jQuery and Web Performance

Dave Methvin

President, jQuery Foundation

Lead Developer, jQuery Core

● Maintains jQuery code and docs

● Supports web developers and standards

● Participates in standards process

○ W3C web standards

○ ECMA 262 (JavaScript)

● Hosts conferences (like this one!)

jQuery Foundation

jQuery Core 1.11 and 2.1

• Shipped last month

• We didn't break a lot of things!

• "jQuery Served Your Way" ™

o Support for IE 6/7/8 (1.11) or not (2.1)

o Custom builds for smaller size

o Use with node, browserify, or inside apps

builtwith.com

jQuery Team - World Wide

http://contribute.jquery.org

PERFORMANCE

"JavaScript / jQuery /

browsers are a bad

developer environment!"

A poor

workman

blames

his tools

How the Programmer Sees It

JavaScript Browser

Web Developer's Reality

Browser JavaScript

Mouse

CSS

HTMLContent caching

Keyboard

Touch

Screen paints

Layout calculationImage decoding

Focus management

Network requests

Web Developer's Reality

Browser JavaScript

Mouse

CSS

HTMLContent caching

Keyboard

Touch

Screen paints

Layout calculationImage decoding

Focus management

Network requests

Optional

How Do I Make My Page Fast?

1)Find slow stuff

2)Make it not slow

What you can

measure using

tools today

How Do I Find the Slow Stuff?

What you can

measure using

tools today

What you

should

measure

How Do I Find the Slow Stuff?

JavaScript Loop Optimization

JavaScript Loop Optimization

Slowest looping style still only

takes 140 microseconds to do

10 iterations of a loop

“Programmers waste enormous amounts of

time thinking about, or worrying about, the

speed of noncritical parts of their programs,

and these attempts at efficiency actually

have a strong negative impact when

debugging and maintenance are considered.

We should forget about small efficiencies,

say about 97% of the time; premature

optimization is the root of all evil. Yet we

should not pass up our opportunities in that

critical 3%.”

--Donald Knuth

“Programmers waste enormous amounts of

time thinking about, or worrying about, the

speed of noncritical parts of their programs,

and these attempts at efficiency actually

have a strong negative impact when

debugging and maintenance are considered.

We should forget about small efficiencies,

say about 97% of the time; premature

optimization is the root of all evil. Yet we

should not pass up our opportunities in that

critical 3%.”

--Donald Knuth

This Should Be You, 97% of the

Time

•• Client-side issues often can be solved by

"peephole" optimizations and don't require

massive architecture changes

• Many — most! — speedups can be done

near the end of the project (or even after

deployment, cough)

Finding and Fixing the 3 Percent

Page Load Performance

How the Browser Loads Pages

1) Browser fetches index.html

2) Pre-fetcher scans HTML for resources (images, CSS,

scripts) and requests them immediately

3) Browser loads / runs JavaScript when encountered

during parsing (since scripts can write out new HTML!)

4) When HTML is fully loaded and parsed, browser calls

DOMContentLoaded handlers (jQuery .ready())

5) Browser does initial rendering of the page (finally the

user sees something!)

Now It May Seem Obvious, But...

• Resources not already in the HTML file can't

be prefetched, resulting in further delayso e.g. stuff injected by your JavaScript/jQuery

• JS frameworks or initial content rendered

from some client-side templates can make

the prefetcher useless

Manual Prefetching

Lets you tell the browser get a running start on

template content or deeper pages in the site.

<link rel="dns-prefetch" href="media.mysite.com">

<link rel="prefetch" href="/img/kitten.jpg">

YSlow

Google PageSpeed

modern.IE

webpagetest.org

Here Are Your Blocking Resources

Here Are Your Blocking Resources

Advertising!

You Have 16 Milliseconds … Begin

60 frames/second ~ 16 milliseconds/frame

• Long-running operations can make the page

appear "janky" rather than smooth

• Really long-running operations can make the

page appear unresponsive to the user

It Happens in 16 Milliseconds?

From High Performance Browser Networking by Ilya Grigorik (O'Reilly)

Adventures in Dirty Layout

:visible

:hidden

"The Dot That Ate Performance"

console.time("init");

$("body").removeClass("activated");

$("p:visible").css("color", "blue");

console.timeEnd("init");

"Hey Browser Can I Bug You?"

30 ms

What If We Track Visibility?

console.time("init");

$("body").removeClass("activated");

$("p.visible").css("color", "blue");

console.timeEnd("init");

"Never Mind Browser, I Know This"

8 ms

Chrome's Yellow Triangle

IE11: Layout after offsetWidth/Height

●

●Look out for :visible or :hidden

● Minimize document-wide style/class

changes

○ Use data- attrs or jQuery `.data()` if non-stylistic

● Get JavaScript out of the path

○ CSS transitions

○ CSS animations

Avoiding Forced Layout

Using Dev Tools Profilers

When JavaScript really is the

problem (or seems to be), a

profiler can find the hot spots.

A Real Site: gimmickbook.com

• Stutters during infinite scroll

• Seems to get worse as the page grows

• Using the jQuery Masonry plugin

What's Wrong?

What's Wrong?

Faster!

Forced Layout/Reflow

Chrome's Event tab shows JavaScript has forced layouts

Chrome Profile ("Tree")

IE 11 Profile ("Call Tree")

What Does This Code Look Like?

Moral of the Story

Infinite scroll should not be used with

full-page layout algorithms!

In this case, the plugin could be

changed to only lay out the new

items, since nothing above them

changed.

You Have the Tools, Use Them!

Twitter: @davemethvin

GitHub: @dmethvin

IRC (Freenode): DaveMethvin #jquery-dev

Email: dave@jquery.com

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