High Performance JavaScript (Amazon DevCon 2011)

High Performance JavaScript Nicholas C. Zakas | Amazon DevCon | April 27, 2011

Whos this guy? Presentation Contributor, Architect Creator of YUI TestAuthor Lead Author Contributor Lead Author

@slicknet(Complaints: @robertduffy)

Does JavaScript performance matter?

After all, all browsers now have optimizing JavaScript enginesTracemonkey/ V8 Squirrelfish Chakra KarakanJaegarMonkey (all) (4+) (9+) (10.5+) (3.5+)

So our scripts are getting really, really fast

Old computers ran slow applications Small amounts of CPU power and memory

New computers are generally faster but slow applications still existMore CPU + more memory = less disciplined application development

Right???

Oh yeah, one more thing

http://jeftek.com/1942/motorola-xoom-sunspider-results/

Its still possible to write slowJavaScript on the new, faster JavaScript engines

JavaScript performance directlyaffects user experience

"Know the enemy and know yourself;in a hundred battles you will never be in peril." -Sun Tzu, The Art of War

The UI ThreadThe brains of the operation

The browser UI thread is responsible forboth UI updates and JavaScript execution Only one can happen at a time

Jobs for UI updates and JavaScript execution are added to a UI queue Each job must wait in line for its turn to execute

<button id="btn" style="font-size: 30px; padding: 0.5em 1em">Click Me</button><script type="text/javascript">window.onload = function(){ document.getElementById("btn").onclick = function(){ //do something };};</script>

Before ClickUI Threadtime UI Queue

When ClickedUI Threadtime UI Queue UI Update onclick UI Update

When ClickedUI Thread UI Updatetime UI Queue onclick Draw down state UI Update

When ClickedUI Thread UI Update onclicktime UI Queue UI Update

When ClickedUI Thread UI Update onclick UI Updatetime UI Queue Draw up state

No UI updates while JavaScript is executing

JavaScript May Cause UI Update<button id="btn" style="font-size: 30px; padding: 0.5em 1em">Click Me</button><script type="text/javascript">window.onload = function(){ document.getElementById("btn").onclick = function(){ var div = document.createElement(“div”); div.className = “tip”; div.innerHTML = “You clicked me!”; document.body.appendChild(div); };};</script>

A UI update must use the latest info available

Long-running JavaScript = Unresponsive UI

Responsive UIUI Thread UI Update JavaScript UI Updatetime

Unresponsive UIUI Thread UI Update JavaScript UI Updatetime

The longer JavaScript runs,the worse the user experience

The runaway script timer prevents JavaScript from running for too long Each browser imposes its own limit (except Opera)

Internet Explorer

Firefox

Safari

Chrome

Runaway Script Timer Limits• Internet Explorer: 5 million statements• Firefox: 10 seconds• Safari: 5 seconds• Chrome: Unknown, hooks into normal crash control mechanism• Opera: none

Does JIT compiling help?

Interpreted JavaScriptUI Thread Interprettime

JITed JavaScript (1st Run)UI ThreadCompile Executetime

JITed JavaScript (After 1st Run)UI Thread Executetime

How Long Is Too Long?“0.1 second [100ms] is about the limit forhaving the user feel that the system is reactinginstantaneously, meaning that no specialfeedback is necessary except to display theresult.” - Jakob Nielsen

Translation: No single JavaScript job shouldexecute for more than 100ms to ensure a responsive UI

Recommendation:Limit JavaScript execution to no more than 50ms measured on IE6 :)

Doing so makes your application awesome

Loadtime TechniquesDont let JavaScript interfere with page load performance

During page load, JavaScripttakes more time on the UI thread

<!doctype html><html><head> <title>Example</title></head><body> <p>Hello world!</p> <script src="foo.js"></script> <p>See ya!</p></body></html>

ResultUI Thread UI Update JavaScript UI Updatetime

ResultUI Thread Hello world! foo.js See ya!time

ResultUI Thread Hello world! Download Parse Run See ya!time The UI thread needs to wait for the script to download, parse, and run before continuing

ResultUI Thread Hello world! Download Parse Run See ya! Variable ConstantDownload time takes the longest and is variable

Translation: The page doesnt render whileJavaScript is downloading, parsing, or executing during page load

<!doctype html><html><head> <title>Example</title></head><body> <script src="foo.js"></script> <p>Hello world!</p> <script src="bar.js"></script> <p>See ya!</p> <script src="baz.js"></script> <p>Uh oh!</p></body></html>

ResultUI Thread JavaScript UI Update JavaScript UI Update JavaScripttime The more scripts to download in between UI updates, the longer the page takes to render

Technique #1: Put scripts at the bottom

<!doctype html><html><head> <title>Example</title></head><body> <p>Hello world!</p> <p>See ya!</p> <script src="foo.js"></script></body></html>

Put Scripts at BottomUI Thread UI Update UI Update JavaScript JavaScript JavaScripttime Even if there are multiple scripts, the page renders quickly

Technique #2: Combine JavaScript files

<!doctype html><html><head> <title>Example</title></head><body> <p>Hello world!</p> <p>See ya!</p> <script src="foo.js"></script> <script src="bar.js"></script> <script src="baz.js"></script></body></html>

UI ThreadUI Update JavaScript JavaScript JavaScripttime Each script has overhead of downloading

UI ThreadUI Update JavaScripttime Combining all of the files limits the networkoverhead and gets scripts onto the page faster

<!doctype html><html><head> <title>Example</title></head><body> <p>Hello world!</p> <p>See ya!</p> <script src="foo-and-bar-and-baz.js"></script></body></html>

Technique #3: Load scripts dynamically

Basic Techniquevar script = document.createElement("script"), body;script.type = "text/javascript";script.src = "foo.js";body.insertBefore(script, body.firstChild);Dynamically loaded scripts are non-blocking

Downloads no longer block the UI thread

<!doctype html><html><head> <title>Example</title></head><body> <p>Hello world!</p> <script src="foo.js"></script> <p>See ya!</p></body></html>

Using HTML <script>UI Thread Hello world! Download Parse Run See ya!time

<!doctype html><html><head> <title>Example</title></head><body> <p>Hello world!</p> <script> var script = document.createElement("script"), body = document.body; script.type = "text/javascript"; script.src = "foo.js"; body.insertBefore(script, body.firstChild); </script> <p>See ya!</p></body></html>

Using Dynamic ScriptsUI Thread Hello world! See ya! Run UI Updatetime Download ParseOnly code execution happens on the UI thread, which means less blocking of UI updates

UsageloadScript("foo.js", function(){ alert("Loaded!");});

Timing Note: Script execution begins immediatelyafter download and parse – timing of execution is not guaranteed

Using Dynamic ScriptsUI Thread Hello world! Run See ya! UI Updatetime Download ParseDepending on time to download and script size, execution may happen before next UI update

Technique #4: Defer scripts

<!doctype html><html><head> <title>Example</title></head><body> <p>Hello world!</p> <script defer src="foo.js"></script> <p>See ya!</p> </body></html>

Support for <script defer>3.5 7.0 5.0 4.0 ?

Deferred scripts begin to download immediately,but dont execute until all UI updates complete

Using <script defer>UI Thread Hello world! See ya! More UI More UI Runtime Download Parse Similar to dynamic script nodes, but with a guarantee that execution will happen last

Timing Note:Although scripts always execute after UI updates complete, the order ofmultiple <script defer> scripts is not guaranteed across browsers

Technique #5: Asynchronous scripts

<!doctype html><html><head> <title>Example</title></head><body> <p>Hello world!</p> <script async src="foo.js"></script> <p>See ya!</p> </body></html>

Support for <script async>3.6 7.0 5.0 ? ?

Asynchronous scripts behave a lot like dynamic scripts

Using <script async>UI Thread Hello world! See ya! Run UI Updatetime Download ParseDownload begins immediately and execution is slotted in at first available spot

Note:Order of execution is explicitly notpreserved for asynchronous scripts

Runtime TechniquesWays to ensure JavaScript doesnt run away

function processArray(items, process, callback){ for (var i=0,len=items.length; i < len; i++){ process(items[i]); } callback();}

Technique #1: Timers

//create a new timer and delay by 500ms setTimeout(function(){ //code to execute here }, 500)setTimeout() schedules a function to be added to the UI queue after a delay

When ClickedUI Thread UI Updatetime UI Queue onclick UI Update

When ClickedUI Thread UI Update onclick UI Updatetime UI Queue

After 25msUI Thread UI Update onclick UI Updatetime UI Queue JavaScript

After 25msUI Thread UI Update onclick UI Update JavaScripttime UI Queue

After Another 25msUI Thread UI Update onclick UI Update JavaScripttime UI Queue JavaScript

After Another 25msUI Thread UI Update onclick UI Update JavaScript JavaScripttime UI Queue

Technique #2: Web Workers

Web Workers● Asynchronous JavaScript execution● Execution happens outside of UI thread ● Not on the UI thread = no UI delays● Data-driven API ● Data is serialized when sending data into or out of Worker ● No access to DOM, BOM ● Completely separate execution environment

//in pagevar worker = new Worker("process.js");worker.onmessage = function(event){ useData(event.data);};worker.postMessage(values);//in process.jsself.onmessage = function(event){ var items = event.data; for (var i=0,len=items.length; i < len; i++){ process(items[i]); } self.postMessage(items);};

When ClickedUI Thread UI Updatetime UI Queue onclick UI Update

When ClickedUI Thread UI Update onclicktime Worker Thread UI Queue UI Update

When ClickedUI Thread UI Update onclick UI Updatetime Worker Thread UI Queue JavaScript

Worker Thread CompleteUI Thread UI Update onclick UI Updatetime UI Queue onmessage

Worker Thread CompleteUI Thread UI Update onclick UI Update onmessagetime UI Queue

Support for Web Workers3.5 4.0 4.0 ? 10.6

Repaint and ReflowHidden performance costs of common operations

Long UI updates =Unresponsive UI

JavaScript can causelong UI updates by triggering repaint and reflow

A repaint occurs when a visual change doesnt require recalculation of layout Changes to visibility, colors (text/background), background images, etc.

Repaint<button id="btn" style="font-size: 30px; padding: 0.5em 1em">Click Me</button><script type="text/javascript">window.onload = function(){ document.getElementById("btn").onclick = function(){ this.style.color = "#ff0"; };};</script> Repaint!

A reflow occurs when a visual change requires a change in layoutInitial page load ▪ browser resize ▪ DOM structure change ▪ layout style change layout information retrieved

Reflow<button id="btn" style="font-size: 30px; padding: 0.5em 1em">Click Me</button><script type="text/javascript">window.onload = function(){ document.getElementById("btn").onclick = function(){ var div = document.createElement(“div”); div.className = “tip”; div.innerHTML = “You clicked me!”; document.body.appendChild(div); };};</script> Reflow!

Repaints and reflows are queued up as JavaScript executes and then executed in order

Reflowvar list = document.getElementsByClassName("items")[0], i, item;for (i=0; i < 10; i++){ item = document.createElement("li"); item.innerHTML = "Item #" + i; list.appendChild(item);} Reflow x 10!

Limiting repaints/reflowsimproves overall performance

Technique #1Perform DOM manipulations off-document

Off-Document Operations• Fast because theres no repaint/reflow• Techniques: – Remove element from the document, make changes, insert back into document – Set elements display to “none”, make changes, set display back to default – Build up DOM changes on a DocumentFragment then apply all at once

DocumentFragment• A document-like object• Not visually represented• Considered to be owned by the document from which it was created• When passed to appendChild(), appends all of its children rather than itself

DocumentFragmentvar list = document.getElementsByClassName("items")[0], fragment = document.createDocumentFragment(), i, item;for (i=0; i < 10; i++){ item = document.createElement("li"); item.innerHTML = "Item #" + i; fragment.appendChild(item);}list.appendChild(fragment); 1 Reflow

Technique #2Group Style Changes

Repaint! Reflow!element.style.color = "red";element.style.height = "100px"; Reflow!element.style.fontSize = "25px";element.style.backgroundColor = "white"; Repaint!

Grouping Style Changes.active { color: red; height: 100px; fontSize: 25px; background-color: white;}element.className = "active"; Reflow!

Grouping Style Changesvar item = document.getElementById("myItem");item.style.cssText = "color:red;height:100px;" + "font-size:25px;background-color: white"); Reflow!

Technique #3Avoid Accidental Reflow

Accidental Reflowelement.width = "100px";var width = element.offsetWidth; Reflow!

What to do?• Minimize access to layout information – offsetTop, offsetLeft, offsetWidth, offsetHeight – scrollTop, scrollLeft, scrollWidth, scrollHeight – clientTop, clientLeft, clientWidth, clientHeight – Most computed styles• If a value is used more than once, store in local variable

Does hardware acceleration help?

Traditional RenderingUI Thread Compositing Drawingtime

Hardware AccelerationUI Thread Prep Waittime Rendering info Signal complete Compositing Drawing GPU

The browser UI thread is responsible forboth UI updates and JavaScript execution Only one can happen at a time

Responsive UIUI Thread UI Update JavaScript UI Updatetime

Avoid Slow Loading JavaScript• Put scripts at the bottom• Concatenate scripts into as few files as possible• Choose the right way to load your scripts – Dynamically created scripts – Deferred scripts – Asynchronous scripts

Avoid Slow JavaScript• Dont allow JavaScript to execute for more than 50ms• Break up long JavaScript processes using: – Timers – Web Workers

Avoid Long UI Updates• Be careful of repaint and reflow• Perform complex DOM operations off- document – Remove elements and re-add them – Use DocumentFragment objects• Group style changes together• Avoid accidental reflow

Dont be horrible

Do be awesome

The End

Etcetera• My blog: www.nczonline.net• Twitter: @slicknet• These Slides: http://slideshare.net/nzakas/

Creative Commons Images Used• http://www.flickr.com/photos/lrargerich/3115367361/• http://www.flickr.com/photos/hippie/2406411610/• http://www.flickr.com/photos/55733754@N00/3325000738/• http://www.flickr.com/photos/eurleif/255241547/• http://www.flickr.com/photos/off_the_wall/3444915939/• http://www.flickr.com/photos/wwarby/3296379139/• http://www.flickr.com/photos/derekgavey/4358797365/• http://www.flickr.com/photos/mulad/286641998/• http://www.flickr.com/photos/torley/2361164281/• http://www.flickr.com/photos/ottoman42/455242/• http://www.flickr.com/photos/goincase/3843348908/

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High Performance JavaScript (Amazon DevCon 2011)

by Nicholas Zakas

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