The DOM is a Mess @ Yahoo

The DOM is a Mess John Resig http://ejohn.org/ - http://twitter.com/jeresig/

A Tour of the DOM A messy DOM ✦ Writing Cross-Browser Code ✦ Common Features ✦ ✦ CSS Selector Engine ✦ DOM Modiﬁcation ✦ Events

Messy Nearly every DOM method is broken in ✦ some way, in some browser. Some old: ✦ ✦ getElementById ✦ getElementsByTagName Some new: ✦ ✦ getElementsByClassName ✦ querySelectorAll

getElementById Likely the most commonly used DOM ✦ method A couple weird bits: ✦ ✦ IE and older versions of Opera returning elements with a name == id ✦ Does not easily work in XML documents

getElementsByTagName Likely tied for most-commonly-used ✦ DOM method Riddled with bugs in IE: ✦ ✦ “*” returns no elements in IE 5.5 ✦ “*” returns no elements on <object> elements in IE 7 ✦ .length gets overwritten in IE if an element with an ID=”length” is found

getElementsByClassName Landed in Firefox 3, Safari 3, Opera 9.6 ✦ A few knotty issues: ✦ ✦ HTMLElement.prototype .getElementsByClassName couldn’t be overwritten in Firefox ✦ Opera doesn’t match a second-speciﬁed class (e.g. class=”a b”, b isn’t found)

querySelectorAll Find DOM elements using CSS selectors ✦ In Firefox 3.1, Safari 3.1, Opera 10, IE 8 ✦ Birthing pains: ✦ ✦ Doesn’t exist in quirks mode, in IE 8 ✦ Safari 3.1 had memory out of bounds problems ✦ Safari 3.2 can’t match uppercase characters in quirks mode ✦ #id doesn’t match in XML documents

Moral If there’s a DOM method, there’s probably ✦ a problem with it somewhere, in some capacity.

Cross-Browser Code

Strategies Pick your browsers ✦ Know your enemies ✦ Write your code ✦

Cost / Beneﬁt IE 7 IE 6 FF 3 Safari 3 Opera 9.5 Cost Benefit Draw a line in the sand.

Graded Support Yahoo Browser Compatibility

Browser Support Grid IE Firefox Safari Opera Chrome Previous 6.0 2.0 3.0 9.5 Current 7.0 3.0 3.2 9.6 Current Next 8.0 3.1 4.0 10.0 jQuery Browser Support

Browser Support Grid IE Firefox Safari Opera Chrome Previous 3.0 9.5 6.0 2.0 Current 7.0 3.0 3.2 9.6 Current Next 3.1 4.0 8.0 10.0 jQuery 1.3 Browser Support

Know Your Enemies Points of Concern for JavaScript Code Browser Bugs Regressions Missing Features JavaScript Code Bug Fixes External Code, Markup

Browser Bugs Generally your primary concern ✦ Your defense is a good test suite ✦ ✦ Prevent library regressions ✦ Analyze upcoming browser releases Your oﬀense is feature simulation ✦ What is a bug? ✦ ✦ Is unspeciﬁed, undocumented, behavior capable of being buggy?

Test, Test, Test 1446 Tests, 9 browsers

External Code Making your code resistant to any ✦ environment ✦ Found through trial and error ✦ Integrate into your test suite ✦ Other libraries ✦ Strange code uses

Environment Testing 100% Passing: ✦ ✦ Standards Mode ✦ Quirks Mode ✦ Inline with Prototype + Scriptaculous ✦ Inline with MooTools Work in Progress: ✦ ✦ XHTML w/ correct mimetype ✦ With Object.prototype ✦ In XUL (Firefox Extensions) ✦ In Rhino (with Env.js)

Object.prototype Object.prototype.otherKey = quot;otherValuequot;; var obj = { key: quot;valuequot; }; for ( var prop in object ) { if ( object.hasOwnProperty( prop ) ) { assert( prop, quot;keyquot;, quot;There should only be one iterated property.quot; ); } }

Greedy IDs <form id=quot;formquot;> <input type=quot;textquot; id=quot;lengthquot;/> <input type=quot;submitquot; id=quot;submitquot;/> </form> document.getElementsByTagName(quot;inputquot;).length

Order of Stylesheets Putting stylesheets before code guarantees ✦ that they’ll load before the code runs. Putting them after can create an ✦ indeterminate situation.

Pollution Make sure your code doesn’t break ✦ outside code ✦ Use strict code namespacing ✦ Don’t extend outside objects, elements Bad: ✦ ✦ Introducing global variables ✦ Extending native objects (Array, Object) ✦ Extending DOM natives

Pollution http://mankz.com/code/ GlobalCheck.htm

Missing Features Typically older browsers missing speciﬁc ✦ features Optimal solution is to gracefully ✦ degrade ✦ Fall back to a simpliﬁed page Can’t make assumptions about ✦ browsers that you can’t support ✦ If it’s impossible to test them, you must provide a graceful fallback Object detection works well here. ✦

Object Detection Check to see if an object or property ✦ exists Useful for detecting an APIs existence ✦ Doesn’t test the compatibility of an API ✦ ✦ Bugs can still exist - need to test those separately with feature simulation

Event Binding function attachEvent( elem, type, handle ) { // bind event using proper DOM means if ( elem.addEventListener ) elem.addEventListener(type, handle, false); // use the Internet Explorer API else if ( elem.attachEvent ) elem.attachEvent(quot;onquot; + type, handle); }

Fallback Detection if ( typeof document !== quot;undefinedquot; && (document.addEventListener || document.attachEvent) && document.getElementsByTagName && document.getElementById ) { // We have enough of an API to // work with to build our application } else { // Provide Fallback }

Fallback Figure out a way to reduce the ✦ experience Opt to not execute any JavaScript ✦ ✦ Guarantee no partial API ✦ (e.g. DOM traversal, but no Events) Redirect to another page, or just work ✦ unobtrusively Working on a ready() fallback for jQuery ✦

Bug Fixes Don’t make assumptions about browser ✦ bugs. ✦ Assuming that a browser will always have a bug is foolhardy ✦ You will become susceptible to ﬁxes ✦ Browsers will become less inclined to ﬁx bugs Look to standards to make decisions about ✦ what are bugs

Failed Bug Fix in FF 3 // Shouldn't work var node = documentA.createElement(quot;divquot;); documentB.documentElement.appendChild( node ); // Proper way var node = documentA.createElement(quot;divquot;); documentB.adoptNode( node ); documentB.documentElement.appendChild( node );

Feature Simulation More advanced than object detection ✦ Make sure an API works as advertised ✦ Able to capture bug ﬁxes gracefully ✦

Verify API // Run once, at the beginning of the program var ELEMENTS_ONLY = (function(){ var div = document.createElement(quot;divquot;); div.appendChild( document.createComment(quot;testquot; ) ); return div.getElementsByTagName(quot;*quot;).length === 0; })(); // Later on: var all = document.getElementsByTagName(quot;*quot;); if ( ELEMENTS_ONLY ) { for ( var i = 0; i < all.length; i++ ) { action( all[i] ); } } else { for ( var i = 0; i < all.length; i++ ) { if ( all[i].nodeType === 1 ) { action( all[i] ); } } }

Figure Out Naming <div id=quot;testquot; style=quot;color:red;quot;></div> <div id=quot;test2quot;></div> <script> // Perform the initial attribute check var STYLE_NAME = (function(){ var div = document.createElement(quot;divquot;); div.style.color = quot;redquot;; if ( div.getAttribute(quot;stylequot;) ) return quot;stylequot;; if ( div.getAttribute(quot;cssTextquot;) ) return quot;cssTextquot;; })(); // Later on: window.onload = function(){ document.getElementsById(quot;test2quot;).setAttribute( STYLE_NAME, document.getElementById(quot;testquot;).getAttribute( STYLE_NAME ) ); }; </script>

Regressions Removing or changing unspeciﬁed APIs ✦ Object detection helps here ✦ Monitor upcoming browser releases ✦ ✦ All vendors provide access to beta releases ✦ Diligence! Example: IE 7 introduced ✦ XMLHttpRequest with ﬁle:// bug Test Suite Integration ✦

Object Failover function attachEvent( elem, type, handle ) { // bind event using proper DOM means if ( elem.addEventListener ) elem.addEventListener(type, handle, false); // use the Internet Explorer API else if ( elem.attachEvent ) elem.attachEvent(quot;onquot; + type, handle); }

Safe Cross-Browser Fixes The easiest form of ﬁx ✦ Uniﬁes an API across browsers ✦ Implementation is painless ✦

Unify Dimensions // ignore negative width and height values if ( (key == 'width' || key == 'height') && parseFloat(value) < 0 ) value = undefined;

Prevent Breakage if ( name == quot;typequot; && elem.nodeName.toLowerCase() == quot;inputquot; && elem.parentNode ) throw quot;type attribute can't be changedquot;;

Untestable Problems Has an event handler been bound? ✦ Will an event ﬁre? ✦ Do CSS properties like color or opacity ✦ actually aﬀect the display? Problems that cause a browser crash. ✦ Problems that cause an incongruous API. ✦

Impractical to Test Performance-related issues ✦ Determining if Ajax requests will work ✦

Battle of Assumptions Cross-browser development is all about ✦ reducing the number of assumptions No assumptions indicates perfect code ✦ ✦ Unfortunately that’s an unobtainable goal ✦ Prohibitively expensive to write Have to draw a line at some point ✦

DOM Traversal Many methods of DOM traversal ✦ One unanimous solution: ✦ ✦ CSS Selector Engine

Traditional DOM getElementsByTagName ✦ getElementById ✦ getElementsByClassName ✦ ✦ in FF3, Safari 3, Opera 9.6 .children ✦ ✦ only returns elements (in all, and FF 3.1) getElementsByName ✦ .all[id] ✦ ✦ Match multiple elements by ID

Top-Down CSS Selector Traditional style of traversal ✦ ✦ Used by all major libraries Work from left-to-right ✦ “div p” ✦ ✦ Find all divs, ﬁnd paragraphs inside Requires a lot of result merging ✦ And removal of duplicates ✦

Bottom-Up Work from right-to-left ✦ ✦ (How CSS Engines work in browsers.) “div p” ✦ ✦ Find all paragraphs, see if they have a div ancestor, etc. Fast for speciﬁc queries ✦ ✦ “div #foo” Deep queries get slow (in comparison) ✦ ✦ “#foo p”

Bottom-Up Some nice features: ✦ ✦ Only one DOM query ✦ No merge/unique required (except for “div, span”) ✦ Everything is a process of ﬁltering

CSS to XPath Browsers provide XPath functionality ✦ Collect elements from a document ✦ Works in all browsers ✦ ✦ In IE it only works on HTML documents Fast for a number of selectors (.class, “div ✦ div div”) ✦ Slow for some others: #id Currently used by Dojo and Prototype ✦

if ( typeof document.evaluate === quot;functionquot; ) { function getElementsByXPath(expression, parentElement) { var results = []; var query = document.evaluate(expression, parentElement || document, null, XPathResult.ORDERED_NODE_SNAPSHOT_TYPE, null); for (var i = 0, length = query.snapshotLength; i < length; i++) results.push(query.snapshotItem(i)); return results; } }

Goal CSS 3 XPath All Elements * //* All P Elements p //p All Child Elements p>* //p/* Element By ID #foo //*[@id='foo'] //*[contains(concat(quot; quot;, Element By Class .foo @class, quot; quot;),quot; foo quot;)] Element With Attribute *[title] //*[@title] First Child of All P p > *:ﬁrst-child //p/*[0] All P with an A descendant //p[a] Not possible Next Element p+* //p/following-sibling::*[0]

querySelectorAll The Selectors API spec from the W3C ✦ Two methods: ✦ ✦ querySelector (ﬁrst element) ✦ querySelectorAll (all elements) Works on: ✦ ✦ document ✦ elements ✦ DocumentFragments Implemented in: ✦ ✦ Firefox 3.1, Safari 3, Opera 10, IE 8

<div id=quot;testquot;> <b>Hello</b>, I'm a ninja! </div> <div id=quot;test2quot;></div> <script> window.onload = function(){ var divs = document.querySelectorAll(quot;body > divquot;); assert( divs.length === 2, quot;Two divs found using a CSS selector.quot; ); var b = document.getElementById(quot;testquot;).querySelector(quot;b:only-childquot;); assert( b, quot;The bold element was found relative to another element.quot; ); }; </script>

<div id=quot;testquot;> <b>Hello</b>, I'm a ninja! </div> <script> window.onload = function(){ var b = document.getElementById(quot;testquot;).querySelector(quot;div bquot;); assert( b, quot;Only the last part of the selector matters.quot; ); }; </script>

DOM Modiﬁcation Injecting HTML ✦ Removing Elements ✦

Injecting HTML HTML 5: ✦ insertAdjacentHTML Already in IE, dicey support, at best ✦ What can we use instead? ✦ ✦ We must generate our own HTML injection ✦ Use innerHTML to generate a DOM

function getNodes(htmlString){ var map = { quot;<tdquot;: [3, quot;<table><tbody><tr>quot;, quot;</tr></tbody></table>quot;], quot;<optionquot;: [1, quot;<select multiple='multiple'>quot;, quot;</select>quot;] // a full list of all element fixes }; var name = htmlString.match(/<w+/), node = name ? map[ name[0] ] || [0, quot;quot;, quot;quot;]; var div = document.createElement(quot;divquot;); div.innerHTML = node[1] + htmlString + node[2]; while ( node[0]-- ) div = div.lastChild; return div.childNodes; } assert( getNodes(quot;<td>test</td><td>test2</td>quot;).length === 2, quot;Get two nodes back from the method.quot; ); assert( getNodes(quot;<td>test</td>quot;).nodeName === quot;TDquot;, quot;Verify that we're getting the right node.quot; );

Element Mappings option and optgroup need to be contained in a ✦ <select multiple=quot;multiplequot;>...</select> legend need to be contained in a ✦ <fieldset>...</fieldset> thead, tbody, tfoot, colgroup, and caption need to be ✦ contained in a <table>...</table> tr need to be in a ✦ <table><thead>...</thead></table>, <table><tbody>...</tbody></table>, or a <table><tfoot>...</tfoot></table> td and th need to be in a ✦ <table><tbody><tr>...</tr></tbody></table> col in a ✦ <table><tbody></tbody><colgroup>...</ colgroup></table> link and script need to be in a ✦ div<div>...</div>

DocumentFragment Fragments can collect nodes ✦ Can be appended or cloned in bulk ✦ Super-fast (2-3x faster than normal) ✦

Inline Script Execution .append(“<script>var foo = 5;</script>”); ✦ Must execute scripts globally ✦ window.execScript() (for IE) ✦ eval.call( window, “var foo = 5;” ); ✦ Cross-browser way is to build a script ✦ element then inject it ✦ Executes globally

function globalEval( data ) { data = data.replace(/^s+|s+$/g, quot;quot;); if ( data ) { var head = document.getElementsByTagName(quot;headquot;)[0] || document.documentElement, script = document.createElement(quot;scriptquot;); script.type = quot;text/javascriptquot;; script.text = data; head.insertBefore( script, head.firstChild ); head.removeChild( script ); } }

Removing Elements Have to clean up bound events ✦ ✦ IE memory leaks Easy to do if it’s managed. ✦

Events Three big problems with Events: ✦ ✦ Memory Leaks (in IE) ✦ Maintaining ‘this’ (in IE) ✦ Fixing event object inconsistencies

Leaks Internet Explorer 6 leaks horribly ✦ ✦ Other IEs still leak, not so badly Attaching functions (that have a closure to ✦ another node) as properties Makes a leak: ✦ elem.test = function(){ anotherElem.className = “foo”; };

‘this’ Users like having their ‘this’ refer to the ✦ target element Not the case in IE ✦ What’s the solution? ✦

Single Handler Bind a single handler to an event ✦ Call each bound function individually ✦ Can ﬁx ‘this’ and event object ✦ How do we store the bound functions in a ✦ way that won’t leak?

Central Data Store Store all bound event handlers in a central ✦ object Link elements to handlers ✦ Keep good separation ✦ One data store per library instance ✦ Easy to manipulate later ✦ ✦ Trigger individual handlers ✦ Easy to remove again, later

Central Data Store Events Data Element function click(){} #43 function mouseover(){} Element Data Element #67 Element Events Element function click(){} #22 function click(){} Data Store

Multiple Stores Element #43 Library A Element #67 Element Element #37 Element #22 Library B

Unique Element ID The structure must hook to an element ✦ Elements don’t have unique IDs ✦ Must generate them and manage them ✦ jQuery.data( elem, “events” ); ✦ Unique attribute name: ✦ ✦ elem.jQuery123456789 = 45; ✦ Prevents collisions with other libraries We can store all sorts of data in here ✦

Questions? http://ejohn.org/ ✦ http://twitter.com/jeresig/ ✦

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The DOM is a Mess @ Yahoo

by jeresig on Feb 02, 2009

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