Performance Optimization and JavaScript Best Practices

transcript

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

• 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

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

<body>The quick brown fox...</body>

// JavaScript logic

-------------------------------------------------------------------------------------

• For multiple views per user, and reusable scripts/css:

href="cart.css">

<body>The quick brown fox...</body>

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*/}

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;

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

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

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

var name = doris'; var lastName = chen'; function doSomething() {...} console.log(name);

var DudeNameSpace = { name : 'Jeffrey', lastName : 'Way', doSomething : function() {...}

} console.log(DudeNameSpace.name);

"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

-----------------------------------------------------------------------------------------------------------------------------

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

setInterval(someFunction, 3000);

• Pass function name

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

> 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

JavaScript Performance and

Best Practices

Doris Chen Ph.D. Developer Evangelist

doris.chen@microsoft.com

http://blogs.msdn.com/b/dorischen/ Twitter @doristchen