Speed Up Your JavaScript
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Describes common JavaScript performance issues and how to work around them.
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Speed Up Your JavaScript
- 1. Speed Up Your JavaScript Nicholas C. Zakas Principal Front End Engineer, Yahoo! x Web E ponents – June 4, 2009
- 2. Who's this guy? • Principal Front End Engineer, Yahoo! Homepage • YUI Contributor • Author
- 4. Why slow?
- 6. No
- 7. No compilation!* * Humor me for now. It'll make this easier.
- 9. Who will help your code?
- 11. JavaScript Performance Issues • Scope management • Data access • Loops • DOM
- 13. Scope Chains
- 14. 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
- 15. Execution Context Identifier Resolution • Start at scope chain position 0 • If not found go to position 1 • Rinse, repeat
- 16. Identifier Resolution • Local variables = fast! • The further into the chain, the slower the resolution
- 17. 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
- 18. 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
- 19. Scope Chain Augmentation • The with statement • The catch clause of try-catch • Both add an object to the front of the scope chain
- 20. Inside of Global Function
- 21. Inside of with/catch Statement • Local variables now in second slot • with variables now in first slot
- 22. “with statement considered harmful” -Douglas Crockford
- 23. Closures • The [[Scope]] property of closures begins with two objects • Calling the closure means three objects in the scope chain (minimum)
- 25. Closures
- 27. 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
- 29. JavaScript Performance Issues • Scope management • Data access • Loops • DOM
- 30. Places to Access Data • Literal value • Variable • Object property • Array item
- 31. 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
- 32. 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
- 33. Property Depth • object.name < object.name.name • The deeper the property, the longer it takes to retrieve
- 34. 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
- 35. Property Notation • Difference between object.name and object[“name”]? – Generally no – Exception: Dot notation is faster in Safari
- 36. 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
- 38. -5% -10% -33%
- 39. JavaScript Performance Issues • Scope management • Data Access • Loops • DOM
- 40. Loops • ECMA-262, 3rd Edition: – for – for-in – do-while – while • ECMA-357, 2nd Edition: – for each
- 42. Which loop?
- 44. 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
- 45. Fixing Loops • Decrease amount of work per iteration • Decrease number of iterations
- 49. Easy Fixes • Eliminate object property/array item lookups
- 51. Easy Fixes • Eliminate object property/array item lookups • Combine control condition and control variable change – Work avoidance!
- 52. Two evaluations: j < len j < len == true
- 53. One evaluation j-- == true -50%
- 54. Easy Fixes • Eliminate object property/array item lookups • Combine control condition and control variable change – Work avoidance!
- 55. 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()
- 56. • Introduces additional function • Function requires execution (execution context created, destroyed) • Function also creates additional object in scope chain 8x
- 57. JavaScript Performance Issues • Scope management • Data Access • Loops • DOM
- 58. DOM
- 59. HTMLCollection
- 60. HTMLCollection Objects • document.images, document.forms, etc. • getElementsByTagName() • getElementsByClassName()
- 61. Note: Collections in the HTML DOM are assumed to be live meaning that they are automatically updated when the underlying document is changed.
- 62. Infinite Loop!
- 63. 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
- 64. 15x 53x 68x
- 65. = = =
- 66. 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
- 67. 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; }
- 68. Reflow
- 69. Reflow is the process by which the geometry of the layout engine's formatting objects are computed. - Chris Waterson, Mozilla
- 70. When Reflow? • Initial page load • Browser window resize • DOM nodes added or removed • Layout styles applied • Layout information retrieved
- 71. Reflow!
- 72. 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
- 73. No reflow! Reflow!
- 74. When Reflow? • Initial page load • Browser window resize • DOM nodes added or removed • Layout styles applied • Layout information retrieved
- 75. Reflow! Reflow! Reflow!
- 76. What to do? • Minimize changes on style property • Define CSS class with all changes and just change className property
- 77. Reflow!
- 78. When Reflow? • Initial page load • Browser window resize • DOM nodes added or removed • Layout styles applied • Layout information retrieved – Only if reflow is cached
- 79. Reflow? Reflow? Reflow?
- 80. What to do? • Minimize access to layout information • If a value is used more than once, store in local variable
- 81. 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
- 83. Will it be like this forever?
- 84. No
- 85. 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.
- 86. Hang in there!
- 87. 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
- 88. Questions?
- 89. Etcetera • My blog: www.nczonline.net • My email: nzakas@yahoo-inc.com • Twitter: @slicknet
- 90. 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/