; } cents = cents % 100; if (cents < 10) money = money + '0'; money = money + cents; return money; } function run_queries(isbn) { var errmsg = 'Either there are no books available,\\' + 'or there is a parsing error because of\\n' + 'some change to their website.\\n\\n' + 'Not everyone has a nice webservice like Amazon'; ////// AJAX for BN.com ///// GM_xmlhttpRequest({method:"POST", url:'http://search.barnesandnoble.com/booksearch/isbninquiry.asp?' + 'isbn='+isbn, data:"", onload:function(result) { try { document.getElementById('burro_bn').innerHTML =  result.responseText.match( 'priceRightBNPrice[^>]*>\([^<]*\)</')[1]; } catch (e) { document.getElementById('burro_bn').parentNode.innerHTML =  '<a href="javascript: alert(\''+errmsg+'\');">none</a>'; } } }); ////// AJAX for Buy.com ///// GM_xmlhttpRequest({method:"POST", url:'http://www.buy.com/retail/GlobalSearchAction.asp?qu='+ isbn, data:"", onload:function(result) { try {  document.getElementById('burro_buy').innerHTML = result.responseText.match( 'productPrice[^>]*>\([^<]*\)</')[1]; } catch (e) {  document.getElementById('burro_buy').parentNode.innerHTML = '<a href="javascript: alert(\''+errmsg+'\');">none</a>'; } } }); ////// AJAX for half.com ///// GM_xmlhttpRequest({method:"POST", url:'http://half.ebay.com/search/search.jsp?' + 'product=books:isbn&query='+isbn, data:"", onload:function(result) { try {  document.getElementById('burro_half').innerHTML =  result.responseText.match( 'Best[^P]*Price[^\$]*\([^<]*\)<')[1]; } catch (e) {  document.getElementById('burro_half').parentNode.innerHTML =  '<a href="javascript: alert(\''+errmsg+'\');">none</a>'; } } }); ////// AJAX for amazon.com ///// GM_xmlhttpRequest({method:"POST",  url:'http://xml.amazon.com/onca/xml3?t=' + amazon_associate_code +   '&dev-t=' + amazon_dev_key +  '&type=lite&f=xml&mode=books&AsinSearch='+isbn, data:"",  onload:function(result) {  var x = str2xml( result.responseText );  var ourprices = x.getElementsByTagName('OurPrice');   if (ourprices.length == 0) { document.getElementById('burro_amazon').parentNode. innerHTML =  '<a href="javascript: alert(\''+errmsg+'\');">none</a> ';  } else { document.getElementById('burro_amazon').innerHTML = ourprices[0].childNodes[0].nodeValue;  }  var usedprices = x.getElementsByTagName('UsedPrice');  if (usedprices.length == 0) { var elmMarket = document.getElementById('burro_     amazonmarket'); elmMarket.parentNode.innerHTML =  '<a href="javascript: alert(\''+errmsg+'\');">none</a>    '; } else { document.getElementById('burro_amazonmarket').innerHTML = usedprices[0].childNodes[0].nodeValue; } } });  var msg = 'We want to check with them regarding the traffic of querying '+  'for prices from their site on every click…'; document.getElementById('burro_powell').parentNode.innerHTML = '<a href="javascript: alert(\''+msg+'\');">(info)</a>'; } function burro( location, isbn ) { var elmWrapper = document.createElement("div"); elmWrapper.setAttribute("title","Click triangle to expand/collapse"); elmWrapper.setAttribute("style",'position:fixed;z-index:99;top:15px;' + 'left:15px;background-color:#ffc;border:1px solid orange;' + 'padding:4px;text-align:left;opacity:.85;font:8pt sans-serif;' + 'overflow:hidden;width:200px;height:15px;margin-bottom:15px;'); var elmCaret = document.createElement("img"); elmCaret.setAttribute("style", "top:-10px"); elmCaret.setAttribute("src", 'data:image/png;base64,iVBORw0KGgoAAA' + 'ANSUhEUgAAAAsAAAALCAYAAACprHcmAAAABmJLR0QA/wD/AP+gvaeTAAAAC' +  'XBIWXMAAAsTAAALEwEAmpwYAAAAB3RJTUUH1QQYCR020Q08hgAAAB10RVh0' + 'Q29tbWVudABDcmVhdGVkIHdpdGggVGhlIEdJTVDvZCVuAAAAiklEQVQY07X' + 'PIQoCURSF4e8NAzYxGicNuAa1WlyCO3AlZnfiNgwahQFxikkcBIsGfaZpzg' + 'ODJ/4c/nMvPyR8g7EsephgH6q6aXnWIelhjkUsi0EL88TqFUfMYlnscMoS5' +  'wUccMYS4yxhfuGNPho88oQ5xxQjrHHpKkcMccMqVPU99eATG2zb4n/zAS4O' + 'HrV1hIB/AAAAAElFTkSuQmCC'); elmCaret.setAttribute("id", "hide_show_elmCaret"); elmWrapper.appendChild( elmCaret ); var elmTitle = document.createElement("img"); elmTitle.setAttribute("style", "padding-left:6px"); elmTitle.setAttribute("src", 'data:image/gif;base64,R0lGODlheAAO' + 'AOYAAAAAAOmUUUOGfx1eVj8/M1ijnqBhLrmFVv//zNi6oCUIAO/vwJqEbGk' + '4EL+/mm9vWZmZmVpaWvfAl7ydiC8vJo+PcwAPCaJtSylNS////4tCC9/fs0' + 'MeAa+vjefy9LR1QpnMzH9/Zny3s7WIazttbG5MMVyVj09PQJ+fgMvO11UwE' + 'wMWGq10R5K6ugg2Nh8fGdiecOaxhdiKTAAACZlqPnWtqu/dxTxlYXNeS1o6' +  'IU8rEZqytlp2d4p4bDZ/dn+Eh4pWL2JNOFKLhf/frtusjtGFS750N8ulfo1' +  'qVM/Ppm2HhsSQa615UvSlYy5HS9rb1hAEDqdvPjgpJg8PDP/vx4S2s6toMz' + 'RERKmlmrS8vkFfXwAhHnI7Ek06K+3x8D92c6/M0DNmZk5nb4xNGr+8s9uNT' + '8x9OysRAtSniJWOlq6GYwAICZGtsP+4dGVCLkaVj4GpqF9fTMuLW8q/ra1r' + 'OV1UTXpKJmaZmb+SabGzstWXZEtMTmg9G4RZN4rFwVZeYSH5BAUUAAgALAA' + 'AAAB4AA4AAAf/gAiCg4MzM4SIgyo5hGcKj1JTiZOUlZaXmJmagmtrPEprlR' + 'N9TA0ICjpIaiMjSEhdm7Gys7SInXd3JiQ3oZN8Bx9jXGpoaEtLFzRumkkEK' + 'LTNz4nRtbWGbHBVJttCJBaTDQYaYzASSzAHQCxEmw4AIbTu8Iny1bMzWTtw' + 'cH4Fbz4DBHhLZGfMmCJy5NCwY0qWuwdxCHQYVIHAiYmCkkR8sABBvQ4hkgx' + '6GBFjCGknPYYkUCGEgwcnNix4QCCOyEsrVoBpUUOEvwIAw4T5RkjFmA8faP' + 'DhU8sdgAcUADhA8ODpCQATN0yhEGKrx3coAMQh5BSqVAQACAhKiyAEgCnv/' + '94CAICAwpQQdjdcmiFmRQuf/nwICDMAA4ZeCLrouBClMR8VJUrMcjd2A4CY' + 'lwVNeYEgDgCRFQCgcHdVLVmxCCyfQGuardvVaD8jCFsBQRLUllY40eICRA0' + 'BwL8I7ZRhUA8VNAx8sIOGSoISy2LVYz2dAF3rguQ5xT3y3Vq1bFm3PSt+/N' + 'TyliyIwbAFhB+BJIinKC5oAh8WBqzAoEKFA5ISONTxRwqZPITAApfdNhYCL' + '0hylV60uUNBVedl99SBmYXnGnnhhfaMapmsgYEY7bWwxRoZZJBCGlAMMgEQ' + 'dMhghR5DxKAADWokYMMTWeyBiTtTdODZM3Z1EBo8HYjlgL9dC8izwBSmWRj' + 'kkAhMMeWG54WnFQUOeFbhJWtoocWJHqj4gyGE6MBCGQHIUIQZetBwAB5qBE' + 'GGF3n8KNYLFx541RQPDIICnxSIVE9Y0nwVB5+BzgbXCRSo5VaWUSYR1QuJh' + 'niFBRlkAQEAiA1yBgtFtNGEEUbQYUAUeKDBABYZPBGBPbTWWs1cP4BKyRk0' + '6NFEAKdawUIUTBBhgxfI+mjrssxiMpclXdhBRxlmmJEUDTQckcAcXnjwRKP' + 'NhltrIAA7'); elmWrapper.appendChild( elmTitle ); var elmCloseBox = document.createElement("img"); elmCloseBox.setAttribute("src", 'data:image/png;base64,iVBORw0KG' + 'goAAAANSUhEUgAAAAwAAAAMCAYAAABWdVznAAAABmJLR0QA/wD/AP+gvaeT' + 'AAAACXBIWXMAAAuJAAALiQE3ycutAAAAB3RJTUUH1QQYCDcSg6d+SAAAAPB' + 'JREFUKM+Fkr1qAkEURs9dnWBrJT5IHkEw4gtEsBQs/AlpkiKlJGnEJlqIjR' + 'brPoAQYhPio1hGsPAHFoW5KSIxo7J7qvlgDty534j6Rolgt987OQnA7XuEs' + 'TuegwIeMYiIkx2hVnsjCL7+su9/0mz2Lox0oNOpUiw+kc2mUVWGww8mkxYi' + 'YK09F4xJMho9kMs9IiJMpy8Y83vFWkUTCVcAWCxWLJcrRIT1OiSTOczuCXi' + 'eK2y3IeXyK4PBPZtNSKn0zGzWJpW6uvyGer1LpVIgn78GYD7/ptHo0e/fHb' + 'emvtHIHv4zvonv4ayXuK9xyg8qt0tfe9qKPAAAAABJRU5ErkJggg=='); elmCloseBox.setAttribute("style", 'position:absolute;left:190px;' + 'top:3px;margin:2px;width:12px;height:12px;background-color:' + '#ffb;border:none;line-height:8px;text-align:center;'); elmCloseBox.setAttribute("title","Click To Remove"); elmCloseBox.addEventListener('click', function() { this.parentNode.style.display = "none"; }, true); elmWrapper.appendChild(elmCloseBox); var elmAbout = document.createElement("a"); var elmAboutImg = document.createElement("img"); elmAboutImg.setAttribute("border", "0"); elmAboutImg.setAttribute("src", 'data:image/png;base64,iVBORw0KG' + 'goAAAANSUhEUgAAAAwAAAAMCAYAAABWdVznAAAABmJLR0QA/wD/AP+gvaeT' + 'AAAACXBIWXMAAAuJAAALiQE3ycutAAAAB3RJTUUH1QQYCDkprC+64gAAAOd' + 'JREFUKM+FkrFOAkEURc9ddrJZCgsqCwtCIiUFJa2df2CwNXRS8AmGisLEaG' + 'Gs3VhTYDUtxZaED/AriGHjPhuEDC5wq3l5czL3vjeyzBlHtC6KoI4BuPk8Q' + 'qx3549rov3+YPBIp3NHq3VLlnkkBf0AWK2+6fevWCze8H7CaPRKUfzgnKsG' + '0jSh272kLEuWyy/a7QvSNKnIsJEk6vWE2SxnPH5nOn3YWopjh9VqIbDBGA5' + 'fyPNnGo2znZVIEEWhJTNDgmbzfHvZzA6H/pP3k3/TqQQkIYle7z6oT74wnz' + '8d3KNOfY19/QKFiTrWqbiPtAAAAABJRU5ErkJggg=='); elmAbout.appendChild(elmAboutImg); elmAbout.setAttribute("style", 'position:absolute;left:175px;top' + ':3px;margin:2px;width:12px;height:12px;background-color:#ff' + 'b;border:none;line-height:12px;text-align:center;text-decor' + 'ation:none;'); elmAbout.setAttribute("title","OverStimulate"); elmAbout.setAttribute("href", 'http://overstimulate.com/articles' + '/2005/04/24/greasemonkey-book-burro-find-cheap-books'); elmWrapper.appendChild(elmAbout); var elmContent = document.createElement("table"); elmContent.setAttribute("style", 'padding:0 5px;width:100%;font:' + '10pt sans-serif;'); elmContent.appendChild( add_site('http://www.amazon.com/exec/obi' + 'dos/ASIN/' + isbn + "/" + amazon_associate_code, "Amazon", "burro_amazon" )); elmContent.appendChild( add_site("http://www.amazon.com/exec/obi" + "dos/redirect?tag="+amazon_associate_code+ "&path=tg/stores/offering/list/-/"+isbn+"/all/", "Amazon (used)", "burro_amazonmarket" )); elmContent.appendChild( add_site( 'http://service.bfast.com/bfas' +  't/click?bfmid=2181&sourceid=' + bn_associate_code +'&bfpid=' + isbn + '&bfmtype=book', "Barnes & Noble", "burro_bn")); elmContent.appendChild( add_site("http://www.buy.com/retail/Glob" + "alSearchAction.asp?qu=" + isbn, "Buy.com", "burro_buy")); elmContent.appendChild( add_site( 'http://www.tkqlhce.com/click-' + half_associate_code+'?ISBN=' + isbn, 'Half.com', 'burro_half' )); elmContent.appendChild( add_site('http://www.powells.com/cgi-bin' + '/biblio?isbn=' + isbn, "Powell's Books", "burro_powell")); elmWrapper.appendChild(elmContent);  elmWrapper.addEventListener('click', function() { var elmCaret = document.getElementById('hide_show_elmCaret'); if (this.style.height != "auto") {  if (this.style.height == "15px") { run_queries( isbn );  } this.style.height = "auto"; elmCaret["src"] = 'data:image/png;base64,iVBORw0KGgoAAAA' + 'NSUhEUgAAAAsAAAALCAYAAACprHcmAAAABmJLR0QA/wD/AP+gva' + 'eTAAAACXBIWXMAAAsTAAALEwEAmpwYAAAAB3RJTUUH1QQYCRoeq' + '/kCuwAAAB10RVh0Q29tbWVudABDcmVhdGVkIHdpdGggVGhlIEdJ' + 'TVDvZCVuAAAAmElEQVQY083QMUoDARSE4W92U2xhkxSpAgY0AXM' +  'Q29zCM3iSXEXQTrCws44Im85CkO1jnq2KKQWnGxiY+Yd/oXw1tT' + 'rr7D86CYdD5Xk3HA8vTi8la7xW1T5Jg8IEN6PvPXnEAkOa5l5Vi' +  'wuc4yk/d9VyfoLrqnpIMmCGu2z79/wGUsv5FFcYY5Nt/wKjI+Bv' + 'uEWnbfu///kTargo75QVC5oAAAAASUVORK5CYII='; } else { elmCaret["src"] = 'data:image/png;base64,iVBORw0KGgoAAAA' + 'NSUhEUgAAAAsAAAALCAYAAACprHcmAAAABmJLR0QA/wD/AP+gva' + 'eTAAAACXBIWXMAAAsTAAALEwEAmpwYAAAAB3RJTUUH1QQYCR020' + 'Q08hgAAAB10RVh0Q29tbWVudABDcmVhdGVkIHdpdGggVGhlIEdJ' + 'TVDvZCVuAAAAiklEQVQY07XPIQoCURSF4e8NAzYxGicNuAa1Wly' + 'CO3AlZnfiNgwahQFxikkcBIsGfaZpzgODJ/4c/nMvPyR8g7Esep' + 'hgH6q6aXnWIelhjkUsi0EL88TqFUfMYlnscMoS5wUccMYS4yxhf' + 'uGNPho88oQ5xxQjrHHpKkcMccMqVPU99eATG2zb4n/zAS4OHrV1' + 'hIB/AAAAAElFTkSuQmCC'; this.style.height = "14px"; } }, true); document.getElementsByTagName("body")[0].appendChild(elmWrapper); } if (document.location.href.match('amazon.com') && !document.location.href.match('rate-this')) { isbn = checkISBN( document.location.href.match(/\/([0-9X]{10})(\/|\?|$)/)[1]); if (isbn) burro( 'amazon', isbn );  } if (document.location.href.match('barnesandnoble.com')) { isbn = checkISBN( document.location.href.match( /[iI][sS][Bb][Nn]=([0-9X]{10})(\&|\?|$)/)[1] ); if (isbn) burro( 'bn', isbn ); } if (document.location.href.match('buy.com')) { var isbn = checkISBN( document.title.match(/ISBN ([0-9X]{10})/)[1] ); if (isbn) burro( 'buy', isbn ); } if (document.location.href.match('powells.com')) { var arBold = document.getElementsByTagName('b'); for (var i=0; i<arBold.length; i++) { if (arBold[i].innerHTML.match('ISBN:')) { isbn = checkISBN(arBold[i].nextSibling.nextSibling.text); if (isbn) burro( 'powells', isbn ); } } } if (document.location.href.match('half.ebay.com')) { var arBold = document.getElementsByTagName('b'); for (var i=0; i<arBold.length; i++) { if (arBold[i].innerHTML.match('ISBN:')) {  isbn = checkISBN(arBold[i].nextSibling.text); if (isbn) burro( 'half', isbn ); } } } 

11.6.2. Running the Hack

After installing this script (Tools Install This User Script), go to http://www.amazon.com and search for Harry Potter Half-Blood Prince. Click through to the book page. In the top-left corner is a small floating window titled Book Burro. Click the triangle to expand the window, and Book Burro will fetch prices from other online retailers, as shown in Figure 11-7.

Figure 11-7. Comparison shopping for Harry Potter

Each of the competitors is a link to buy that book on another site. Click Barnes & Noble to go to the book page at http://barnesandnoble.com, as shown in Figure 11-8.

Figure 11-8. Harry Potter on BarnesAndNoble.com

The script will not fetch prices from other sites unless you expand the Book Burro window. You will need to expand the pricing window manually each time you go to a book page to see competitors' prices.