High Performance JavaScript - jQuery Conference SF Bay Area 2010

High Performance JavaScript Nicholas C. Zakas Principal Front End Engineer, Yahoo! jQuery Conference – SF Bay Area, April 24 2010

Principal Front End Engineer

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Things I Know About JavaScript Professional Wrestling YUI jQuery

Does this matter?

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 on the new, faster JavaScript engines

How can I improve JavaScript performance?

How can I improve JavaScript performance? How can I make this fast???

How can I improve JavaScript performance? How can I make this fast??? Why is this so slow?????

Many aspects conspire to make JavaScript slow Awareness is the first step to solution

The UI Thread The brains of the operation

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 if the UI thread is busy 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"> $(document).ready(function(){ $("#btn").bind("click",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 UI Update onclick time UI Queue UI Update

When Clicked UI Thread UI Update onclick 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"> $(document).ready(function(){ $("#btn").bind("click",function(){ $("body").append("<div class="tip">You " + "clicked me!</div>”); }); }); </script>

A UI update must use the latest info available

Long-running JavaScript = Unresponsive UI

Responsive UI UI Thread UI Update JavaScript UI Update time

Unresponsive UI UI Thread UI Update JavaScript UI Update time

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

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 50ms

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

Timers to the rescue!

JavaScript Timers • Created using setTimeout() • Schedules a new JavaScript execution job for some time in the future • When the delay is up, the job is added to the UI queue – Note: This does not guarantee execution after the delay, just that the job is added to the UI queue and will be executed when appropriate

JavaScript Timers • For complex processing, split up into timed functionality • Use timers to delay some processing for later

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

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

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

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

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

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

Web Workers to the rescue!

Web Workers • Asynchronous JavaScript execution • Execution happens in a separate process – 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 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 UI Update time UI Queue onclick UI Update

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

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

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

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

Web Workers Support 3.5 4.0 4.0

Repaint and Reflow UI update jobs

Long UI updates = Unresponsive UI

Long UI updates = Unresponsive UI = Frustrated users

JavaScript can cause long UI updates

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"> $(document).ready(function(){ $("#btn").bind("click",function(){ $(this).css({ color: "#f00" }); }); }); </script> Repaint!

A reflow occurs 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"> $(document).ready(function(){ $("#btn").bind("click",function(){ $("body").append("<div class="tip">You " + "clicked me!</div>”); }); }); </script> Reflow!

Repaints and reflows are queued up as JavaScript executes

Reflow var list = $("ul.items"); for (var i=0; i < 10; i++){ list.append("<li>Item #" + i + "</li>"); } Reflow x 10!

Limiting repaints/reflows improves overall performance

Solution #1 Perform 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

No reflow! Reflow!

Solution #2 Group Style Changes

Repaint! Reflow! $("span.example").css({ color: "red" }); $("span.example").css({ height: "100px" }); Reflow! $("span.example").css({ fontSize: "25px" }); $("span.example").css({ backgroundColor: "white" }); Repaint!

Grouping Style Changes .active { color: red; height: 100px; fontSize: 25px; background-color: white; } $("span.example").addClass("active"); Reflow!

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

Solution #3 Avoid Accidental Reflow

Accidental Reflow $("div.example").css({ width: "100px"}); var width = $("div.example").width(); 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

Things I Know About JavaScript Professional Wrestling YUI jQuery

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

Responsive UI UI Thread UI Update JavaScript UI Update time

Avoid Slow JavaScript • Don't 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

Questions?

Etcetera • My blog: www.nczonline.net • My email: nzakas@yahoo-inc.com • Twitter: @slicknet

Creative Commons Images Used • http://www.flickr.com/photos/lrargerich/3115367361/ • http://www.flickr.com/photos/photomonkey/93277011/ • 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/

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High Performance JavaScript - jQuery Conference SF Bay Area 2010

by Nicholas Zakas on Apr 24, 2010

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