Secrets of
JavaScript Libraries
(Left to Right)
Sam Stephenson (Prototype)
Alex Russell (Dojo)
Thomas Fuchs (Script.aculo.us)
Andrew Dupont (Prototype)
John Resig (jQuery)

What to Cover
✦ Topics:
✦ JavaScript Language
✦ Cross-Browser Code
✦ Events
✦ DOM Traversal
✦ Style
✦ Animations
✦ Distribution
✦ HTML Insertion

Secrets of the
JavaScript Language

// Set up a class and create an element
var Clock = Class.create({
initialize: function() {
this.createElement();
},
createElement: function() {
this.element = new Element(\"div\");
}
});

// Display the time
var Clock = Class.create({
initialize: function() {
this.createElement();
},
createElement: function() {
this.element = new Element(\"div\");
var date = new Date();
this.element.update(
date.getHours() + \":\" +
date.getMinutes().toPaddedString(2) + \".\" +
date.getSeconds().toPaddedString(2)
);
}
});
$(document.body).insert(new Clock().element);

// Add the timer
var Clock = Class.create({
initialize: function() {
this.createElement();
this.createTimer();
},
createElement: function() {
this.element = new Element(\"div\");
},
updateElement: function() {
var date = new Date();
this.element.update(
date.getHours() + \":\" +
date.getMinutes().toPaddedString(2) + \".\" +
date.getSeconds().toPaddedString(2)
);
},
createTimer: function() {
window.setInterval(500, this.updateElement.bind(this));
}
});

// Add some options
var Clock = Class.create({
color: \"black\",
format: \"#{hour}:#{minute}.#{second}\",
initialize: function(options) {
Object.extend(this, options);
this.createElement();
this.createTimer();
},
createElement: function() {
this.element = new Element(\"div\");
this.element.setStyle({ color: Clock().element);
$(document.body).insert(new this.color });
}, $(document.body).insert(new Clock({ color: \"red\" }).element);
$(document.body).insert(new Clock({ format: \"#{hour}:#{minute}\" }).element);
updateElement: function() {
this.element.update(this.format.interpolate(this.getTime()));
},
getTime: function() {
var date = new Date();
return {
hour: date.getHours(),
minute: date.getMinutes().toPaddedString(2),
second: date.getSeconds().toPaddedString(2)
}
}, ...

// Use #toElement
var Clock = Class.create({
...
toElement: function() {
return this.element;
}
});
$(document.body).insert(new Clock());
$(document.body).down(\"div.clock > div\").replace(new Clock());
$(document.body).down(\"div.clock\").update(new Clock());

// Subclass it
var AmPmClock = Class.create(Clock, {
format: \"#{hour}:#{minute}:#{second} #{ampm}\",
getTime: function($super) {
var time = $super();
time.ampm = time.hour < 12 ? \"am\" : \"pm\";
if (time.hour == 0) {
time.hour = 12;
} else if (time.hour > 12) {
time.hour -= 12;
}
return time;
}
});
$(document.body).insert(new AmPmClock());

// Or monkeypatch it
Object.extend(Clock.prototype, {
format: \"#{hour}:#{minute}:#{second} #{ampm}\",
getTime: Clock.prototype.getTime.wrap(function(original) {
var time = original();
time.ampm = time.hour < 12 ? \"am\" : \"pm\";
if (time.hour == 0) {
time.hour = 12;
} else if (time.hour > 12) {
time.hour -= 12;
}
return time;
}
});
$(document.body).insert(new Clock());

Secrets of
Cross-Browser Code

Browser Sniffing
(wait, hear me out)

Conditionally evil

In order of desirability:

capabilities
&
quirks

Capabilities
are easy to sniff

Quirks
are... more troublesome

Object detection
(for capabilities)
if (document.evaluate) {
// ...
}

Distill to a boolean
(for stuff in the gray area)
var thinksCommentsAreElements = false;
if ( document.createElement('!') ) {
thinksCommentsAreElements = true;
}

...then sniff
(for outright bugs/quirks)
if (Prototype.Browser.IE) {
element.style.filter =
\"alpha(opacity=50);\";
}

Try to do the right thing,
but don’t stand on principle

The social contract
Good faith from browser makers ➝
good faith from web authors

Secrets
of Quality
Assurance

1,500
750
0
1.5.0
1.5.1
1.6.0.2

Debugging
✦ Localize & Reproduce
✦ Find the smallest possible code that
generates the problem
✦ Easy test-case generation
✦ ./gen.sh 1245 ajax
./gen.sh 1246 dom
✦ Simple logging, all browsers:
document.getElementById(“output”)
.innerHTML += “<br>” + msg;

Secrets of
Events

Event handlers
are tricky

Don’t store them on the
element itself
circular references = sadness

Build a global hashtable
(like jQuery does it)

Element Data Store
✦ Each element gets a unique ID
(bound w/ a unique property)
elem.jQuery12345 = 1;
✦ Points back to large data structure:
data[1] = { ... all element data here ... };
✦ Data (like event handlers) are stored here
data[1] = {
handlers: { click: [ function(){...} ], ... }
};

Then clean up
on page unload
So that IE doesn’t keep them
in memory in perpetuum

Fixing memory leaks

Internet Explorer 6
red-headed stepchild

Don’t “prove” your code
has no leaks
(view the results!)

Drip
is awesome

Test page:
Create a bunch of elements, assign each an event
handler, then remove each one from the page

Demonstrating the Leak
Notice the stair-step effect

Plugging the leak
// In Prototype, will remove all event listeners from an element
Event.stopObserving(someElement);
Event.purgeObservers = function(element, includeParent) {
Element.select(element, \"*\").each(Event.stopObserving);
if ( includeParent ) Event.stopObserving( element );
};

Redefining functions
add “before advice” to functions
that need to remove elements

Code sample
Element.Methods.update = Element.Methods.update.wrap(
function(proceed, element, contents) {
Event.purgeObservers(element);
return proceed(element, contents);
}
);
Element.Methods.replace = Element.Methods.replace.wrap(
function(proceed, element, contents) {
Event.purgeObservers(element, true);
return proceed(element, contents);
}
);
Element.Methods.remove = Element.Methods.remove.wrap(
function(proceed, element) {
Event.purgeObservers(element, true);
return proceed(element);
}
);
Element.addMethods();

Drop-in fix
for Prototype 1.6

Custom Events
• Everything is event based if you squint
• DOM is a good foundation
• Terrible for stitching together non-DOM
components and code
• Composition == good, inheritance == bad
• Custom events let us join loosely
• When to use them? Pitfalls?

Custom Events (contd.)
// in Dojo:
dojo.subscribe(“/foo”, function(e, arg){ ... });
dojo.publish(“/foo”, [{ data: “thinger”}, “second arg”]);
// in Prototype:
document.observe(“event:foo”, function(e){ ... });
$(“nodeId”).fire(“event:foo”, { data: “thinger” });
// in jQuery:
$(document).bind(“foo”, function(e, data, arg){ ... });
$(document).trigger(“foo”, [{ data: “thinger”}, “second”]);

Secrets of
DOM Traversal

Selector Internals
• Optimized DOM
• Top-down vs. bottom-up
• Caching + winnowing
• XPath
• Native, aka: querySelectorAll()

Selector Internals
• Tradeoffs: size vs. speed vs. complexity
• Prototype: XPath when possible, else DOM
• Good perf, lower complexity, hackable
• Dojo: all 3 methods, picks best available
• Best perf, biggest, highest complexity
• JQuery: DOM
• Good perf, small size, extensiblity vs.
forward-compat tradeoff

Secrets of
Style

Computed Style
✦ IE way vs. Everyone else
✦ IE returns “actual value”
✦ Everyone else returns “pixel value”
✦ font-size: 2em;
IE: “2em”
Other: 24
✦ Need to convert to common base
✦ Performance: Very costly, generally avoided
wherever possible

Pixel Values in IE
✦ if ( !/^\\d+(px)?$/i.test( ret ) && /^\\d/.test( ret ) ) {
// Remember the original values
var style = elem.style.left, runtimeStyle = elem.runtimeStyle.left;
// Put in the new values to get a computed value out
elem.runtimeStyle.left = elem.currentStyle.left;
elem.style.left = ret || 0;
ret = elem.style.pixelLeft + “px”;
// Revert the changed values
elem.style.left = style;
elem.runtimeStyle.left = runtimeStyle;
}
✦ From Dean Edwards:
http://erik.eae.net/archives/2007/07/27/18.54.15/#comment-102291

Computed Style
✦ Safari 2 & 3:
Giant mess for display: none elements
✦ Safari 2
✦ getComputedStyle() returns undefined
✦ Safari 3
✦ Always return empty strings for value
✦ Much harder to detect
var ret = document.defaultView.getComputedStyle( elem, null );
return !ret || ret.getPropertyValue(”color”) == “”;

Finding dimensions

Dimensions of what?
content box, border box, margin box...

DHTML properties
clientWidth, offsetWidth

The value you want is not captured by any property
Lorem ipsum dolor sit
amet.
width
clientWidth
offsetWidth

Computed styles
getting padding & border value

The fool-proof,
painful way
Take the offsetWidth,
then subtract computed padding & border

Code example
Element.getCSSWidth = function(element) {
element = $(element);
return element.offsetWidth -
parseFloat(element.getStyle(\"borderLeft\")) -
parseFloat(element.getStyle(\"paddingLeft\")) -
parseFloat(element.getStyle(\"paddingRight\")) -
parseFloat(element.getStyle(\"borderRight\"));
};

Secrets
of Animation

Old-School
“Effects”

setInterval

Events-based

Secrets
ofJavaScript
Deployment

CONTENT
EXPIRATION

Concatenation

GZIP
4-5x smaller

Packaging
• Dev vs. deployment constraints
• No library a single file, but all ship that way
• # of requests largest constraint
• Sync vs. async
• Static resource servers + CDNs
• Dependency management matters!
• Runtime vs. deployment

Packaging
// in Dojo:
dojo.provide(“foo.bar.Baz”);
dojo.require(“dojox.dtl”);
// in GWT:
package com.foo.bar;
import com.foo.bar.Blah;
// in JSAN:
JSAN.use(“foo.bar.Blah”);
// exports handled by build tools

Packaging
• The build-process divide
• Library support vs. server concat/shrink
• Can we strip “dead” code?
• Social artifacts of packaging systems

HTML Insertion
✦ $(“div”).append(“<b>foo</b>”);
✦ Convert HTML String
✦ innerHTML
✦ Range: .createContextualFragment()
✦ Must purify first:
✦ Fix <table>s for IE (<tbody> wrap)
✦ Handle <option>s (contain in <select>)

HTML Insertion
✦ // Fix “XHTML ”-style tags in all browsers
elem = elem.replace(/(<(\\w+)[^>]*?)\\/>/g, function(all, front, tag){
return tag.match(/^(abbr|br|col|img|input|link|meta|param|
hr|area|embed)$/i) ?
all :
front + “></” + tag + “>”;
});
✦ $(“<abbr/>”).html(“hello!”).appendTo(“#foo”);

Script Execution
✦ Execute Script in Global Scope
✦ var head = document.getElementsByTagName(”head”)[0] ||
document.documentElement,
script = document.createElement(”script”);
script.type = “text/javascript”;
if ( jQuery.browser.msie )
script.text = data;
else
script.appendChild( document.createTextNode( data ) );
head.appendChild( script );
head.removeChild( script );

Questions
✦ Panelists:
✦ John Resig (ejohn.org)
✦ Sam Stephenson (conio.net)
✦ Alex Russell (alex.dojotoolkit.org)
✦ Thomas Fuchs (script.aculo.us/thomas)
✦ Andrew Dupont (andrewdupont.net)
✦ Frameworks:
✦ Prototype (prototypejs.org)
✦ jQuery (jquery.com)
✦ Dojo (dojotoolkit.org)
✦ Script.aculo.us (script.aculo.us)