The document discusses techniques for improving frontend web performance, with a focus on optimizing JavaScript loading. It recommends splitting code downloads into above-the-fold and enhancements sections, loading scripts asynchronously and in parallel without blocking, and avoiding scattered inline scripts. Key techniques include loading scripts with XMLHttpRequest, in iframes, or dynamically creating script elements.

1. Steve Souders [email_address] http://stevesouders.com/ Even Faster Web Sites best practices for faster pages 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 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. The Performance Golden Rule 80-90% of the end-user response time is spent on the frontend. Start there. greater potential for improvement simpler proven to work

5. 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

6. High Performance Web Sites YSlow

7. http://conferences.oreilly.com/velocity/ June 22-24, 2009

8. High Performance Web Sites, Vol 2 Split the initial payload Load scripts without blocking Don't scatter inline scripts Split dominant domains Make static content cookie-free Reduce cookie weight Minify CSS Optimize images Use iframes sparingly To www or not to www

9. Why focus on JavaScript? AOL eBay Facebook MySpace Wikipedia Yahoo! YouTube

10. Scripts Block <script src=&quot;A.js&quot;> blocks parallel downloads and rendering What's &quot;Cuzillion&quot;? http://stevesouders.com/cuzillion/?ex=10008

11. Cuzillion 'cuz there are a zillion pages to check a tool for quickly constructing web pages to see how components interact Open Source http://stevesouders.com/cuzillion/

12. Initial Payload and Execution 26% avg 252K avg JavaScript Functions Executed before onload www.aol.com 115K 30% www.ebay.com 183K 44% www.facebook.com 1088K 9% www.google.com/search 15K 45% search.live.com/results 17K 24% www.msn.com 131K 31% www.myspace.com 297K 18% en.wikipedia.org/wiki 114K 32% www.yahoo.com 321K 13% www.youtube.com 240K 18%

13. Split the initial payload split your JavaScript between what's needed to render the page and everything else load &quot;everything else&quot; after the page is rendered separate manually (Firebug); tools needed to automate this (Doloto from Microsoft) load scripts without blocking – how?

14. MSN.com: Parallel Scripts Scripts and other resources downloaded in parallel! How? 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. Advanced 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 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;<scri&quot; + &quot;ipt type='text/javascript' src='A.js'>&quot; + &quot;</scri&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. Summary of Traits * Only other document.write scripts are downloaded in parallel (in the same script block).

26. and the winner is...

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?

28. Inline Scripts Block long executing inline scripts block rendering and downloads http://stevesouders.com/cuzillion/?ex=10035 workarounds: initiate execution with setTimeout (>250 for FF, nglayout.initialpaint.delay ) move JavaScript to external script with advanced downloading techniques use Defer attribute (IE only)

29. Inline Scripts after Stylesheets Block Downloading Firefox blocks parallel downloads when downloading stylesheets IE doesn't... ...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

30. Examples of Scattered Scripts eBay MSN MySpace Wikipedia

31. Don't Scatter Inline Scripts remember inline scripts carry a cost avoid long-executing inline scripts don't put inline scripts between stylesheets and other resources

32. Takeaways focus on the frontend run YSlow: http://developer.yahoo.com/yslow this year's focus: JavaScript Split the Initial Payload Load Scripts without Blocking Don't Scatter Inline Scripts speed matters you CAN make your site even faster!

33. Announcement 1: HTTPWatch for FF http://httpwatch.com/ previously IE only Firefox private beta now Firebug 1.05 Net Panel buggy

34. Announcement 2: Firebug Lite 1.2 http://getfirebug.com/lite.html console.log Inspect DOM and CSS

35. Announcement 3: Mozilla & Firebug John Resig, Rob Campbell, Jan Odvarko Firebug Working Group stability, performance, bug fixes

36. Steve Souders [email_address] http://stevesouders.com/