High performance java script why everything youve been taught is wrong

High-Performance
JavaScript:
Why Everything You’ve
Been Taught is Wrong
Joseph Smarr
Plaxo, Inc.

Joseph Smarr, Plaxo, Inc.
About me
- Chief Platform Architect at Plaxo
- First employee (March 2002)
- Architect and lead developer for Plaxo Online
- Abusing web browsers since 1993 (Mosaic)
- Plaxo Online 2.0 (AJAX via iframes in 2004)
- JavaScript Wormhole (OSCON 06)
http://JosephSmarr.com

About Plaxo
- Smart Address Book
- Syncs address book and calendar with lots of places
- User updates their contact info  you get it automatically
- Founded in 2002, ~50 employees, 15M+ users
- Backed by Sequoia, Ram Shriram,Tim Koogle, et al
http://www.plaxo.com

Plaxo Online
AJAX Desktop
beta.plaxo.com
• Flexible desktop
• Contacts
• Calendar
• Tasks
• Notes
• Sync dashboard
• Pulse
• Profile / settings

Looks great…but it almost didn’t ship!
- Spring 06: “Let’s really push the envelope for Plaxo 3.0”
- Summer 06: “Wow, these are great UI ideas, keep em coming”
- Fall 06: “Let’s put 7 great web devs full time on this”’
- Winter 06: “Ok, we built a ton…now let’s optimize it, no problem”
- March 07: “Uh oh, making it fast is way harder than we thought”
- April 07: “We can’t ship this as is—do we need to start over?!?”
- June 07: “After a heroic effort, it’s just barely fast enough (phew!)”

Where did we go wrong??
- Didn’t take performance seriously from day one
- Didn’t think the browser’s limitations were significant
- Didn’t use the app daily as we were developing it
- Didn’t push back on feature requests for performance
- Didn’t value perceived performance / responsiveness
…overcoming these challenges required unlearning a lot of
standard assumptions about building software…

Why EverythingYou’ve Been Taught is Wrong
- AJAX euphoria
- Web browsers can be made to do anything now!
- Use desktop / OOP programming style
- Why it’s wrong
- Browsers are being pushed beyond their comfort zone
- JavaScript code is parsed & interpreted every time  cost per line
of source code

Why High-Performance JavaScript Matters
- Everyone is amazed by fast apps
- It hardly matters what else they do!
- Everyone hates slow apps
- It hardly matters what else they do…
- AJAX was supposed to be all about responsiveness!!
Having tried and almost failed, we now have a mantra:

The High-Performance JS Mantra
- Be Lazy
- Be Responsive
- Be Pragmatic
- BeVigilant

- Be Lazy: Nothing is faster than doing nothing
- Be Responsive
- Be Pragmatic
- BeVigilant

Write less code
- Initial parsing of JavaScript is often a major bottleneck
- No JIT, no cached object code, interpreted every time
- Parse time is non-linear in the size of total JavaScript?
- Can’t rely on browser caching to excuse large code size
- Yahoo study: surprising number of hits with empty cache
- Frequent code releases  frequently need to re-download
- More code = more to download, execute, maintain, etc.
- Ideal for large AJAX apps is <500K JS uncompressed
Be Lazy: Nothing is faster than doing nothing

0
500
1,000
1,500
2,000
2,500
Zimbra Yahoo!
Mail
Plaxo
(before)
Renkoo Plaxo
(after)
Remember
the Milk
Meebo Gmail Google
Calendar
Plaxo
(startup)
JScodesize(KB,uncompressed)
Total code size of some AJAX apps

Write less code
- Minimize the JavaScript code you send down
- Minify = good, obfuscate = not much better
- Strip debug / logging lines (don’t just set log-level = 0)
- Remove unnecessary OOP boilerplate
- Get/Set functions don’t actually protect member vars! etc.
- Minimize dependency on third-party library code
- Lots of extra code comes along that you don’t need
- Libraries solve more general problems  use like scaffolding

Load JavaScript on-demand
- Once you’ve written less code, load it only as-needed
- Break into classes / modules; use require / provide (ala dojo)
- Bundle classes into packages to minimize server round-trips
- Packages should ignore pre-loaded dependencies
- Tradeoff of downloading shared code twice vs.
multiple round trips (e.g. for common widgets)
- Build packages with error-handler hook for development
- Will re-build from source every time if you don’t write
“I work hard at
being lazy”
“I work hard at
being lazy”
Ryan “Roger” Moore

Draw UI as late as possible
- Draw less DOM = faster to draw, browser less saturated
- Never pre-draw hidden UI if you can avoid it
- Cache previously drawn HTML when appropriate
- But have to know when to invalidate the cache
- Don’t keep hidden UI up-to-date behind the scenes
- Just re-draw next time you show it (simpler, one-time cost)
- Consider re-drawing vs. partial dynamic UI updates
- Redraw is often faster / easier / less code

Never pre-draw hidden UI

How to Be Lazy
√ Write less code!
√ Load JS on-demand
√ Draw UI as late as possible

- Be Lazy
- Be Responsive: Jump when the user says jump
- Be Pragmatic
- BeVigilant

Minimize initial perceived load time
- Put CSS at the top of your page and JS at the bottom
- Draw major placeholder UI with “loading…” first
- Load / draw your app in stages (lazy, on-demand)
Be Responsive: Jump when the user says jump

Load your app in stages

Yield early and often
- Always want to show a quick response acknowledgement
- But browser often doesn’t update UI until your code returns!
- Solution: do minimum work, use setTimeout(0) to yield
- Use closures to chain state together with periodic pauses
- Draw UI progressively, with loading messages as needed
- Use onmousedown instead of onclick (~100msec faster!)
- Demo: http://josephsmarr.com/oscon-js/yield.html

Cache backend responses
- All data requests should go through data-manager code
- Request as needed and cache results for subsequent asks
- Requesting code always assumes async response
- Use range caches  only fill in missing pieces
- Ideal for partial views into long lists of data
- Balance local updates vs. re-fetching from APIs
- Do the easy cases, but beware of too much update code
- Worst case = trash cache and re-fetch = first-time case
“Data structures and
AJAX—together at last!”
“Data structures and
AJAX—together at last!”
Glenn “Fiddich” Dixon

How to Be Responsive
√ Minimize initial perceived loading time
√ Yield early and often for responsive UI
√ Cache API responses with data-manager layer

- Be Lazy
- Be Responsive
- Be Pragmatic: Don’t make things even harder
- BeVigilant

Play to the browser’s strengths
- Avoid DOM manipulation; use innerHTML and array.join(“”)
- Avoid dynamic CSS-class definitions & CSS math
- Avoid reflow when possible (esp. manually on browser resize)
- Avoid memory allocation (e.g. string-splitting)
- Do DOM manipulation off-DOM, then re-insert at the end
Be Pragmatic: Don’t make things even harder

Cheat when you can / should
- Use global functions or IDs when reasonable
- Finding by class / attaching event handlers is slow
- Protect modularity only when needed (e.g. widgets)
- Directly attach onclick, etc. handlers instead of using event
listeners where appropriate
- Use fastest find-elems available when you need to scan the
DOM (don’t rely on general-purpose code)

Inline initial API calls & HTML
- Tempting to load blank page and do everything in JavaScript
- Have to redraw UI dynamically; don’t want two copies of UI code
- Problem: initial load is usually too slow
- Too many round-trips to the server; too long before initial UI shows up
- Solution: if you have to do it every time, do it statically
- Save out initial API responses in web page
- Use data-manager to hide pre-fetching (can change your mind later)
- Download initial HTML in web page

How to Be Pragmatic
√ Play to the browser’s strengths
√ Cheat when you can / should
√ Inline initial API calls / HTML for faster load time

- Be Lazy
- Be Responsive
- Be Pragmatic
- BeVigilant: Only you can prevent slow web apps

Profile like crazy
- Bottlenecks abound and are usually not obvious
- Use firebug’s profiler (Joe Hewitt, you rule! )
- Use timestamp diffs and alerts
- Comment-out blocks of code
- Measure with a consistent environment
- Browsers bog down  always restart first
- Try multiple runs and average (and don’t forget the cache)
Be Vigilant: Only you can prevent slow web apps

Firebug is your friend

Consider performance from day one
- Apps get slow when you make them do many / slow things!
- Consider how much code / work is needed for each feature
- Is it making the browser work against the grain?
- What else is suffering for this feature? Is it worth it?
- Make sure everyone remembers how important speed is

- Building high-performance apps requires the right attitude
- Must consider and prioritize speed in every decision
- Ask “what features can I add within this size / speed?” vs.“how
small / fast can I get this set of features?”
- I had to learn this the hard way
(Plaxo 3.0 almost didn’t ship!)
Get your priorities straight
“Performance first,
features second!”
“Performance first,
features second!”
Todd & Cam

How to BeVigilant
√ Profile like crazy
√ Consider performance from day one
√ Get your priorities straight

Conclusion:
Avoid making the same mistakes we did
- Make the browser happy … and it will make you happy
- Web browsers are more like mobile phones than desktops
- Limited, flimsy, temperamental platform being stretched beyond its
initial design goals
- But everyone’s got one, so it’s still the best place to be
- Don’t push the limits unless you have to
- Often, small quick-loading pages with AJAX interactions is best
- Sometimes you really do need rich, interactive controls

Just Remember:
EverythingYou’ve Been Taught is Wrong
- Think about performance — early and often
- Write as little code as you need — each line has a cost
- Do what the browser wants (whenever possible)
- Remember the high-performance JavaScript mantra:
- Be lazy
- Be responsive
- Be pragmatic
- Be vigilant
http://JosephSmarr.com

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