jQuery Anti-Patterns for
Performance &
Compression
Paul Irish
jQuery Conf ’09

Me.
Interaction Designer at Molecular, Inc.
@paul_irish
http://paulirish.com Front-end development blog
http://aurgasm.us Eclectic music blog

Performance

Performance

... but, jQuery is so sexy!
Taskspeed Test Lines of Code
200
150
100
50
0
YUI Dojo 1.3.1 Dojo 1.2.3 Qooxdoo MooTools Prototype.js jQuery PureDOM

Oft cited best practices
Cache length during loops
Cache your selections
Leverage documentFragment
Append new content outside the loop

Oft cited
BORING best practices
Cache length during loops
Cache your selections
Leverage documentFragment
Append new content outside the loop

Oft cited
BORING best practices
Cache length during loops
// appending inside. bad.
$.each(reallyLongArray, function(count, item) {
Cache your selections
var newLI = '<li>' + item + '</li>';
$('#ballers').append(newLI);
Leverage documentFragment
});
Append new content outside the loop
// documentFragment off-DOM
var frag = document.createDocumentFragment();
$.each(reallyLongArray, function(count, item) {
var newLI = '<li>' + item + '</li>';
frag.appendChild(newLI[0]);
});
$('#ballers')[0].appendChild(frag);

var newLI = '<li>' + item + '</li>';
$('#ballers').append(newLI);
Oft cited best practices
});
BORING
// documentFragment off-DOM
var frag = document.createDocumentFragment();
Cache length during loops
$.each(reallyLongArray, function(count, item) {
var newLI = '<li>' + item + '</li>';
Cache your selections
frag.appendChild(newLI[0]);
});
Leverage documentFragment
$('#ballers')[0].appendChild(frag);
Append new content outside the loop
// string concatenate and set innerHTML
var myhtml = '';
$.each(reallyLongArray, function(count, item) {
myhtml += '<li>' + item + '</li>';
});
$('#ballers').html(myhtml);

Keep things DRY
If you’re repeating
yourself, you’re doing it
wrong

Moar DRY plz?
if ($ventfade.data('currently') != 'showing') {
$ventfade.stop();
}
if ($venthover.data('currently') != 'showing') {
$venthover.stop();
}
if ($spans.data('currently') != 'showing') {
$spans.stop();
}
from http://mt-ventures.com/_js/global.js

All clean! Thx
var elems = [$ventfade,$venthover,$spans];
$.each(elems,function(k,v){
if (v.data('currently') != 'showing'){
v.stop();
}
})

Architecture Anti-Patterns
Anonymous functions bound everywhere suck
$(document).ready(function(){
...
$('#magic').click(function(e){
$('#yayeffects').slideUp(function(){
...
});
});
$('#happiness').load(url+' #unicorns',function(){
...
})
});

Architecture - Object Literal
var PI = {
onReady : function(){
...
$('#magic').click(PI.candyMtn);
$('#happiness').load(url+' #unicorns',PI.unicornCb);
},
candyMtn : function(e){
$('#yayeffects').slideUp(PI.slideCb);
},
slideCb : function(){
...
},
unicornCb : function(){
...
}
}
$(document).ready(PI.onReady);

Architecture - Object Literal
Advantages:
Easier to navigate and discuss
Profilers give you actual names to work with
You can execute these from firebug console
You can write unit tests against them

Anti-Pattern: The requery
// create and append your element
$(document.body).append("<div class='baaron'/>");
// requery to bind stuff
$("div.baaron").click(function(){});
// better:
// swap to appendTo to hold your elem
$("<div class='baaron'/>")
.appendTo(document.body)
.click(function(){});

$(‘#whats .the’,context)

This is not the .context property
// find all stylesheets in the body
var bodySheets = $('style',document.body);
bodySheets.context // ==> BODY element
Ignore that for the moment, I know no one that’s
found a use

$(‘#whats .the’,context)
Never pass it a string selector. Ever.
No performance gain vs $(root).find(selector)
var arms = $('div.robotarm', '#container');
// instead
var arms = $('#container').find('div.robotarm');

$(‘#whats .the’,context)
You typically pass it this, but it’s purely a
convenience to avoid find()
$('form.comments',this).submit(captureSubmit);
// exact same as
$(this).find('form.comments').submit(captureSubmit);
Which is more readable?
$('.reply_form', $(this).closest('.comment')).hide();
$(this).closest('.comment').find('.reply_form').hide();

The Crowd Say Bo Selector

Come on, my selector
Selector engines have come a long, long way.

Come on, my selector
Selector engines have come a long, long way.

Come on, my selector
Engines work in different ways
Top-down, bottom-up, function creation, other crazy
shit
// from NWMatcher:
// selecting '.outmost #outer span'
T=e.nodeName;if(T=="SPAN"||T=="span")
{while((e=e.parentNode)&&e.nodeType==1)
{if((n=e.getAttributeNode("id"))&&n.value=="outer")
{if((e=e.parentNode)&&e.nodeType==1)
{C=e.className;if(C&&(" "+C+" ").indexOf(" outmost ")>-1)
{r[X++]=N;continue main;}}}}}

Selector engines, parse direction
Left to right (Top-down) Right to left (Bottom-up)
Mootools Sizzle
Sly YUI 3
Peppy NWMatcher
Dojo Acme
Ext JS
Prototype.js
details: http://alexsexton.com/selectors/

Selector engines, parse direction
div.data table.attendees .gonzalez
Left to right (Top-down) Right to left (Bottom-up)
Mootools Sizzle
Sly YUI 3
Peppy NWMatcher
Dojo Acme
Ext JS
Prototype.js
details: http://alexsexton.com/selectors/

Selector engines, parse direction
Left to right (Top-down) Right to left (Bottom-up)
Mootools Sizzle
Sly YUI 3
Peppy NWMatcher
Dojo Acme
Ext JS
Prototype.js
details: http://alexsexton.com/selectors/

Selector engines, parse direction
Left to right (Top-down) Right to left (Bottom-up)
Mootools Sizzle
Sly YUI 3
Peppy NWMatcher
Dojo Acme querySelectorAll (qSA)
Ext JS
Prototype.js
details: http://alexsexton.com/selectors/

Selector Optimization
Specific on the right, light on the left
// let's find scott
div.data .gonzalez
// specific on right, light on the left
.data td.gonzalez
tag.class if possible on your right-most selector.
just tag or just .class on left.

Selector Optimization
Of course, descending from an #id is best
// basic #id-based selector
var arms = $('#container div.robotarm');
// hyper-optimized #id case first, then find:
var arms = $('#container').find('div.robotarm');

Selector Optimization
Don’t be needlessly specific
// let's find scott
.data table.attendees td.gonzalez
// better: drop the middle
.data td.gonzalez
A flatter DOM helps, so move to HTML5
Also a wider range of tags speeds up filters

Selector Optimization
Avoid the universal selector
Avoid the implied universal selector
$('.buttons > *') // terribly costly
$('.buttons').children() // much better
$('.gender :radio') // implied universal
$('.gender *:radio') // exact same, explicit now
$('.gender input:radio') // much better

Selector Optimization
Google PageSpeed’s efficient selectors analysis
MDC: Writing Efficient CSS
https://developer.mozilla.org/en/Writing_Efficient_CSS
Benchmark.js
http://code.paulirish.com/sandbox/benchmark.js

Event delegation
function delegate(type, delegate, handler) {
return $(document).bind(type, function(event) {
var target = $(event.target);
if (target.is(delegate)) {
return handler.apply(target, arguments);
}
});
}
delegate('click','td.jehl',createRockstar);
// and with live():
$('td.jehl').live('click',createRockstar);

Event Delegation
live() isn’t just for dynamic content
Speeds up page load
Only one event handler is bound vs many
Good for >3 elements all getting the same handler
// speed up live on page load
var jqElem = $(document);
jqElem.selector = 'li.ui';
jqElem.live('dblclick', dblhandler);

The DOM is slow
Pull elements off the DOM while you toy with them
var table = $('#some-table');
var parent = table.parent();
table.remove();
table.addLotsAndLotsOfRows();
parent.append(table);

Minimize DOM touches
Use classes, but if a style change user-selected:
jQuery('a.swedberg').css('color', '#BADA55');
jQuery('<style type="text/css"> a.swedberg { color: BADA55; } </style>')
.appendTo('head');
Timings for X elements
3000
2250 (1000 iterations)
1500 css()
style tag
750
0
1 5 10 20 50

Minimize DOM touches

Don’t treat jQuery as a Black Box
Use the source as your documentation
Add this to your bookmark bar, NOW!
http://ajax.googleapis.com/ajax/libs/jquery/1/jquery.js
http://bit.ly/jqsource
Determine which are convenience methods:
getScript: function( url, callback ) {
return jQuery.get(url, null, callback, "script");
},
getJSON: function( url, data, callback ) {
return jQuery.get(url, data, callback, "json");
},

Don’t treat jQuery as a Black Box
Learn the lesser-known methods
map(), slice(), stop(), (de)queue(),
prevAll(), pushStack(), inArray() , etc
// index() in jQuery <= 1.3.2
$('#rdworth').parent().children().index( $('#rdworth')[0] )
// using prevAll() is 10% faster (also sexier)
$('#rdworth').prevAll().length
// in jQuery 1.3.3
$('#rdworth').index()

Don’t act on absent elements
jQuery is very kind and doesn’t throw errors at you
Don’t assume it’s just fine to do
$('#doesntexist').slideUp()
// this will execute genFx(), speed() and animate()
// before it hits an each()
jQuery UI widgets have a lot of overhead you’ll hit

Don’t act on absent elements
jQuery.fn.doOnce = function(func){
this.length && func.apply(this);
return this;
}
$('li.cartitems').doOnce(function(){
// make it ajax! o/
});

Don’t act on absent elements
$.fn.plugin = function(opts){
if(!this.length) return this;
var opts = $.extend(......
...
return this.each(...

Compression

Compression
YUI Compressor
Sits on Rhino.
Comments, whitespace, variable replacement
//it already does these micro-optimizations:
object['prop'] ==> object.prop
{'key':123} ==> {key:123}
'jon's apostophes' ==> "jon's apostrophes"
'bigass ' + 'string' ==> 'bigass string'

Variable definition
// old 'n busted // new hotness
var test1 = 1; var test1 = 1,
var test2 = function() { test2 = function() {
// function code // function code
}; },
var test3 = test2(test1); test3 = test2(test1);

Munge the primitives
Define shortcuts at the top of your scope
Good for both compression and scope chain traversal
var TRUE = true,
FALSE = false,
NULL = null,
window = self,
undefined = undefined;

Munge the primitives
Define shortcuts at the top of your scope
Good for both compression and scope chain traversal
var TRUE = true,
FALSE = false,
NULL = null,
window = self,
undefined;
undefined = undefined;

Arrays & Strings
//For arrays, typically:
['a','b','c','d','e','f']
// split if you have => 6 items
'a b c d e f'.split(' ')
str.slice(2) // smaller
str.substr(2) // faster

var str=‘Let’s put this into action’
// html.no-js html>
<!doctype ==> html.js
var elem = document.getElementsByTagName('html')[0];
elem.className = elem.className.replace('no-js','js');
<html class="no-js">
// quicker reference, safer replace
<head>
var elem = document.documentElement;
elem.className = elem.className.replace(/bno-jsb/,'js');
<script>
// one// change the html class to 'js'
line ftw!
// in the head, no FOUC
document.documentElement.className =
document.documentElement.className.replace(/bno-jsb/,
</script>
'js');
</body>
// shorter with a self-executing anonymous function
(function(B){B.className=B.className.replace(/bno-jsb/,

var str=‘Let’s put this into action’
// html.no-js ==> html.js
var elem = document.getElementsByTagName('html')[0];
elem.className = elem.className.replace('no-js','js');
// quicker reference, safer replace
var elem = document.documentElement;
elem.className = elem.className.replace(/bno-jsb/,'js');
// one line ftw!
document.documentElement.className =
document.documentElement.className.replace(/bno-jsb/,
'js');
// shorter with a self-executing anonymous function
(function(B){B.className=B.className.replace(/bno-jsb/,

// html.no-js ==> html.js
var elem = document.getElementsByTagName('html')[0];
var str=‘Let’s put this into action’
elem.className = elem.className.replace('no-js','js');
// quicker reference, safer replace
var elem = document.documentElement;
elem.className = elem.className.replace(/bno-jsb/,'js');
// one line ftw!
document.documentElement.className =
document.documentElement.className.replace(/bno-jsb/,
'js');
// shorter with a self-executing anonymous function
(function(B){B.className=B.className.replace(/bno-jsb/,
'js')})(document.documentElement);
// pass className, object string notation
(function(H,C){H[C]=H[C].replace(/bno-jsb/,'js')})
(document.documentElement,'className')

Conditionals
// old 'n busted
if ( type === 'foo' || type === 'bar' ) {}
// regex test
if ( /^(foo|bar)$/.test(type) ) {}
// obj literal lookup (smaller if <5 items)
if ( ({foo:1,bar:1})[type] ) {}

Logic and Ternary operands
// basic function detection
document.querySelectorAll && document.querySelectorAll('a:nth-child(2)')
// assignment is legal, but it evaluates to the right expression
callback && (isCallbackCalled = true) && callback(returnVal);
// call or cache the callback function
(isCallbackCalled || returnVal) ? fn(returnVal) : (callback = fn);
// inline function calls
isToday('Saturday') && Math.round(Math.random()) && $('#winnar').show()
// if JSON2.js or Native JSON is present, otherwise eval.
data = window.JSON && JSON.parse(data) || eval('('+data +')');

Write maintainable code
As a developer,
you should work first and foremost
for the user of your products.
The second most important person to work for is
the developer that takes over from you.
- Christian Heilmann

Comments
/*!
* Will not be removed by YUI Compressor
*/
// for quick toggling on and off:
/* */
aaaahYeah();
/* */
/* * /
ohHellNo();
/* */

Compression Tools
CompressorRater
http://compressorrater.thruhere.net/
YUI Compressor front-end
http://refresh-sf.com/yui/

Case Study: Modernizr

Thanks, ya’ll.
Slides at http://paulirish.com/perf
@paul_irish
thx:
Alex Sexton, Ben Alman, Adam Sontag, James
Padolsey, temp01, #jquery on Freenode

todo
shadow effect to code samples
more context research and this: http://
groups.google.com/group/jquery-dev/msg/
b4b7935a4013dfe7 and http://ispeakwebstuff.co.uk/
web-design-development-tutorials/clever-jquery-
selectors/

// pngfix for IE6
// e.g. FL.pngfix('img.bigProdShot,a.thumb');
pngfix : function(sel){
// conditional comments for inclusion of that js.
if (typeof DD_belatedPNG == 'undefined'){ return;
} else {
// delay pngfix until window onload
$(window).load(function(){ $(sel).each(function()
{ DD_belatedPNG.fixPng(arguments[1]); }); });
}
} // end of FL.pngfix()