High-Performance
Kick-Ass
Web Apps
(with focus on JavaScript)
Stoyan Stefanov, @stoyanstefanov
April 25, 2009
JSConf, Washington, D.C.

About me
• Yahoo! Search
• Yahoo! Exceptional
Performance
• YSlow 2.0 architect
• http://smush.it
• Books, articles
• http://phpied.com

Importance of performance
• 500 ms slower = 20% drop in
traffic (Google)

Importance of performance
• 500 ms slower = 20% drop in
traffic (Google)
• 400 ms slower = 5-9% drop in
full-page traffic (Yahoo!)

Importance of performance
• 500 ms slower = 20% drop in
traffic (Google)
• 400 ms slower = 5-9% drop in
full-page traffic (Yahoo!)
• 100 ms slower = 1% drop in
sales (Amazon)

Importance of performance
• Self-regulating system
• Slow down = lose users
• It’s about user experience

“The premature optimization…
• … is the root of all evil”
Knuth
• “Make it right before you
make it fast”
Crockford

Pick your battles
• measure
• profile
• monitor

On trade-offs
“…everything has its drawbacks,
as the man said when his
mother-in-law died, and they
came upon him for the funeral
expenses.”
Jerome K. Jerome
Three Man in a Boat

The Life of Page 2.0
HTML page
request onload settles request
sent
marriage? R.I.P.
conception birth graduation
User perceived
“onload” happens
somewhere here

The waterfall

The Waterfall
1. Less stuff
2. Smaller stuff
3. Out of the way
4. Start early

The Waterfall
1. Less stuff
2. Smaller stuff
3. Out of the way
4. Start early

Less HTTP requests
• Combine components

Less HTTP requests
• Before:
<script src=\"jquery.js\"></script>
<script src=\"jquery.twitter.js\"></script>
<script src=\"jquery.cookie.js\"></script>
<script src=\"myapp.js\"></script>

Less HTTP requests
• After:
<script
src=\"all.js\"
type=\"text/javascript\">
</script>

Less HTTP requests
• You just saved 3 HTTP requests

Less HTTP requests
• repeat for CSS:
<link
href=\"all.css\"
rel=\"stylesheet\"
type=\"text/css”
/>

Less HTTP requests
• Inline images:
CSS sprites
with data: URI scheme

Less HTTP requests
• data: URI scheme
$ php ‐r \"echo base64_encode(file_get_contents('my.png'));”
iVBORw0KGgoAAAANSUhEUgAAAAQAAAADCAIAAAA7ljmRAAAAGElEQVQIW2P4
DwcMDAxAfBvMAhEQMYgcACEHG8ELxtbPAAAAAElFTkSuQmCC

Less HTTP requests
• data: URI scheme
background‐image: url(\"data:image/png;base64,iVBORw0KG...\");

Less HTTP requests
• data: URI scheme
<img src=\"data:image/png;base64,iVBOR...\" />

Less HTTP requests
• data: URI scheme
• works in IE!...

Less HTTP requests
• data: URI scheme
• works in IE8!

Less HTTP requests
• data: URI scheme
• MHTML for IE < 8
http://www.phpied.com/mhtml-when-you-need-data-uris-in-ie7-and-under/
http://www.hedgerwow.com/360/dhtml/base64-image/demo.php

Less stuff? Cache
• Cache is less universal than
we think
• You can help
http://yuiblog.com/blog/2007/01/04/performance-research-part-2/

“never expire” policy
• Static components with far-
future Expires header
• JS, CSS, img
ExpiresActive On
ExpiresByType image/png \"access plus 10 years\"

Inline vs. external
• a.k.a. less http vs. more
cache
• how about both?

Inline vs. external
• First visit:
1. Inline
2. Lazy-load the external file
3. Write a cookie

Inline vs. external
• Later visits:
1. Read cookie
2. Refer to the external file

The Waterfall
1. Less stuff ✔
2. Smaller stuff
3. Out of the way
4. Start early

The Waterfall
1. Less stuff
2. Smaller stuff
3. Out of the way
4. Start early

Gzip
Source: Bill Scott, Netflix

Minify
• Before
/**
* The dom module provides helper methods for
* manipulating Dom elements.
* @module dom
*
*/
(function() {
var Y = YAHOO.util, // internal shorthand
getStyle, // for load time browser branching
setStyle, // ditto
propertyCache = {}, // for faster hyphen converts
reClassNameCache = {}, // cache regexes for className
document = window.document; // cache for faster lookups
YAHOO.env._id_counter = YAHOO.env._id_counter || 0;

Minify
• After
(function(){var
B=YAHOO.util,K,I,J={},F={},M=window.document;YAHOO.env._id_counter=YAHOO.en
v._id_counter||0;

Minify
• YUI Compressor
• Minifies JS and CSS
• Tolerates * and _ hacks
• More than minification

Minify
• Minify inline code too

Gzip or minification?
• 62,885 bytes - original jQuery (back in
Aug 2007)
• 31,822 - minified with the YUI
Compressor
• 19,758 - original gzipped
• 10,818 - minified and gzipped FTW
http://www.julienlecomte.net/blog/2007/08/13/

204
• The world’s smallest component?
• 204 No Content
<?php
header(\"HTTP/1.0 204 No Content\");
// .... do your job, e.g. logging
?>
http://www.phpied.com/204-no-content/

The Waterfall
1. Less stuff ✔
2. Smaller stuff ✔
3. Out of the way
4. Start early

The Waterfall
1. Less stuff
2. Smaller stuff
3. Out of the way
4. Start early

Free-falling waterfalls
• Less DNS lookups – fetch
components from not more
than 2-4 domains
• Less redirects
• Blocking JavaScript

Not free-falling

JavaScript rocks!
• But also blocks
html
js
png
png

Non-blocking JavaScript
• Include via DOM
html
js
png
png
var js = document.createElement('script');
js.src = 'myscript.js';
var h = document.getElementsByTagName('head')[0];
h.appendChild(js);

Non-blocking JavaScript
• And what about my inline
scripts?
• Setup a collection (registry)
of inline scripts

Step 1
• Inline in the <head>:
var myapp = {
stuff: []
};

Step 2
• Add to the registry
Instead of:
<script>alert('boo!');</script>
Do:
<script>
myapp.stuff.push(function(){
alert('boo!');
});
</script>

Step 3
• Execute all
var l = myapp.stuff.length;
for(var i = 0, i < l; i++) {
myapp.stuff[i]();
}

Blocking CSS?
But they do block:
• In FF2
• When followed by a script

The Waterfall
1. Less stuff ✔
2. Smaller stuff ✔
3. Out of the way ✔
4. Start early

The Waterfall
1. Less stuff
2. Smaller stuff
3. Out of the way
4. Start early

flush() early
html
png
js 
css
html
js
png
✔
css

flush()
<html>
<head>
<script src=\"my.js\"
type=\"text/javascript\"></script>
<link href=\"my.css\"
type=\"text/css\" rel=\"stylesheet\" />
</head>
<?php flush() ?>
<body>
....

The Waterfall
1. Less stuff ✔
2. Smaller stuff ✔
3. Out of the way ✔
4. Start early ✔

Life after onload

Life after onload
1. Lazy-load
2. Preload
3. XHR
4. JavaScript optimizations

Lazy-load
• bells & whistles
• badges & widgets

Preload
• to help next page’s
waterfall
• img, CSS, JS, DNS lookups

XHR (Ajax)
• small – gzip, JSON
• less – Expires
• GET over POST

GET vs. POST for XHR
var url = 'test.php';
var request = new XMLHttpRequest();
request.open(\"POST\", url, false);
// …
request.send('test=1');

GET vs. POST for XHR

JavaScript optimizations
• local vars
• DOM
• garbage collection
• init-time branching
• memoization
• threads

Local variables
• globals are all sorts of bad
• use var
• localize globals

Local variables
var a = 1;
(function(){
var a = 2;
function b(){
var a = 3;
alert(a);
}
b();
})(); // 3

Local variables
var a = 1;
(function(){
var a = 2;
function b(){
// var a = 3;
alert(a);
}
b();
})(); // 2

Local variables
var a = 1;
(function(){
// var a = 2;
function b(){
// var a = 3;
alert(a);
}
b();
})(); // 1

Local variables
• less crawling up the scope chain
• localize
• function pointers too
• help YUI compressor (it won’t rename
globals)
Wait!
Isn’t that a
micro-optimization?

Localize DOM access
function foo(){
for (var i = 0; i < 100000; i++) {
document.getElementsByTagName('head');
}
}
foo();

Localize DOM access
function foo(){
var get = document.getElementsByTagName;
for (var i = 0; i < 100000; i++) {
get('head');
}
}
4
foo();
times
faster

Touching the DOM
function foo() {
for (var count = 0; count < 1000; count++) {
document.body.innerHTML += 1;
}
}

Touching the DOM
function foo() {
var inner = '';
for (var count = 0; count < 1000; count++) {
inner += 1;
}
document.body.innerHTML += inner;
1000
}
times
faster

Cleaning up after yourself
• Properties you no longer need
var myApp = {
prop: huge
};
// ...
delete myApp.prop;

Cleaning up after yourself
• DOM elements you no longer
need
var el = $('mydiv');
el.parentNode.removeChild(el);

Cleaning up after yourself
• DOM elements you no longer
need
var el = $('mydiv');
delete el.parentNode.removeChild(el);

Init-time branching
• Instead of…
function myEvent(el, type, fn) {
if (window.addEventListener) {
el.addEventListener(type, fn, false);
} else if (window.attachEvent) {
el.attachEvent(\"on\" + type, fn);
} else {…
}

Init-time branching
• Do…
if (window.addEventListener) {
var myEvent = function (el, type, fn) {
el.addEventListener(type, fn, false);
}
} else if (window.attachEvent) {
var myEvent = function (el, type, fn) {
el.attachEvent(\"on\" + type, fn);
}
}

Lazy definition
function myEvent(el, type, fn) {
if (window.addEventListener) {
myEvent = function(el, type, fn) {
el.addEventListener(type, fn, false);
};
} else if (window.attachEvent) {
//...
}
return myEvent(el, type, fn);
}

Memoization
• for expensive, repeating tasks
function myFunc(param){
if (!myFunc.cache) {
myFunc.cache = {};
}
if (!myFunc.cache[param]) {
var result = {}; // …
myFunc.cache[param] = result;
}
return myFunc.cache[param];
}

Threads
• Web Workers for modern browsers
var myWorker = new Worker('my_worker.js');
myWorker.onmessage = function(event) {
alert(\"Called back by the worker!\");
};
https://developer.mozilla.org/en/Using_DOM_workers

Threads
• … or setTimeout() for the rest
1. Do a chunk of work
2. setTimeout(chunk, 1) and return/yield

Life after onload
1. Lazy-load ✔
2. Preload ✔
3. XHR ✔
4. JavaScript optimizations ✔

YUI3
http://developer.yahoo.com/yui/3

YUI3
• Lighter
less KB, modules, sub-modules
• Faster
opportunity to refactor
• A la carte modules

YUI3 a la carte
• Combo handler
http://yui.yahooapis.com/combo?oop‐min.js&event‐min.js
• Self-populating
YUI().use(“anim”, function(Y) {
var a = new Y.Anim({...});
a.run();
});

Thank you!
Stoyan Stefanov
@stoyanstefanov
http://www.phpied.com

Credits/Further reading
• http://looksgoodworkswell.blogspot.com/2008/06/velocity-conference-improving-netflix.html
• http://developer.yahoo.com/yui/compressor/
• http://www.julienlecomte.net/blog/2007/12/39/
• http://webo.in/articles/habrahabr/46-cross-browser-data-url/
• http://yuiblog.com/blog/2008/07/22/non-blocking-scripts
• http://hitchhikers.wikia.com/wiki/Mostly_Harmless
• http://developer.yahoo.com/performance/
• http://oreilly.com/catalog/9780596522308/
• http://oreilly.com/catalog/9780596529307/
• http://www.nczonline.net/blog/tag/performance/