High Performance JavaScript 2011

High Performance JavaScript Nicholas C. Zakas| nczonline.net

Who's this guy? Co-Creator csslint.net Contributor, Creator of YUI Test Ex-tech lead Yahoo! Author Lead Author Contributor Lead Author

@slicknet (complaints: @brendaneich)

Does JavaScript performance matter?

All browsers now haveoptimizing JavaScript engines Tracemonkey/ 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 applications Small amounts of CPU power and memory

New computers are generally faster but slow applications still exist More 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 for both 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 Click UI Thread time UI Queue

When Clicked UI Thread time UI Queue UI Update onclick UI Update

When Clicked UI Thread UI Update time UI Queue onclick Draw down state UI Update

When Clicked UI Thread onclick UI Update time UI Queue UI Update

When Clicked UI Thread onclick UI Update UI Update time 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 UI UI Thread JavaScript UI Update UI Update time

Unresponsive UI UI Thread JavaScript UI Update UI Update time

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)

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

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 JavaScript UI Thread Interpret time

JITed JavaScript (1st Run) UI Thread Compile Execute time

JITed JavaScript (After 1st Run) UI Thread Execute time

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>

Result UI Thread JavaScript UI Update UI Update time

Result UI Thread foo.js See ya! Hello world! time

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

Result UI Thread Download See ya! Hello world! Parse Run Variable Constant Download 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>

Result UI Thread JavaScript UI Update UI Update JavaScript JavaScript time 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 Bottom UI Thread JavaScript UI Update UI Update JavaScript JavaScript time 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 Thread JavaScript UI Update JavaScript JavaScript time Each script has overhead of downloading

UI Thread JavaScript UI Update time Combining all of the files limits the network overhead 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 Technique var 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 Thread Download See ya! Hello world! Parse Run 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 Scripts UI Thread See ya! Hello world! UI Update Run time Download Parse Only code execution happens on the UI thread, which means less blocking of UI updates

Usage loadScript("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> ? 5.0 3.5 7.0 4.0

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

Using <script defer> UI Thread See ya! Hello world! More UI Run More UI time 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 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> ? 5.0 3.6 7.0 10

Asynchronous scripts behave a lot like dynamic scripts

Using <script async> UI Thread See ya! Hello world! UI Update Run time Download Parse Download begins immediately and execution is slotted 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 500ms setTimeout(function(){ //code to execute here }, 500); setTimeout() schedules a function to be added to the UI queue after a delay

When Clicked UI Thread UI Update time UI Queue onclick UI Update

When Clicked UI Thread UI Update UI Update onclick time UI Queue

After 25ms UI Thread UI Update UI Update onclick time UI Queue JavaScript

After 25ms UI Thread JavaScript UI Update UI Update onclick time UI Queue

After Another 25ms UI Thread JavaScript UI Update UI Update onclick time UI Queue JavaScript

After Another 25ms UI Thread JavaScript JavaScript UI Update UI Update onclick time UI Queue

Technique #2: Script Yielding NEW!

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

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

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 Update UI Thread UI Update UI Update onclick time UI Queue JavaScript

After Last UI Update UI Thread JavaScript UI Update UI Update onclick time UI Queue

No Other UI Updates UI Thread JavaScript UI Update UI Update onclick time UI Queue JavaScript

No Other UI Updates UI Thread JavaScript JavaScript UI Update UI Update onclick time UI Queue

Support for Web Workers 10.6 4.0 3.5 4.0 10

Web Workers Asynchronous JavaScript execution Execution happens outside the UI thread Doesn’t block UI updates Data-Driven API Data is serialized going into and out of the worker No access to DOM or BOM Separate execution environment

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

When Clicked UI Thread onclick UI Update time UI Queue Worker Thread UI Update

When Clicked UI Thread UI Update UI Update onclick time UI Queue Worker Thread JavaScript

Worker Thread Complete UI Thread UI Update UI Update onclick time UI Queue onmessage

Worker Thread Complete UI Thread onmessage UI Update UI Update onclick time UI 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't 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 reflowoccurs when a visual change requires a change in layout Initial 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 executesand then executed in order

Reflow var 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 Operations Fast because there's no repaint/reflow Techniques: Remove element from the document, make changes, insert back into document Set element's 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

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

Technique #3Avoid Accidental Reflow

Accidental Reflow element.style.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 Rendering UI Thread Compositing Drawing time

Hardware Acceleration UI Thread Prep Wait time Rendering info Signal complete Compositing Drawing GPU

High Performance JavaScript 2011

Recommended

Recommended

More Related Content

What's hot

What's hot (20)

Viewers also liked

Viewers also liked (20)

Similar to High Performance JavaScript 2011

Similar to High Performance JavaScript 2011 (20)

More from Nicholas Zakas

More from Nicholas Zakas (11)

Recently uploaded

Recently uploaded (20)

High Performance JavaScript 2011

Editor's Notes