Unlearning and Relearning jQuery - Client-side Performance Optimization

Unlearning and Relearning jQuery
Client-side Performance Optimization
Oct 14, 2010
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Created for Magma Rails 2010 - www.magmarails.com

Purpose and Audience
Purpose
Understand why client-side performance optimization is important
Better understand jQuery and how to maximize performance when using it
Audience
Has a very basic understanding of client-side web development
JavaScript, HTML, CSS
Ideally, has used some JavaScript framework before
jQuery, Prototype, YUI, MooTools, etc.
If you haven’t specifically used jQuery before that’s fine. The API documentation will fill in the blanks later!
If you’ve never used a framework before that’s fine too, this will begin to make more sense later as you write your first jQuery application.
Oct 14, 2010
Unlearning and Relearning jQuery (Client-side performance Optimization) by Jonathan Dean - www.jonathandean.com
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Why care about the performance of client-side code?
Myth: “It doesn’t run on my servers so it’s not important to the speed of my application.”
Fact: Users of your website judge the performance of your application based on how long it takes for them to begin using it, not how long it takes to process and render a request on your servers
This is includes:
Number of requests for files to the server (this is a big deal)
Use sprites: http://css-tricks.com/css-sprites/
Compress JavaScript and CSS files into a single file of each type when possible: http://developer.yahoo.com/yui/compressor/
The total size of content download
The amount of time it takes to render the page
The amount of time it takes until the page is ready to be interacted with
How long it takes to process each interaction with the page
This means that perceived performance is just as important as actual performance! We’ll talk about techniques that apply to both
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Why use a JavaScript framework?
Deals with browser inconsistencies for you
You have to write a lot less code
Lots of community contribution and therefore a more stable core for your JavaScript application
You can upgrade to a newer version later and improve your application for free
Did I mention that you get to write less code?
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Why jQuery and not Prototype?
Easier to use than Prototype
Usually results in fewer lines of code
jQuery has a smaller footprint
By far the most popular JavaScript framework
About 28% of all websites on the internet use it
The big players use it (google, twitter, amazon, etc.)
This means more community support and more contribution to the open source code
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Using jQuery with Ruby on Rails
For Rails 3
Use the Unobtrusive JavaScript library at http://github.com/rails/jquery-ujs
In Rails 3 the JavaScript helpers no longer mix JS with your markup (this is a very good thing!) allowing us to easily swap out JS frameworks with just a single JS file
More instructions at http://joshhuckabee.com/jquery-rails-3
For older versions of Rails
Use the jRails plugin from http://github.com/aaronchi/jrails
Replaces the default helpers in Rails to output jQuery code instead of Prototype
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What you probably already know
You can access elements in a page using a CSS selector
$(‘#myDiv’) instead of document.getElementById(‘myDiv’)
In Prototype this would be $(‘myDiv’) since the $ method looks up an element based on its ID
$(‘.someClass’) gives you an array of all elements having a particular class
In Prototype this would be $$(‘.someClass’) since the $$ method is the one used for selecting elements via a CSS-style selector
You can do fun things like:
Show and hide elements on a page
$(‘#myDiv’).show()
$(‘#myDiv’).hide()
Make things happen when you click on an element
$(‘#myDiv’).click(function(){ alert(‘clicked!’) })
…and a bunch of other stuff too
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What you may not know
Some selectors can be very slow
You don’t need to use $() every time you want to select an element
You can chain jQuery method calls
Usually a good idea, but not always (more on this later)
You may be writing fewer lines of code with a JS framework but it can still make your application much slower if used improperly
This goes for all frameworks, including Prototype
jQuery !== JavaScript
jQuery is NOT all-powerful!
You still need to understand how JavaScript works without it!
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The DOM (Document Object Model)
Think of it as a JavaScript API for accessing HTML elements and manipulating them
Get an element with a particular ID
document.getElementById(‘myDiv’)
Redirect to another page
window.location = ‘http://example.com’
Get all elements with a particular tag name
document.getElementsByTagName(’div’);
All browsers support these
You want to favor jQuery use that directly maps to a built in DOM method because they will be the fastest
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Image courtesy of John M. Kennedy T. http://en.wikipedia.org/wiki/File:JKDOM.SVG

The DOM (Document Object Model)
Terminology I’ll use
Descendent: All elements contained within that element
LINK and ANCHOR are descendents of DOCUMENT
TEXT and RADIO are descents of FORM
TEXT and RADIO are also descendents of DOCUMENT
Ancestor: The opposite of descendent
FORM is an ancestor of TEXT and RADIO
DOCUMENT is the ancestor of FORM, TEXT, RADIO, LINK, and ANCHOR
Parent: The direct ancestor of an element (only one)
FORM is the parent of TEXT and RADIO
DOCUMENT is the parent of LINK and ANCHOR
Children: The direct descendents of an element
TEXT and RADIO are children of FORM
TEXT and RADIO are not children of DOCUMENT (they are descendents)
LINK and ANCHOR are children of DOCUMENT
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Note: Examples are for the image to the right, not true for all pages
Image courtesy of John M. Kennedy T. http://en.wikipedia.org/wiki/File:JKDOM.SVG

Using Firebug to look at the DOM
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Firebug is a free extension for Firefox: http://getfirebug.com/Webkit browsers (like Safari and Chrome) have similar built-in functionality.

The difference between DOM Ready and onload
You can’t access a DOM element until it has been fully loaded and rendered by the browser
onload occurs after everything on the page has been loaded
HTML
Stylesheets
JavaScript (including 3rd party!)
Images
Content of iframes
Comes with the browser
window.onload = myFunction;
<body onload=“myFunction();”>
DOM Ready occurs when the document has been processed and all DOM objects are ready for manipulation
Doesn’t wait for assets since they aren’t part of the DOM
jQuery and Prototype give you this functionality
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How do I choose?
Put application-critical JavaScript in a DOM Ready block
When you can’t use your application without it
Example: critical event listeners
$(‘#myButton’).click(loadImportantForm);
Use onload for anything non-critical
Deferring to onload is a good idea when possible because too much code in DOM Ready can cause the rendering of your page to stall, adding additional time until the user can interact with it
Example: initializing animations
$(‘#myDiv’).hover(sillyFadeOver, sillyFadeOut);
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When do I need DOM Ready / onload?
Simple answer: when the browser loads your JavaScript before it loads the DOM elements you need
When you define your JavaScript in external files
Usually you include these in the header. This means that the JavaScript file is fully loaded an begins to execute before the browser begins to load the body of your document.
Sometimes you also include JS files within the body of your document. In this case you don’t know when the file will be ready to execute. Could happen before your DOM is ready or after it.
When you use inline JavaScript before all required elements have been loaded
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Does that mean I don’t always need it?
Yes, but be careful. When in doubt, use DOM Ready or onload
This will work
<a id=“myAnchor” href=“#”>click me if you can</a>
<script>document.getElementById(‘myAnchor’);</script>
This will not
<script>document.getElementById(‘myAnchor’);</script>
<a id=“myAnchor” href=“#”>click me if you can</a>
This will work in some browsers but IE < 8 will have a fit
<body>
<div>
<script type="text/javascript">
varnewElem = document.createElement(’div');
document.body.appendChild(newElem);
</script>
</div>
</body>
(so don’t try to use element before it has been closed)
This applies to jQuery syntax as well
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Using DOM Ready and onload
Onload
function myFunction(){
// your code
}
window.onload = myFunction;// replaces onload with this function, so can only have this once
Onload (jQuery style)
$(window).load(function(){ // can have multiple on a page
// your code
});
DOM Ready
jQuery only and can have multiple DOM Ready blocks on a page
Longer format
$(document).ready(function(){
// your code
});
Short version
$(function(){
// your code
});
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Common mistake with DOM Ready
Don’t define methods in a Dom Ready block, just call them
Works
// your code
}
myFunction();
});
Doesn’t work
// your code
}
myFunction();
});
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Getting elements faster
Always start with an ID
$(‘#myDiv’)
Behind the scenes, jQuery can use the native document.getElementById() which is very fast
Fastest way to get a DOM element
Avoid the class selector on a large DOM node
$(‘.myClass’)
Very slow over a large DOM node
No native DOM call common to all browsers
Use a tag name when possible
$(‘div.myClass’) is much faster than $(‘.myClass’)
Uses document.getElementsByTagName() behind the scenes
But remember that ID is the fastest so $(‘div#myDiv’) is actually slower than $(‘#myDiv’)
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Getting elements faster
Optimize selectors using the ‘right to left’ model
As of jQuery 1.3 this the method the selector engine uses
This means the most specific (right-most) selector is found first and then its ancestors are checked starting on the left
Make your right-most selector very efficient
Example
$(‘.myClassdiv.otherClass’) is actually more efficient than
$(‘div.myClass .otherClass’)
However, descending from an ID is still the fastest:
$(‘#myDivdiv.otherClass’)
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Cache your DOM lookups
A call to $() returns a valid jQuery object, so store it for later use
This is bad because each line does a DOM lookup
$(‘#myDiv .myClass’).click(myFunc);
$(‘#myDiv .myClass’).css(‘width’, ‘250px’);
$(‘#myDiv .myClass’).css(‘color’, ‘red’);
This is much better
varmyStuff = $(‘#myDiv .myClass’);
myStuff.click(myFunc);
myStuff.css(‘width’, ‘250px’);
myStuff.css(‘color’, ‘red’);
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Work with the most specific DOM element possible
$(‘#myDiv .myClass’) is much faster than $(‘.myClass’)
Use the find() jQuery method to search for matching elements within your cached object
varmyDiv = $(‘#myDiv’);
myDiv.find(‘.myClass’).hide();
myDiv.find(‘input’).click(myFun);
A lot of times it makes sense to cache the main elements of your page up front and use them later
content = $(‘#content’);
sidebar = content.find(‘#sidebar’);
article = content.find(‘#article’);
commentWrapper = article.find(‘#comments’);
comments = commentWrapper.find(‘div.comment’);
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Use chaining
Remember that a call to $() returns a valid jQuery object, which means you can chain jQuery methods
You can shorten the previous example:
$(‘#myDiv .myClass’).click(myFunc);
$(‘#myDiv .myClass’).css(‘width’, ‘250px’);
$(‘#myDiv .myClass’).css(‘color’, ‘red’);
To the lighter-weight:
$(‘#myDiv .myClass’).click(myFunc).css(‘width’, ‘250px’).css(‘color’, ‘red’);
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When not to use chaining
Be careful chaining the DOM traversal methods
Doing so means that your application may be too dependent on the DOM structure
Example of what not to do:
$(‘#myDiv’).parent().parent(); // fails if you add another element surrounding this one
Potential alternatives:
$(‘#myDiv’).parents(‘someTag’)
$(‘#myDiv’).parents(‘.someClass’)
Not about performance, but about writing good code
Yes, the DOM structure still matters in the last two examples but it isn’t so rigid that you can’t add or remove elements in between
And yes, the last two examples may be a little slower but not so much that it’s worth writing unsafe code
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Avoid as much DOM manipulation as possible
Rendering changes to the DOM is very expensive, so do as few manipulations as possible
Example of what not to do:
<div id=“myDiv”></div>
<script>
for(vari=0; i < 100; i++){
$(‘#myDiv’).append(‘<a href=“#”>link</a>’); // The browser has to re-render this and any other effected elements at each iteration of the loop
}
</script>
Oct 14, 2010
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It would be much more efficient to store all elements you wish to create in a string and then insert them one time:
<script>
myLinks = ‘’;
for(vari=0; i < 100; i++){
myLinks = myLinks + ‘<a href=“#”>link</a>’;
}
$(‘#myDiv’).append(myLinks); // this is the only DOM manipulation
</script>
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It would be even more efficient if we wrap many individual elements in a single element and then insert that in the DOM:
<script>
myLinks = ‘<div id=“myDiv”>’;
for(vari=0; i < 100; i++){
myLinks = myLinks + ‘<a href=“#”>link</a>’;
}
myLinks = myLinks + ‘</div>’;
$(‘#myDiv’).replaceWith(myLinks); // instead of putting a string of 100 elements in this div we are replacing a single DOM node
</script>
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Don’t like the idea of creating a string for your DOM elements?
Since rendering is the most expensive part, use the clone() jQuery method to make a copy of the DOM object that you can manipulate without being rendered and then insert that object into the DOM when finished
<script>
clonedDiv = $(‘#myDiv’).clone();
for(vari=0; i < 100; i++){
clonedDiv.append(‘<a href=“#”>link</a>’); // clonedDiv is not visible on the page so manipulating it doesn’t cause the browser to re-render
}
$(‘#myDiv’).replaceWith(clonedDiv); // still the only time re-rendering occurs
</script>
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You can also improve performance of DOM manipulations by using the DocumentFragment object
DocumentFragment is a lightweight Document object you can create, manipulate and then insert into the DOM later (much like the previous example)
More information at: http://www.devguru.com/technologies/xmldom/quickref/obj_documentFragment.html
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Event delegation
Events in jQuery “bubble” up to their parent DOM elements unless you tell them not to
You can use this to your advantage by creating a single event listener on a parent object and letting that listener determine which of its children triggered the event
This is much more efficient than creating a many listeners on individual elements
If you’re only binding one or two event listeners then this method isn’t really necessary
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Event delegation
Let’s say you have a div with 100 anchor tags and you want to hide the anchor when it’s clicked
Instead of:
$(‘#myDiva’).bind(‘click’, function(){
$(this).hide();
});
Do this:
$(‘#myDiv’).bind(‘click’, function(e){
if(e.target.nodeName.toLowerCase() === 'a'){
$(e.target).hide()
}
});
e is the Event object
Notice that e.target returns the actual DOM node, not a jQuery object, so you need to pass it to $() to get jQuery functionality
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jQuery.delegate()
This is the same example using jQuery’s built in event delegation method (1.4.2 and above)
$('#myDiv').delegate('a', 'click', function(e){
$(this).hide();
});
Tip: for this and the last example, use e.preventDefault() to prevent the default action of the event. In this case, following the hyperlink:
$('#myDiv').delegate('a', 'click', function(e){
e.preventDefault();
$(this).hide();
});
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Lazy-loading assets
Load the bare minimum up front and load the rest as needed
Initial load and rendering of the page is much faster, allowing the user to interact with the page as quickly as possible
Works well in many cases but sometimes waiting for something to load after the user begins to interact with the page is more frustrating, so choose wisely when it’s appropriate
An excellent choice for non-critical 3rd party tracking scripts, especially ones that are notorious slow
Can apply to JavaScript files, images, even CSS
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Lazy-loading assets
Method 1: document.write()
Can also apply this method to other types of assets such as images and CSS
<script>
document.write('<script src="', ’someScript.js', '" type="text/javascript"></script>');
</script>
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Lazy-loading assets
Method 2: Change the src property of an existing script tag
This example is just for JavaScript but you could do something similar for other assets
<script id=”lazy" type="text/JavaScript"></script>
<script>
document.getElementById(’lazy').src = ‘someScript.js';
</script>
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Lazy-loading assets
Method 3: Use AJAX
Applies to any type of asset
Just add an AJAX call on DOM Ready to grab what you need
Check out the jQuery documentation on using AJAX: http://api.jquery.com/category/ajax/
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Lazy-loading assets
Method 4: Use the Lazy Load jQuery plugin: http://www.appelsiini.net/projects/lazyload
When you want to lazy load images on a very long page
Let’s you load images visible in the browser first and delay loading of images “below the fold” (below the current browser view)
Simplest example:
$("img").lazyload(); // causing images below the fold to be lazy loaded
Start loading images when they are 200px below the fold:
$("img").lazyload({ threshold : 200 });
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Unobtrusive JavaScript
Keep JavaScript in external files and avoid inline JavaScript (Rails 3 does it!)
Less code for the browser to load and interpret while rendering
JavaScript in external files can be cached by the browser. Inline JavaScript can’t be cached
Avoid:
<a href=“#” onclick=“myFunc();”>run myFunc</a>
In favor of:
<a href=“#” id=“myLink”>run myFunc</a>
$(‘#myLink’).bind(‘click’, myFunc); // in external file on DOM Ready
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Unobtrusive JavaScript
Rails 3 is a great inspiration for more advanced uses
Uses custom attributes to indentify what events to bind on elements
Rails helper code:
<%= link_to "delete", domain_path(@domain), :method => :delete, :confirm => "Are you sure?" %>
In Rails 2 this outputs a real mess:
<a href="/domains/1" class="destroy" onclick="if (confirm('Are you sure?')) { varf = document.createElement('form'); f.style.display = 'none'; this.parentNode.appendChild(f); f.method = 'POST'; f.action = this.href;varm = document.createElement('input'); m.setAttribute('type', 'hidden'); m.setAttribute('name', '_method'); m.setAttribute('value', 'delete'); f.appendChild(m);vars = document.createElement('input'); s.setAttribute('type', 'hidden'); s.setAttribute('name', 'authenticity_token'); s.setAttribute('value', 'pKvg9hsnQ33uk='); f.appendChild(s);f.submit(); };return false;">delete</a>
In Rails 3 the markup just looks like this:
<a href="/domains/1" data-confirm="Are you sure?" data-method="delete" rel="nofollow">delete</a>
An external JavaScript file (remember jquery-ujs?) uses the attributes and binds the appropriate JavaScript to the element
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Examples from http://www.simonecarletti.com/blog/2010/06/unobtrusive-javascript-in-rails-3/

Storing data on DOM objects
Sometimes you need to store extra information about an element to improve efficiency, keep yourself from having to pass a bunch of variables through method calls, etc.
It’s tempting to do one of the following:
$(‘#myElement’).attr(‘rel’, ‘some data’); // improper use of attribute
var data = $(‘#myElement’).attr(‘rel’);
$(‘#hiddenInput’).val(‘some data’); // unnecessary form data
var data = $(‘#hiddenInput’).val();
jQuery gives you a much nicer alternative:
$(‘#myElement’).data(‘key’, ‘value’);
var data = $(‘#myElement’).data(‘key’);
Note: you can also store data on a DOM object directly using standard object syntax since a DOM object is the same as any other object. Just don’t store it on a jQuery object unless you use data()
varelem = $(‘#myElement’).get(0);
elem.foo = ‘bar’;
alert(elem.foo);
But data() is safer so you generally just want to use that
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Use for instead of each()
The native for loop is much faster than jQuery’s each() iterator so use it when possible for large arrays
Instead of:
$.each (array, function (i) {
// do something with array[i]
});
The same can be done with a native for loop:
for (vari=0;i<array.length; i++) {
// do something with array[i]
}
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Use short variable names and create aliases
Reduces overall size of file, especially if not minified
Only useful when these items are used frequently in a large JS application
Does reduce code readability so good minification may be a better option in many cases, but some of these may not be minified so watch out
varw = window,
l = w.location,
d = document,
t = true,
f = false,
n = null,
M = Math,
o = "object",
a = "array",
s = "string”,
u = "undefined";
l = ‘http://jonathandean.com’; // redirect to another page
if(myVar == f || myVar == n) // test if myVar is false or null
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Other client-side performance tips
Make the fewest number of requests to the server as possible
Each request adds overhead and additional time to load
Combine multiple small images into a single sprite
Combine JS and CSS into a single file of each type when possible
Don’t load a file you don’t need
Compress JS and CSS to remove whitespace characters and shorten long variable and method names
You don’t necessarily want to create “packed” files
These files will have the smallest total size
However, the browser needs to read in the entire file and parse it before it can be used. The parsing step is not cached and adds overhead to each page load. Often the benefit does not outweigh the cost
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Download for later
You can find these slides sometime tomorrow at http://www.jonathandean.com/2010/10/unlearning-and-relearning-jquery-client-side-performance-optimization/
The short URL is http://tinyurl.com/jquery-magma
Check http://jonathandean.com soon for an HTML version of the presentation (after a few more caguamas and a long flight home!)
Oct 14, 2010
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Unlearning and Relearning jQuery - Client-side Performance Optimization

by Jon Dean on Oct 14, 2010

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Unlearning and Relearning jQuery - Client-side Performance Optimization — Presentation Transcript