Performance Optimization and JavaScript Best Practices

JavaScript Performance and
Best Practices

Doris Chen Ph.D.
Developer Evangelist
doris.chen@microsoft.com
http://blogs.msdn.com/b/dorischen/
Twitter @doristchen

Who am I?
• Developer Evangelist at Microsoft based in Silicon Valley, CA
– Blog: http://blogs.msdn.com/b/dorischen/
– Twitter @doristchen
– Email: doris.chen@microsoft.com
• Has over 13 years of experience in the software industry focusing on
web technologies
• Spoke and published widely at JavaOne, O'Reilly, SD West, SD
Forum and worldwide User Groups meetings
• Before joining Microsoft, Doris Chen was a Technology Evangelist at
Sun Microsystems
• Doris received her Ph.D. from the University of California at Los
Angeles (UCLA)

Agenda
• Optimization Strategies

• JavaScript Best Practices

Ajaxify
• The client and server are in a dialog
• Make the messages between them as small as
possible
• The client does not need a copy of the
database
– just enough information to serve the user
• Don't rewrite the server application in
JavaScript

Don't Optimize Without Measuring
• Intuitions are often wrong
• A single trial is unreliable. Timers can be off by
as much as 15 msec
• Even accurate measurements can lead to
wrong conclusions

Manual Timing Method

function myFunctionToTest() {
var start = new Date().getTime();
... // body of the function
var totalTime = new Date().getTime() - start;
}

Only speed up things that take a lot of
time
• If profiling shows that you are spending most
of your time in A, don't bother optimizing C.

Don't Tune For Quirks
• Some browsers have surprising inefficiencies
• A trick that is faster on Browser A might be
slower on Browser B
• The performance characteristics of the next
generation may be significantly different
• Avoid short-term optimizations

Put stylesheets at the top (css)
> Want the browser to display whatever content
it has as soon as possible
 Avoids flash of unstyled content or blank white
screen
> Solution: put stylesheets in document head
 allows the page to render progressively
> CSS at the bottom:
 prohibits progressive rendering in many browsers,
including Internet Explorer
 browsers block rendering to avoid having to redraw
elements of the page if their styles change

Move scripts to the bottom (javascript)
> Scripts block parallel downloads across all
hostnames

> Scripts block rendering of everything below them
in the page

JavaScript Best Practices
> Provide a clean separation of content, CSS, and
JavaScript
> De-reference unused objects

> Think Asynchronous

> Working with Objects

> Defer Loading Resources

> General JavaScript Coding Best Practices

11

Separation of content, CSS, and
JavaScript
> A rich web application user interface is made up
of
 content (HTML/XHTML)
 styles (CSS)
 JavaScript
> Place CSS and JavaScript code in separate files
 Optimize the bandwidth usage by having CSS and
JavaScript file loaded only once
> Variables to decide using external or inline
 multiple page views per user per session
 many of pages re-use the same scripts and stylesheets

Inline or External
• Inline JavaScript and CSS in front page, but dynamically
download the external files after the page has finished loading

<style>#Styles</style>
<body>The quick brown fox...</body>
<script type="text/javascript">
// JavaScript logic
<script>
-------------------------------------------------------------------------------------
• For multiple views per user, and reusable scripts/css:
<link rel="stylesheet" type="text/css"
href="cart.css">
<body>The quick brown fox...</body>
<script type="text/javascript" src="cart.js">

De-reference unused objects
//delete objects
var foo='Delete Me'; //do something with foo
delete foo;

// detach listeners
someElement.removeEventListener(type, fn, false);

// remove DOM elements
someElement.parentNode.removeChild(someElement);

// page onunload
window.onunload= function() { /*do something*/}

14

Think Asynchronous
> Prevents browsers from halting execution of a
code block that might take a long time
> Semi-Asynchronous – Like an Ajax request
 Use a callback
> use setTimeout() in conjunction with a callback
function longProcess({ name : value}, callback) {
// do what needs to be done
callback.apply({});
}

setTimeout(function() {
longProcess({ name : “doris”}, function() {
alert(“all done”);
}
}, 0);

Working with Objects (I)
var i;
for (i = 0; i < divs.length; i += 1) {
divs[i].style.color = "black";
divs[i].style.border = thickness + 'px solid blue';
divs[i].style.backgroundColor = "white";
}
-----------------------------------------------------------------------------------------------------------------------------------------------
var border = thickness + 'px solid blue';
var nrDivs = divs.length;
var ds, i;
for (i = 0; i < nrDivs; i += 1) {
ds = divs[i].style;
ds.color = "black";
ds.border = border; Good
ds.backgroundColor = "white";
}

Strings
• Concatenation with +
– Each operation allocates memory
– foo = a + b;
• Concatenate with array.join('')
– The contents of an array are concatenated into a
single string
– foo = [a, b].join('');

Working with Objects (II)
String Concatenation
window.tablebuilder = function() {
var _header, _rows = [];
this.setHeader = function(_headers) {
_header = "<tr><th>" +
_headers.join("</th><th>") + "</tr>";
};
this.addRow = function(_cells) {
_rows.push("<tr><td>" +
_cells.join("</td><td>") + "</td></tr>");
};
this.toString = function() {
return "<table>" + _header +
"<tbody>" + _rows.join("") + "</tbody>" +
"</table>";
};
};

Defer Loading Resources
> If you have a large library or set of libraries,
you don't need to load everything when a
page is loaded
> Resources can be html, scripts, or css
 Use Ajax Request
 Add script/css links using DOM

Example: Defer Loading Resources

<script>
var deferredLibs = [ '/resources/jquery.js' ,
'/resources/base.js'];
addLibraries(libs : deferredLibs,
function(args) {
// initialize components
});
</script>

Example: Defer Loading Resources (cont.)

function addLibraries( libs, callback) {
for(var i=0; i < libs.length; i+=1) {
var head =
document.getElementsByTagName("head")[0];
var s = document.createElement("script");
// add callback tracking logic
s.src = libs[i];
head.appendChild(s);
}
}

JavaScript Best Practices
> Provide a clean separation of content, CSS, and
JavaScript
> De-reference unused objects

> Think Asynchronous

> Working with Objects

> Defer Loading Resources

> General JavaScript Coding Best Practices

22

Use === Instead of ==

• two different kinds of equality operators: ===
| !== and == | !=
• same type and value
– === true
– !== false

Eval = Bad
eval(string)

• eval function compiles and executes a string and
returns the result
– gives us access to JavaScript’s compiler
• what the browser uses to convert strings into actions
• most misused feature of the language
– decreases script’s performance substantially
– also poses a huge security risk because it grants far too much
power to the passed in text
• Avoid it if you can!

Don’t Use Short-Hand
• Technically, get away with omitting most curly
braces and semi-colons
if(someVariableExists)
x = false
anotherfunctionCall();
-----------------------------------------------------------------------------------------------------------------------------
Interpreted by some browsers

if(someVariableExists) {
x = false;}
anotherFunctionCall();
----------------------------------------------------------------------------------------------------------------------------- -----

if(someVariableExists) {
x = false;
anotherFunctionCall();
}

Declare Variables Outside of the For
Statement
for(var i = 0; i < someArray.length; i++) {
var container = document.getElementById('container');
container.innerHtml += 'my number: ' + i;
console.log(i);
}
----------------------------------------------------------------------------------------------------------------------
var container = document.getElementById('container');
for(var i = 0, len = someArray.length; i < len; i++) {
container.innerHtml += 'my number: ' + i;
console.log(i);
}

Reduce Globals
"By reducing your global footprint to a single name, you significantly reduce
the chance of bad interactions with other applications, widgets, or libraries."
- Douglas Crockford

var name = doris';
var lastName = chen';
function doSomething() {...}
console.log(name);
-----------------------------------------------------------------------------------------------------------------------------
var DudeNameSpace = {
name : 'Jeffrey',
lastName : 'Way',
doSomething : function() {...}
}
console.log(DudeNameSpace.name);

Don't Pass a String to "SetInterval"
or "SetTimeOut"
• Never pass a string to SetInterval and
SetTimeOut

setInterval(
"document.getElementById('container').innerHTML +=
'My new number: ' + i", 3000
);

• Pass function name
setInterval(someFunction, 3000);

Use {} Instead of New Object()
var o = new Object();
o.name = 'Jeffrey';
o.lastName = 'Way';
o.someFunction = function() {
console.log(this.name);
}
----------------------------------------------------------------------------------------------------------------------

var o = {
name: 'Jeffrey',
lastName = 'Way',
someFunction : function() {
console.log(this.name);
}
};

var o = {}; //create empty object

Use [] Instead of New Array()

var a = new Array();
a[0] = "Joe";
a[1] = 'Plumber';
----------------------------------------------------------------------------------------------------------------------

var a = ['Joe','Plumber'];

Code Quality
• High quality code is most likely to avoid
platform problems.
• Code Conventions for the JavaScript
Programming Language
– http://javascript.crockford.com/code.html
• Use JSLint.com. Pass with no warnings

JSLint -- Code Quality Tool
• JSLint can help improve the robustness and
portability of your programs
– enforces style rules
– spot some errors that are very difficult to find in
debugging
– It can help eliminate implied globals
• Integrated with tools, Visual Studio 2010
– http://jslint4vs2010.codeplex.com/
• Resources
– http://www.jslint.com/
– http://www.javascriptlint.com/download.htm

DOM Manipulation
• If JavaScript were infinitely fast, most pages
would run at about the same speed.
• The bottleneck tends to be the DOM interface
• There is a significant cost every time you
touch the DOM tree
• Each touch can result in a reflow computation,
which is expensive

Make good use of Ajax Libraries
• Effective code reuse will make widgets more
effective
• JavaScript Toolkits
– Wrap up ajax details in javascript libraries
– jQuery, Dojo, prototype+scriptaculous, YUI,...

Reduce the size of JavaScript file
• Reduce the amount of source code to reduce
download time.
• Minification deletes whitespace and
comments
– While in development mode keep your scripts
readable so that they may be debugged easier
• Consider compressing your JavaScript
resources when you deploy your application
– If you use a 3rd party JavaScript library use the
compressed version if one is provided.

Gzip components (server)
> you can affect users' download times
 Gzip supported in more browsers
 Gzip generally reduces the response size by 70%
> Not just for html, gzip all scripts, stylesheets, XML, JSON
but not images, PDF Content-Encoding: gzip

> Gzip configuration
 HTTP request
Accept-Encoding: gzip, deflate

 HTTP response
Content-Encoding: gzip

Resources
> Best practices and Guidelines
 http://developer.yahoo.com/performance/rules.ht
ml
> Useful Sites
 http://stevesouders.com/
 http://javascript.crockford.com/

Upcoming Web Camps
2 Days HTML5 and Web Development
WebCamp
May 20-May 21st, 2011, Mountain View, CA
Free, learning innovative web technology,
hands on experience

Performance Optimization and JavaScript Best Practices

Recommended

More Related Content

What's hot (20)

Viewers also liked (7)

Similar to Performance Optimization and JavaScript Best Practices (20)

More from Doris Chen (20)

Recently uploaded (20)

Performance Optimization and JavaScript Best Practices