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Speed Up Your JavaScript

Nicholas C. Zakas discusses common JavaScript performance issues and recommendations to address them. He covers scope management, data access, loops, and DOM manipulation. The key points are to minimize scope chain lookups, store frequently accessed data in local variables, optimize loops, avoid reflows from DOM changes, and leverage modern browser optimizations. Overall, the talk provides practical tips to speed up JavaScript code by addressing issues related to scope, data access patterns, loops, and DOM interactions.

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Speed Up Your JavaScript Nicholas C. Zakas Principal Front End Engineer, Yahoo! x Web E ponents – June 4, 2009
Who's this guy? • Principal Front End Engineer, Yahoo! Homepage • YUI Contributor • Author
Why slow?
Bad compilation?
No
No compilation!* * Humor me for now. It'll make this easier.
Browsers won't help your code!!!!
Who will help your code?
JavaScript Performance Issues • Scope management • Data access • Loops • DOM
Scope Chains
When a Function Executes • An execution context is created • The context's scope chain is initialized with the members of the function's [[Scope]] collection • An activation object is created containing all local variables • The activation object is pushed to the front of the context's scope chain
Execution Context Identifier Resolution • Start at scope chain position 0 • If not found go to position 1 • Rinse, repeat
Identifier Resolution • Local variables = fast! • The further into the chain, the slower the resolution
Identifier Resolution (Reads) 200 180 160 140 Firefox 3 Time (ms) per 200,000 reads Firefox 3.1 Beta 3 120 Chrome 1 Chrome 2 Beta Internet Explorer 7 100 Internet Explorer 8 Opera 9.64 80 Opera 10 Alpha Safari 3.2 Safari 4 Beta 60 40 20 0 1 2 3 4 5 6 Identifier Depth
Identifier Resolution (Writes) 200 180 160 140 Firefox 3 Time (ms) per 200,000 writes Firefox 3.1 Beta 3 120 Chrome 1 Chrome 2 Beta Internet Explorer 7 100 Internet Explorer 8 Opera 9.64 80 Opera 10 Alpha Safari 3.2 Safari 4 Beta 60 40 20 0 1 2 3 4 5 6 Identifier Depth
Scope Chain Augmentation • The with statement • The catch clause of try-catch • Both add an object to the front of the scope chain
Inside of Global Function
Inside of with/catch Statement • Local variables now in second slot • with variables now in first slot
“with statement considered harmful” -Douglas Crockford
Closures • The [[Scope]] property of closures begins with two objects • Calling the closure means three objects in the scope chain (minimum)
Closures
Inside of Closure
Recommendations • Store out-of-scope variables in local variables – Especially global variables • Avoid the with statement – Adds another object to the scope chain, so local function variables are now one step away – Use local variables instead • Be careful with try-catch – The catch clause also augments the scope chain • Use closures sparingly • Don't forget var when declaring variables
JavaScript Performance Issues • Scope management • Data access • Loops • DOM
Places to Access Data • Literal value • Variable • Object property • Array item
Data Access Performance • Accessing data from a literal or a local variable is fastest – The difference between literal and local variable is negligible in most cases • Accessing data from an object property or array item is more expensive – Which is more expensive depends on the browser
Data Access 100 90 80 70 Time (ms) per 200,000 reads 60 Literal Local Variable 50 Array Item Object Property 40 30 20 10 0 Firefox 3 Firefox 3.1 Chrome 1 Chrome 2 Internet Internet Opera 9.64 Opera 10 Safari 3.2 Safari 4 Beta 3 Beta Explorer 7 Explorer 8 Alpha Beta
Property Depth • object.name < object.name.name • The deeper the property, the longer it takes to retrieve
Property Depth 250 200 Firefox 3 Time (ms) per 200,000 reads Firefox 3.1 Beta 3 150 Chrome 1 Chrome 2 Beta Internet Explorer 7 Internet Explorer 8 Opera 9.64 100 Opera 10 Alpha Safari 3.2 Safari 4 Beta 50 0 1 2 3 4 Property Depth
Property Notation • Difference between object.name and object[“name”]? – Generally no – Exception: Dot notation is faster in Safari
Recommendations • Store these in a local variable: – Any object property accessed more than once – Any array item accessed more than once • Minimize deep object property/array item lookup
-5% -10% -33%
JavaScript Performance Issues • Scope management • Data Access • Loops • DOM
Loops • ECMA-262, 3rd Edition: – for – for-in – do-while – while • ECMA-357, 2nd Edition: – for each
Which loop?
It doesn't matter!
What Does Matter? • Amount of work done per iteration – Includes terminal condition evaluation and incrementing/decrementing • Number of iterations • These don't vary by loop type
Fixing Loops • Decrease amount of work per iteration • Decrease number of iterations
Easy Fixes • Eliminate object property/array item lookups
Easy Fixes • Eliminate object property/array item lookups • Combine control condition and control variable change – Work avoidance!
Two evaluations: j < len j < len == true
One evaluation j-- == true -50%
Easy Fixes • Eliminate object property/array item lookups • Combine control condition and control variable change – Work avoidance!
Things to Avoid for Speed • ECMA-262, 3rd Edition: – for-in nd • ECMA-357, 2 Edition: – for each • ECMA-262, 5th Edition: – array.forEach() • Function-based iteration: – jQuery.each() – Y.each() – $each – Enumerable.each()
• Introduces additional function • Function requires execution (execution context created, destroyed) • Function also creates additional object in scope chain 8x
JavaScript Performance Issues • Scope management • Data Access • Loops • DOM
DOM
HTMLCollection
HTMLCollection Objects • document.images, document.forms, etc. • getElementsByTagName() • getElementsByClassName()
Note: Collections in the HTML DOM are assumed to be live meaning that they are automatically updated when the underlying document is changed.
Infinite Loop!
HTMLCollection Objects • Look like arrays, but aren't – Bracket notation – length property • Represent the results of a specific query • The query is re-run each time the object is accessed – Include accessing length and specific items – Much slower than accessing the same on arrays – Exceptions: Opera, Safari
15x 53x 68x
= = =
HTMLCollection Objects • Minimize property access – Store length, items in local variables if used frequently • If you need to access items in order frequently, copy into a regular array
function array(items){ try { return Array.prototype.slice.call(items); } catch (ex){ var i = 0, len = items.length, result = Array(len); while (i < len){ result[i] = items[i]; i++; } } return result; }
Reflow
Reflow is the process by which the geometry of the layout engine's formatting objects are computed. - Chris Waterson, Mozilla
When Reflow? • Initial page load • Browser window resize • DOM nodes added or removed • Layout styles applied • Layout information retrieved
Reflow!
DocumentFragment • A document-like object • Not visually represented • Considered a child of the document from which it was created • When passed to addChild(), appends all of its children rather than itself
No reflow! Reflow!
When Reflow? • Initial page load • Browser window resize • DOM nodes added or removed • Layout styles applied • Layout information retrieved
Reflow! Reflow! Reflow!
What to do? • Minimize changes on style property • Define CSS class with all changes and just change className property
Reflow!
When Reflow? • Initial page load • Browser window resize • DOM nodes added or removed • Layout styles applied • Layout information retrieved – Only if reflow is cached
Reflow? Reflow? Reflow?
What to do? • Minimize access to layout information • If a value is used more than once, store in local variable
Speed Up Your DOM • Be careful using HTMLCollection objects • Perform DOM manipulations off the document • Change CSS classes, not CSS styles • Be careful when accessing layout information
Will it be like this forever?
No
Browsers With Optimizing Engines • Chrome (V8) • Safari 4+ (Nitro) • Firefox 3.5+ (TraceMonkey) • Opera 10? 11? (Carakan) All use native code generation and JIT compiling to achieve faster JavaScript execution.
Hang in there!
Summary • Mind your scope • Local variables are your friends • Function execution comes at a cost • Keep loops small • Avoid doing work whenever possible • Minimize DOM interaction • Use a good browser and encourage others to do the same
Questions?
Etcetera • My blog: www.nczonline.net • My email: nzakas@yahoo-inc.com • Twitter: @slicknet
Creative Commons Images Used • http://www.flickr.com/photos/blackbutterfly/3051019058/ • http://www.flickr.com/photos/23816315@N07/2528296337/ • http://www.flickr.com/photos/37287477@N00/515178157/ • http://www.flickr.com/photos/ottoman42/455242/ • http://www.flickr.com/photos/crumbs/2702429363/ • http://flickr.com/photos/oberazzi/318947873/