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/

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Performance Optimization and JavaScript Best Practices

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