High Performance JavaScript 2011

High Performance JavaScriptNicholas C. Zakas| nczonline.net

Who's this guy?Co-Creator csslint.netContributor,Creator of YUI TestEx-tech leadYahoo!AuthorLead AuthorContributorLead Author

@slicknet(complaints: @brendaneich)

Does JavaScript performance matter?

All browsers now haveoptimizing JavaScript enginesTracemonkey/JaegarMonkey(3.5+)V8(all)Nitro(4+)Chakra(9+)Karakan(10.5+)

http://ie.microsoft.com/testdrive/benchmarks/sunspider/default.html

Old computers ran slow applicationsSmall amounts of CPU power and memory

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

It's still possible to write slow JavaScript

JavaScript performancedirectlyaffects user experience

The UI Threadaka Browser Event Loop

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

Jobs for UI updates and JavaScript executionare added to a UI queueEach 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 ThreadtimeUI Queue

When ClickedUI ThreadtimeUI QueueUI UpdateonclickUI Update

When ClickedUI ThreadUI UpdatetimeUI QueueonclickDraw down stateUI Update

When ClickedUI ThreadonclickUI UpdatetimeUI QueueUI Update

When ClickedUI ThreadonclickUI UpdateUI UpdatetimeUI QueueDraw 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 ThreadJavaScriptUI UpdateUI Updatetime

Unresponsive UIUI ThreadJavaScriptUI UpdateUI Updatetime

The longer JavaScript runs,the worse the user experience

The runaway script timer prevents JavaScriptfrom running for too longEach browser imposes its own limit (except Opera)

Runaway Script Timer LimitsInternet Explorer: 5 million statementsFirefox: 10 secondsSafari: 5 secondsChrome: Unknown, hooks into normal crash control mechanismOpera: none

How Long Is Too Long?“0.1 second [100ms] is about the limit for having the user feel that the system is reacting instantaneously, meaning that no special feedback is necessary except to display the result.”- Jakob Nielsen

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

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

Interpreted JavaScriptUI ThreadInterprettime

JITed JavaScript (1st Run)UI ThreadCompileExecutetime

JITed JavaScript (After 1st Run)UI ThreadExecutetime

Loadtime TechniquesDon't let JavaScript interfere with page load performance

During page load, JavaScript takes 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 ThreadJavaScriptUI UpdateUI Updatetime

ResultUI Threadfoo.jsSee ya!Hello world!time

ResultUI ThreadDownloadSee ya!Hello world!ParseRuntimeThe UI thread needs to wait for the script todownload, parse, and run before continuing

ResultUI ThreadDownloadSee ya!Hello world!ParseRunVariableConstantDownload time takes the longest and is variable

Translation:The page doesn't render while JavaScript 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 ThreadJavaScriptUI UpdateUI UpdateJavaScriptJavaScripttimeThe more scripts to download in between UIupdates, 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 ThreadJavaScriptUI UpdateUI UpdateJavaScriptJavaScripttimeEven if there are multiple scripts, the pagerenders 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 ThreadJavaScriptUI UpdateJavaScriptJavaScripttimeEach script has overhead of downloading

UI ThreadJavaScriptUI UpdatetimeCombining 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

Using HTML <script>UI ThreadDownloadSee ya!Hello world!ParseRuntime

<!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 ThreadSee ya!Hello world!UI UpdateRuntimeDownloadParseOnly 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 immediately after download and parse – timing of execution is not guaranteed

<!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>?5.03.57.04.0

Deferred scripts begin to download immediately, but don't execute until all UI updates complete

Using <script defer>UI ThreadSee ya!Hello world!More UIRunMore UItimeDownloadParseSimilar to dynamic script nodes, but with aguarantee that execution will happen last

Timing Note:Although scripts always execute after UI updates complete, the order of multiple <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>?5.03.67.010

Asynchronous scripts behave a lot like dynamic scripts

Using <script async>UI ThreadSee ya!Hello world!UI UpdateRuntimeDownloadParseDownload begins immediately and execution isslotted in at first available spot

Note:Order of execution is explicitly not preserved for asynchronous scripts

Runtime TechniquesWays to ensure JavaScript doesn't run away

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

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

When ClickedUI ThreadUI UpdatetimeUI QueueonclickUI Update

When ClickedUI ThreadUI UpdateUI UpdateonclicktimeUI Queue

After 25msUI ThreadUI UpdateUI UpdateonclicktimeUI QueueJavaScript

After 25msUI ThreadJavaScriptUI UpdateUI UpdateonclicktimeUI Queue

After Another 25msUI ThreadJavaScriptUI UpdateUI UpdateonclicktimeUI QueueJavaScript

After Another 25msUI ThreadJavaScriptJavaScriptUI UpdateUI UpdateonclicktimeUI Queue

Technique #2: Script YieldingNEW!

//delay a function until after UI updates are donesetImmediate(function(){//code to execute here});setImmediate() adds code to the UI queue after pending UI updates are finished

Support for Script Yielding????10msSetIntermediate()

functionyieldingProcessArray(items, process, callback){//create a clone of the originalvartodo = items.concat();setImmediate(function(){var start = +new Date();do { process(todo.shift()); } while (todo.length > 0 && (+new Date() - start < 50));if (todo.length > 0){setImmediate(arguments.callee); } else { callback(items); }});}

After Last UI UpdateUI ThreadUI UpdateUI UpdateonclicktimeUI QueueJavaScript

After Last UI UpdateUI ThreadJavaScriptUI UpdateUI UpdateonclicktimeUI Queue

No Other UI UpdatesUI ThreadJavaScriptUI UpdateUI UpdateonclicktimeUI QueueJavaScript

No Other UI UpdatesUI ThreadJavaScriptJavaScriptUI UpdateUI UpdateonclicktimeUI Queue

Support for Web Workers10.64.03.54.010

Web WorkersAsynchronous JavaScript executionExecution happens outside the UI threadDoesn’t block UI updatesData-Driven APIData is serialized going into and out of the workerNo access to DOM or BOMSeparate 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 ThreadonclickUI UpdatetimeUI QueueWorker ThreadUI Update

When ClickedUI ThreadUI UpdateUI UpdateonclicktimeUI QueueWorker ThreadJavaScript

Worker Thread CompleteUI ThreadUI UpdateUI UpdateonclicktimeUI Queueonmessage

Worker Thread CompleteUI ThreadonmessageUI UpdateUI UpdateonclicktimeUI Queue

Repaint and ReflowHidden performance costs of common operations

Long UI updates=Unresponsive UI

JavaScript can cause long UI updates by triggeringrepaint and reflow

A repaint occurs when a visual change doesn'trequire recalculation of layoutChanges 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 reflowoccurs when a visual changerequires a change in layoutInitial page load ▪ browser resize ▪ DOM structure change ▪ layout style changelayout 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 executesand 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/reflows improves overall performance

Technique #1Perform DOM manipulations off-document

Off-Document OperationsFast because there's no repaint/reflowTechniques:Remove element from the document, make changes, insert back into documentSet element's display to “none”, make changes, set display back to defaultBuild up DOM changes on a DocumentFragment then apply all at once

DocumentFragmentA document-like objectNot visually representedConsidered to be owned by the document from which it was createdWhen 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";element.style.fontSize = "25px";element.style.backgroundColor = "white";Reflow!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.style.width= "100px";var width = element.offsetWidth;Reflow!

What to do? Minimize access to layout informationoffsetTop, offsetLeft, offsetWidth, offsetHeightscrollTop, scrollLeft, scrollWidth, scrollHeightclientTop, clientLeft, clientWidth, clientHeightMost computed styles If a value is used more than once, store in local variable

Does hardware acceleration help?

Traditional RenderingUI ThreadCompositingDrawingtime

Hardware AccelerationUI ThreadPrepWaittimeRendering infoSignal completeCompositingDrawingGPU

High Performance JavaScript 2011

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