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For much of its existence, JavaScript has been slow. No one complained until developers created complex web applications with thousands of lines of JavaScript code. Although newer JavaScript engines have improved the situation, there’s still a lot to understand about what makes JavaScript slow and what you can do to speed up your code.
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High Performance JavaScriptNicholas C. Zakas | @slicknetNicholas C. Zakas | @slicknet
Who's this guy?
Co-Creator csslint.net
Contributor,Creator of YUI Test
Author Lead Author Contributor Lead Author
@slicknet
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 butslow applications still exist
More CPU + more memory = less disciplined application development
Oh yeah, one more thing
http://jeftek.com/1942/motorola-xoom-sunspider-results/
It's still possible to write slow JavaScript
JavaScript performancedirectly
affects user experience
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 executionare 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
UI Queuetimetime
When Clicked
UI Thread
UI Queuetimetime
onclickonclick
UI UpdateUI Update
UI UpdateUI Update
When Clicked
UI Thread
UI Queuetimetime
onclickonclick
UI UpdateUI Update
UI UpdateUI Update
Draw down stateDraw down state
When Clicked
UI Thread
UI Queuetimetime
onclickonclick
UI UpdateUI Update
UI UpdateUI Update
When Clicked
UI Thread
UI Queuetimetime
onclickonclick UI UpdateUI UpdateUI UpdateUI Update
Draw up stateDraw 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
timetime
JavaScriptJavaScript UI UpdateUI UpdateUI UpdateUI Update
Unresponsive UI
UI Thread
timetime
JavaScriptJavaScript UI UpdateUI UpdateUI UpdateUI Update
The longer JavaScript runs,the worse the user experience
The runaway script timer prevents JavaScriptfrom 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 JavaScript
UI Thread
timetime
InterpretInterpret
JITed JavaScript (1st Run)
UI Thread
timetime
CompileCompile ExecuteExecute
JITed JavaScript (After 1st Run)
UI Thread
timetime
ExecuteExecute
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
measured on IE6 :)
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
timetime
JavaScriptJavaScript UI UpdateUI UpdateUI UpdateUI Update
Result
UI Thread
timetime
foo.jsfoo.js See ya!See ya!Hello world!Hello world!
Result
UI Thread
timetime
DownloadDownload See ya!See ya!Hello world!Hello world! ParseParse RunRun
The UI thread needs to wait for the script todownload, parse, and run before continuing
Result
UI Thread
DownloadDownload See ya!See ya!Hello world!Hello world! ParseParse RunRun
Download time takes the longest and is variable
Variable Constant
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
timetime
JavaScriptJavaScript UI UpdateUI UpdateUI UpdateUI Update
The more scripts to download in between UIupdates, the longer the page takes to render
JavaScriptJavaScript JavaScriptJavaScript
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
timetime
JavaScriptJavaScriptUI UpdateUI UpdateUI UpdateUI Update
Even if there are multiple scripts, the pagerenders quickly
JavaScriptJavaScript JavaScriptJavaScript
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
timetime
JavaScriptJavaScriptUI UpdateUI Update
Each script has overhead of downloading
JavaScriptJavaScript JavaScriptJavaScript
UI Thread
timetime
JavaScriptJavaScriptUI UpdateUI Update
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
var script = document.createElement("script"), body;script.type = "text/javascript";script.src = "foo.js";body.insertBefore(script, body.firstChild);
Basic Technique
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
timetime
DownloadDownload See ya!See ya!Hello world!Hello world! ParseParse RunRun
<!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><!-- more content --></body></html>
Using Dynamic Scripts
UI Thread
timetime
DownloadDownload
See ya!See ya!Hello world!Hello world!
ParseParse
RunRun
Only code execution happens on the UI thread,which means less blocking of UI updates
UI UpdateUI Update
function loadScript(url, callback){var script = document.createElement("script"), body = document.body;script.type = "text/javascript";
if (script.readyState){ //IE <= 8 script.onreadystatechange = function(){ if (script.readyState == "loaded" || script.readyState == "complete"){ script.onreadystatechange = null; callback(); } };} else { //Others script.onload = function(){ callback(); };}script.src = url;body.insertBefore(script, body.firstChild);
}
loadScript("foo.js", function(){ alert("Loaded!");});
Usage
Timing Note:Script execution begins immediately after download and parse – timing of
execution is not guaranteed
Using Dynamic Scripts
UI Thread
timetime
DownloadDownload
See ya!See ya!Hello world!Hello world!
ParseParse
RunRun
Depending on time to download and script size,execution may happen before next UI update
UI UpdateUI 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> <!-- even more markup --></body></html>
7.07.03.53.5 5.05.0 ??4.04.0
Support for <script defer>
Deferred scripts begin to download immediately,
but don't execute until all UI updates complete
Using <script defer>
UI Thread
timetime
DownloadDownload
See ya!See ya!Hello world!Hello world!
ParseParse
RunRun
Similar to dynamic script nodes, but with aguarantee that execution will happen last
More UIMore UI More UIMore UI
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> <!-- even more markup --></body></html>
7.07.03.63.6 5.05.0 ??1010
Support for <script async>
Asynchronous scripts behave a lot like dynamic scripts
Using <script async>
UI Thread
timetime
DownloadDownload
See ya!See ya!Hello world!Hello world!
ParseParse
RunRun
Download begins immediately and execution isslotted in at first available spot
UI UpdateUI Update
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();}
Technique #1: Timers
//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
function timedProcessArray(items, process, callback){ //create a clone of the original
var todo = items.concat(); setTimeout(function(){ var start = +new Date(); do { process(todo.shift()); } while (todo.length > 0 && (+new Date() - start < 50)); if (todo.length > 0){ setTimeout(arguments.callee, 25); } else { callback(items); } }, 25);}
When Clicked
UI Thread
UI Queuetimetime
onclickonclick
UI UpdateUI Update
UI UpdateUI Update
When Clicked
UI Thread
UI Queuetimetime
onclickonclick
UI UpdateUI Update
UI UpdateUI Update
When Clicked
UI Thread
UI Queuetimetime
onclickonclick
UI UpdateUI Update
UI UpdateUI Update
When Clicked
UI Thread
UI Queuetimetime
UI UpdateUI UpdateUI UpdateUI Update onclickonclick
After 25ms
UI Thread
UI Queuetimetime
UI UpdateUI UpdateUI UpdateUI Update onclickonclick
JavaScriptJavaScript
After 25ms
UI Thread
UI Queuetimetime
UI UpdateUI UpdateUI UpdateUI Update onclickonclick JavaScriptJavaScript
After Another 25ms
UI Thread
UI Queuetimetime
UI UpdateUI UpdateUI UpdateUI Update onclickonclick JavaScriptJavaScript
JavaScriptJavaScript
After Another 25ms
UI Thread
UI Queuetimetime
UI UpdateUI UpdateUI UpdateUI Update onclickonclick JavaScriptJavaScript JavaScriptJavaScript
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 Clicked
UI Thread
UI Queuetimetime
onclickonclick
UI UpdateUI Update
UI UpdateUI Update
When Clicked
UI Thread
UI Queuetimetime
onclickonclick
UI UpdateUI Update
UI UpdateUI Update
When Clicked
UI Thread
UI Queuetimetime
onclickonclick
UI UpdateUI Update
UI UpdateUI Update
When Clicked
UI Thread
UI Queuetimetime
onclickonclick
UI UpdateUI Update
UI UpdateUI Update
Worker Thread
When Clicked
UI Thread
UI Queuetimetime
UI UpdateUI UpdateUI UpdateUI Update onclickonclick
Worker Thread
JavaScriptJavaScript
Worker Thread Complete
UI Thread
UI Queuetimetime
UI UpdateUI UpdateUI UpdateUI Update onclickonclick
onmessageonmessage
Worker Thread Complete
UI Thread
UI Queuetimetime
UI UpdateUI UpdateUI UpdateUI Update onclickonclick onmessageonmessage
4.04.03.53.5 4.04.0 10.610.61010
Support for Web Workers
Repaint and ReflowHidden performance costs of common operations
Long UI updates=
Unresponsive UI
Unresponsive UI
UI Thread
timetime
JavaScriptJavaScript UI UpdateUI UpdateUI UpdateUI Update
JavaScript can cause long UI updates by triggering
repaint 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!Repaint!
A reflow occurs when a visual change requires a change in layout
Initial 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!Reflow!
Repaints and reflows are queued up as JavaScript executes
and 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!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
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 Reflow1 Reflow
Technique #2Group Style Changes
element.style.color = "red";element.style.height = "100px";element.style.fontSize = "25px";element.style.backgroundColor = "white";
Repaint!Repaint! Reflow!Reflow!
Reflow!Reflow!
Repaint!Repaint!
.active { color: red; height: 100px; fontSize: 25px; background-color: white;}
element.className = "active";
Reflow!Reflow!
Grouping Style Changes
var item = document.getElementById("myItem");item.style.cssText = "color:red;height:100px;" + "font-size:25px;background-color: white");
Reflow!Reflow!
Grouping Style Changes
Technique #3Avoid Accidental Reflow
element.width = "100px";
var width = element.offsetWidth;
Reflow!Reflow!
Accidental 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
timetime
CompositingCompositing DrawingDrawing
timetime
Hardware Acceleration
UI Thread
PrepPrep
GPU
CompositingCompositing
Rendering info
WaitWait
DrawingDrawing
Signal complete
Recap
The browser UI thread is responsible forboth UI updates and JavaScript execution
Only one can happen at a time
Responsive UI
UI Thread
timetime
JavaScriptJavaScript UI UpdateUI UpdateUI UpdateUI Update
Unresponsive UI
UI Thread
timetime
JavaScriptJavaScript UI UpdateUI UpdateUI UpdateUI Update
Unresponsive UI
UI Thread
timetime
JavaScriptJavaScript UI UpdateUI UpdateUI UpdateUI Update
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• 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
Don't be horrible
Do be awesome
Etcetera• My blog: www.nczonline.net• Twitter: @slicknet• These Slides: slideshare.net/nzakas• Hire us: projects@stubbornella.org
Questions?
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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