The document summarizes Steve Souders' presentation on optimizing frontend performance. It discusses the importance of frontend optimization, common techniques like minimizing HTTP requests and leveraging a CDN. It then covers strategies for loading scripts asynchronously without blocking parallel downloads or rendering, including techniques like script DOM element, XHR injection, and coupling asynchronous scripts with dependencies.

1. Steve Souders [email_address] http://stevesouders.com/docs/sxsw-20090314.ppt Even Faster Web Sites Disclaimer: This content does not necessarily reflect the opinions of my employer.

2. the importance of frontend performance 17% 83% iGoogle, primed cache 9% 91% iGoogle, empty cache

3. time spent on the frontend April 2008 Empty Cache Primed Cache www.aol.com 97% 97% www.ebay.com 95% 81% www.facebook.com 95% 81% www.google.com/search 47% 0% search.live.com/results 67% 0% www.msn.com 98% 94% www.myspace.com 98% 98% en.wikipedia.org/wiki 94% 91% www.yahoo.com 97% 96% www.youtube.com 98% 97%

4. 14 RULES MAKE FEWER HTTP REQUESTS USE A CDN ADD AN EXPIRES HEADER GZIP COMPONENTS PUT STYLESHEETS AT THE TOP PUT SCRIPTS AT THE BOTTOM AVOID CSS EXPRESSIONS MAKE JS AND CSS EXTERNAL REDUCE DNS LOOKUPS MINIFY JS AVOID REDIRECTS REMOVE DUPLICATE SCRIPTS CONFIGURE ETAGS MAKE AJAX CACHEABLE

8. 25% discount code: "ssouders25"

9. Sept 2007

10. June 2009

11. Even Faster Websites Split the initial payload Load scripts without blocking Couple asynchronous scripts Don't scatter inline scripts Split the dominant domain Flush the document early Use iframes sparingly Simplify CSS Selectors Ajax performance (Doug Crockford) Writing efficient JavaScript (Nicholas Zakas) Creating responsive web apps (Ben Galbraith, Dion Almaer) Comet (Dylan Schiemann) Beyond Gzipping (Tony Gentilcore) Optimize Images (Stoyan Stefanov, Nicole Sullivan)

12. why focus on JavaScript? AOL eBay Facebook MySpace Wikipedia Yahoo! YouTube

13. scripts block <script src=&quot;A.js&quot;> blocks parallel downloads and rendering http://stevesouders.com/cuzillion/?ex=10008

14. MSN.com: parallel scripts Scripts and other resources downloaded in parallel! How? Secret sauce?! var p= g.getElementsByTagName(&quot;HEAD&quot;)[0]; var c=g.createElement(&quot;script&quot;); c.type=&quot;text/javascript&quot;; c.onreadystatechange=n; c.onerror=c.onload=k; c.src=e; p.appendChild(c) MSN

15. asynchronous script loading XHR Eval XHR Injection Script in Iframe Script DOM Element Script Defer document.write Script Tag

16. XHR Eval script must have same domain as main page must refactor script var xhrObj = getXHRObject(); xhrObj.onreadystatechange = function() { if ( xhrObj.readyState != 4 ) return; eval(xhrObj.responseText); }; xhrObj.open('GET', 'A.js', true); xhrObj.send(''); http://stevesouders.com/cuzillion/?ex=10009

17. XHR Injection var xhrObj = getXHRObject(); xhrObj.onreadystatechange = function() { if ( xhrObj.readyState != 4 ) return; var se=document.createElement('script'); document.getElementsByTagName('head') [0].appendChild(se); se.text = xhrObj.responseText; }; xhrObj.open('GET', 'A.js', true); xhrObj.send(''); script must have same domain as main page http://stevesouders.com/cuzillion/?ex=10015

18. Script in Iframe <iframe src='A.html' width=0 height=0 frameborder=0 id=frame1></iframe> iframe must have same domain as main page must refactor script: // access iframe from main page window.frames[0].createNewDiv(); // access main page from iframe parent.document.createElement('div'); http://stevesouders.com/cuzillion/?ex=10012

19. Script DOM Element var se = document.createElement('script'); se.src = 'http://anydomain.com/A.js'; document.getElementsByTagName('head')[0] .appendChild(se); script and main page domains can differ no need to refactor JavaScript http://stevesouders.com/cuzillion/?ex=10010

20. Script Defer <script defer src='A.js'></script> only supported in IE (just landed in FF 3.1) script and main page domains can differ no need to refactor JavaScript http://stevesouders.com/cuzillion/?ex=10013

21. document.write Script Tag document.write (&quot;<scr&quot; + &quot;ipt type='text/javascript' src='A.js'>&quot; + &quot;</scr&quot; + &quot;ipt>&quot;); parallelization only works in IE parallel downloads for scripts, nothing else all document.write s must be in same script block http://stevesouders.com/cuzillion/?ex=10014

22. browser busy indicators

23. browser busy indicators good to show busy indicators when the user needs feedback bad when downloading in the background

24. ensure/avoid ordered execution Ensure scripts execute in order: necessary when scripts have dependencies IE: http://stevesouders.com/cuzillion/?ex=10017 FF: http://stevesouders.com/cuzillion/?ex=10018 Avoid scripts executing in order: faster – first script back is executed immediately http://stevesouders.com/cuzillion/?ex=10019

25. load scripts without blocking * Only other document.write scripts are downloaded in parallel (in the same script block).

26. and the winner is... XHR Eval XHR Injection Script in iframe Script DOM Element Script Defer Script DOM Element Script Defer Script DOM Element Script DOM Element (FF) Script Defer (IE) XHR Eval XHR Injection Script in iframe Script DOM Element (IE) XHR Injection XHR Eval Script DOM Element (IE) Managed XHR Injection Managed XHR Eval Script DOM Element Managed XHR Injection Managed XHR Eval Script DOM Element (FF) Script Defer (IE) Managed XHR Eval Managed XHR Injection Script DOM Element (FF) Script Defer (IE) Managed XHR Eval Managed XHR Injection different domains same domains no order preserve order no order no busy show busy show busy no busy preserve order

27. load scripts without blocking don't let scripts block other downloads you can still control execution order, busy indicators, and onload event What about inline scripts?

29. synchronous JS example: menu.js <script src=&quot;menu.js&quot; type=&quot;text/javascript&quot;></script> <script type=&quot;text/javascript&quot;> var aExamples = [ ['couple-normal.php', 'Normal Script Src'], ['couple-xhr-eval.php', 'XHR Eval'], ... ['managed-xhr.php', 'Managed XHR'] ]; function init() { EFWS.Menu.createMenu('examplesbtn', aExamples); } init(); </script>

30. asynchronous JS example: menu.js <script type=&quot;text/javascript&quot;> var domscript = document.createElement('script'); domscript.src = &quot;menu.js&quot;; document.getElementsByTagName('head')[0].appendChild(domscript); var aExamples = [ ['couple-normal.php', 'Normal Script Src'], ['couple-xhr-eval.php', 'XHR Eval'], ... ['managed-xhr.php', 'Managed XHR'] ]; function init() { EFWS.Menu.createMenu('examplesbtn', aExamples); } init(); </script> script DOM element approach

31. before after

32. load scripts without blocking * Only other document.write scripts are downloaded in parallel (in the same script block). !IE

33. asynchronous scripts wrap-up what about inlined code that depends on the script?

34. what about inlined code that depends on the script?

35. baseline coupling results (not good) * Scripts download in parallel regardless of the Defer attribute. need a way to load scripts asynchronously AND preserve order

36. coupling techniques hardcoded callback window onload timer degrading script tags script onload

37. technique 1: hardcoded callback <script type=&quot;text/javascript&quot;> var aExamples = [['couple-normal.php', 'Normal Script Src'], ...]; function init() { EFWS.Menu.createMenu('examplesbtn', aExamples); } var domscript = document.createElement('script'); domscript.src = &quot;menu.js&quot;; document.getElementsByTagName('head')[0].appendChild(domscript); </script> init() is called from within menu.js not very flexible doesn't work for 3 rd party scripts

38. technique 2: window onload <iframe src=&quot;menu.php&quot; width=0 height=0 frameborder=0> </iframe> <script type=&quot;text/javascript&quot;> var aExamples = [['couple-normal.php', 'Normal Script Src'], ...]; function init() { EFWS.Menu.createMenu('examplesbtn', aExamples); } if ( window.addEventListener ) { window.addEventListener(&quot;load&quot;, init, false); } else if ( window.attachEvent ) { window.attachEvent(&quot;onload&quot;, init); } </script> init() is called at window onload must use async technique that blocks onload: Script in Iframe does this across most browsers init() called later than necessary

39. technique 3: timer <script type=&quot;text/javascript&quot;> var domscript = document.createElement('script'); domscript.src = &quot;menu.js&quot;; document.getElementsByTagName('head')[0].appendChild(domscript); var aExamples = [['couple-normal.php', 'Normal Script Src'], ...]; function init() { EFWS.Menu.createMenu('examplesbtn', aExamples); } function initTimer(interval) { if ( &quot;undefined&quot; === typeof(EFWS) ) { setTimeout(initTimer, interval); } else { init(); } } initTimer(300); </script> load if interval too low, delay if too high slight increased maintenance – EFWS

40. John Resig's degrading script tags <script src=&quot;menu-degrading.js&quot; type=&quot;text/javascript&quot;> var aExamples = [['couple-normal.php', 'Normal Script Src'], ...]; function init() { EFWS.Menu.createMenu('examplesbtn', aExamples); } init(); </script> at the end of menu-degrading.js: var scripts = document.getElementsByTagName(&quot;script&quot;); var cntr = scripts.length; while ( cntr ) { var curScript = scripts[cntr-1]; if (curScript.src.indexOf(&quot;menu-degrading.js&quot;) != -1) { eval( curScript.innerHTML ); break; } cntr--; } http://ejohn.org/blog/degrading-script-tags/ cleaner clearer safer – inlined code not called if script fails no browser supports it

41. technique 4: degrading script tags <script type=&quot;text/javascript&quot;> var aExamples = [['couple-normal.php', 'Normal Script Src'],...]; function init() { EFWS.Menu.createMenu('examplesbtn', aExamples); } var domscript = document.createElement('script'); domscript.src = &quot;menu-degrading.js&quot;; if ( -1 != navigator.userAgent.indexOf(&quot;Opera&quot;) ) { domscript.innerHTML = &quot;init();&quot;; } else { domscript.text = &quot;init();&quot;; } document.getElementsByTagName('head')[0].appendChild(domscript); </script> elegant, flexible (cool!) not well known doesn't work for 3 rd party scripts (unless...)

42. technique 5: script onload <script type=&quot;text/javascript&quot;> var aExamples = [['couple-normal.php', 'Normal Script Src'], ...]; function init() { EFWS.Menu.createMenu('examplesbtn', aExamples); } var domscript = document.createElement('script'); domscript.src = &quot;menu.js&quot;; domscript.onloadDone = false; domscript.onload = function() { if ( ! domscript.onloadDone ) { init(); } domscript.onloadDone = true; }; domscript.onreadystatechange = function() { if ( &quot;loaded&quot; === domscript.readyState ) { if ( ! domscript.onloadDone ) { init(); } domscript.onloadDone = true; } } document.getElementsByTagName('head')[0].appendChild(domscript); </script> pretty nice, medium complexity

43. what about multiple scripts that depend on each other, and inlined code that depends on the scripts? two solutions: Managed XHR DOM Element and Doc Write

44. multiple script example: menutier.js <script src=&quot;menu.js&quot; type=&quot;text/javascript&quot;></script> <script src=&quot;menutier.js&quot; type=&quot;text/javascript&quot;></script> <script type=&quot;text/javascript&quot;> var aRaceConditions = [['couple-normal.php', 'Normal...]; var aWorkarounds = [['hardcoded-callback.php', 'Hardcod...]; var aMultipleScripts = [['managed-xhr.php', 'Managed XH...]; var aLoadScripts = [['loadscript.php', 'loadScript'], ...]; var aSubmenus = [[&quot;Race Conditions&quot;, aRaceConditions], [&quot;Workarounds&quot;, aWorkarounds], [&quot;Multiple Scripts&quot;, aMultipleScripts], [&quot;General Solution&quot;, aLoadScripts]]; function init() { EFWS.Menu.createTieredMenu('examplesbtn', aSubmenus); } </script>

45. technique 1: managed XHR <script type=&quot;text/javascript&quot;> var aRaceConditions = [['couple-normal.php', 'Normal...]; var aWorkarounds = [['hardcoded-callback.php', 'Hardcod...]; var aMultipleScripts = [['managed-xhr.php', 'Managed XH...]; var aLoadScripts = [['loadscript.php', 'loadScript'], ...]; var aSubmenus = [[&quot;Race Conditions&quot;, aRaceConditions], ...]; function init() { EFWS.Menu.createTieredMenu('examplesbtn', aSubmenus); } EFWS.Script.loadScriptXhrInjection(&quot;menu.js&quot;, null, true); EFWS.Script.loadScriptXhrInjection(&quot;menutier.js&quot;, init, true); </script> XHR Injection asynchronous technique does not preserve order – we have to add that before after

46. EFWS.loadScriptXhrInjection // Load an external script. // Optionally call a callback and preserve order. loadScriptXhrInjection: function(url, onload, bOrder) { var iQ = EFWS.Script.queuedScripts.length; if ( bOrder ) { var qScript = { response: null, onload: onload, done: false }; EFWS.Script.queuedScripts[iQ] = qScript; } var xhrObj = EFWS.Script.getXHRObject(); xhrObj.onreadystatechange = function() { if ( xhrObj.readyState == 4 ) { if ( bOrder ) { EFWS.Script.queuedScripts[iQ].response = xhrObj.responseText; EFWS.Script.injectScripts(); } else { eval(xhrObj.responseText); if ( onload ) { onload(); } } } }; xhrObj.open('GET', url, true); xhrObj.send(''); } process queue (next slide) or... eval now, call callback save response to queue add to queue (if bOrder)

47. EFWS.injectScripts // Process queued scripts to see if any are ready to inject. injectScripts: function() { var len = EFWS.Script.queuedScripts.length; for ( var i = 0; i < len; i++ ) { var qScript = EFWS.Script.queuedScripts[i]; if ( ! qScript.done ) { if ( ! qScript.response ) { // STOP! need to wait for this response break; } else { eval(qScript.response); if ( qScript.onload ) { qScript.onload(); } qScript.done = true; } } } } preserves external script order non-blocking works in all browsers couples with inlined code works with scripts across domains ready for this script, eval and call callback bail – need to wait to preserve order if not yet injected

48. technique 2: DOM Element and Doc Write Firefox & Opera – use Script DOM Element IE – use document.write Script Tag Safari, Chrome – no benefit; rely on Safari 4 and Chrome 2

49. EFWS.loadScripts loadScripts: function(aUrls, onload) { // first pass: see if any of the scripts are on a different domain var nUrls = aUrls.length; var bDifferent = false; for ( var i = 0; i < nUrls; i++ ) { if ( EFWS.Script.differentDomain(aUrls[i]) ) { bDifferent = true; break; } } // pick the best loading function var loadFunc = EFWS.Script.loadScriptXhrInjection; if ( bDifferent ) { if ( -1 != navigator.userAgent.indexOf('Firefox') || -1 != navigator.userAgent.indexOf('Opera') ) { loadFunc = EFWS.Script.loadScriptDomElement; } else { loadFunc = EFWS.Script.loadScriptDocWrite; } } // second pass: load the scripts for ( var i = 0; i < nUrls; i++ ) { loadFunc(aUrls[i], ( i+1 == nUrls ? onload : null ), true); } }

50. multiple scripts with dependencies <script type=&quot;text/javascript&quot;> var aRaceConditions = [['couple-normal.php', 'Normal...]; var aWorkarounds = [['hardcoded-callback.php', 'Hardcod...]; var aMultipleScripts = [['managed-xhr.php', 'Managed XH...]; var aLoadScripts = [['loadscript.php', 'loadScript'], ...]; var aSubmenus = [[&quot;Race Conditions&quot;, aRaceConditions], ...]; function init() { EFWS.Menu.createTieredMenu('examplesbtn', aSubmenus); } EFWS.Script.loadScripts([&quot;menu.js&quot;, &quot;menutier.js&quot;], init); </script> scripts on same domain: order preserved, no blocking scripts on different domain: order preserved: all loads scripts in parallel: all except Saf3, Chr1 load script and image in parallel: FF, Saf4, Chr2

51. asynchronous scripts wrap-up

52. case study: Google Analytics recommended pattern: 1 <script type=&quot;text/javascript&quot;> var gaJsHost = ((&quot;https:&quot; == document.location.protocol) ? &quot;https://ssl.&quot; : &quot;http://www.&quot;); document.write(unescape(&quot;%3Cscript src='&quot; + gaJsHost + &quot;google-analytics.com/ga.js' type='text/javascript'%3E%3C/script%3E&quot;)); </script> <script type=&quot;text/javascript&quot;> var pageTracker = _gat._getTracker(&quot;UA-xxxxxx-x&quot;); pageTracker._trackPageview(); </script> document.write Script Tag approach blocks other resources 1 http://www.google.com/support/analytics/bin/answer.py?hl=en&answer=55488

53. case study: dojox.analytics.Urchin 1 _loadGA: function(){ var gaHost = (&quot;https:&quot; == document.location.protocol) ? &quot;https://ssl.&quot; : &quot;http://www.&quot;; dojo.create('script', { src: gaHost + &quot;google-analytics.com/ga.js&quot; }, dojo.doc.getElementsByTagName(&quot;head&quot;)[0]); setTimeout(dojo.hitch(this, &quot;_checkGA&quot;), this.loadInterval); }, _checkGA: function(){ setTimeout(dojo.hitch(this, !window[&quot;_gat&quot;] ? &quot;_checkGA&quot; : &quot;_gotGA&quot;), this.loadInterval); }, _gotGA: function(){ this.tracker = _gat._getTracker(this.acct); ... } Script DOM Element approach &quot;timer&quot; coupling technique (script onload better) 1 http://docs.dojocampus.org/dojox/analytics/Urchin

54. asynchronous loading & coupling async technique: Script DOM Element easy, cross-browser doesn't ensure script order coupling technique: script onload fairly easy, cross-browser ensures execution order for external script and inlined code

55. bad: stylesheet followed by inline script browsers download stylesheets in parallel with other resources that follow... ...unless the stylesheet is followed by an inline script http://stevesouders.com/cuzillion/?ex=10021 best to move inline scripts above stylesheets or below other resources use Link, not @import

56. don't scatter inline scripts eBay MSN MySpace Wikipedia

57. iframes: most expensive DOM element load 100 empty elements of each type tested in all major browsers 1 1 IE 6, 7, 8; FF 2, 3.0, 3.1b2; Safari 3.2, 4; Opera 9.63, 10; Chrome 1.0, 2.0

58. iframes block onload parent's onload doesn't fire until iframe and all its components are downloaded workaround for Safari and Chrome: set iframe src in JavaScript <iframe id=iframe1 src=&quot;&quot;></iframe> <script type=&quot;text/javascript&quot;> document.getElementById('iframe1').src=&quot; url &quot;; </script>

59. scripts block iframe no surprise – scripts in the parent block the iframe from loading IE Firefox Safari Chrome Opera script script script

60. stylesheets block iframe (IE, FF) surprise – stylesheets in the parent block the iframe or its resources in IE & Firefox IE Firefox Safari Chrome Opera stylesheet stylesheet stylesheet

61. stylesheets after iframe still block (FF) surprise – even moving the stylesheet after the iframe still causes the iframe's resources to be blocked in Firefox IE Firefox Safari Chrome Opera stylesheet stylesheet stylesheet

62. iframes: no free connections iframe shares connection pool with parent (here – 2 connections per server in IE 7) iframe parent

63. flush the document early gotchas: PHP output_buffering – ob_flush() Transfer-Encoding: chunked gzip – Apache's DeflateBufferSize before 2.2.8 proxies and anti-virus software browsers – Safari (1K), Chrome (2K) other languages: $| or FileHandle autoflush (Perl), flush (Python), ios.flush (Ruby) call PHP's flush() html image image script html image image script

64. flushing and domain blocking you might need to move flushed resources to a domain different from the HTML doc case study: Google search blocked by HTML document different domains html image image script html image image script google image image script image 204

65. takeaways focus on the frontend run YSlow: http://developer.yahoo.com/yslow this year's focus: JavaScript speed matters

66. impact on revenue Google: Yahoo: Amazon: 1 http://home.blarg.net/~glinden/StanfordDataMining.2006-11-29.ppt 2 http://www.slideshare.net/stoyan/yslow-20-presentation +500 ms  -20% traffic 1 +400 ms  -5-9% full-page traffic 2 +100 ms  -1% sales 1

67. cost savings hardware – reduced load bandwidth – reduced response size http://billwscott.com/share/presentations/2008/stanford/HPWP-RealWorld.pdf

68. if you want better user experience more revenue reduced operating expenses the strategy is clear Even Faster Web Sites

69. Steve Souders [email_address] http://stevesouders.com/docs/sxsw-20090314.ppt