Javascript high performance

by Guilherme Trindade @ Avenue Code
ggoncalves@avenuecode.com

Jan 30th, 2014

AGENDA

Why?

When?

How?

Load Time
Data Storage
DOM Scripting
Algorithms and Flow Control
Strings
Tools

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Javascript High Performance

PRerequisites

Too Late! No one is leaving!

But if you insist, Javascript.

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Javascript High Performance

WHY?

How many times did you give up browsing through a page because it was too slow?
Browser's optimization won't ever handle everything
Each day, a new kind of device gets access to the internet.

Battery life
Response time

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Javascript High Performance

WHEN?

Before your code is ready
But in what kind of applications?

All of them.

Most optimizations have no additional cost
What if my code is done already?

Effort
Risk
Benefit

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Javascript High Performance

HOW?

Watch the rest of this presentation

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Javascript High Performance

DAta Load

Put scripts at the bottom.

a script tag blocks rendering
defer attribute

Load scripts together.

1 x 100kb is faster than 4 x 25kb
Concatenate and minify.
Extra: Use Sprites to combine Images

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Javascript High Performance

DATA LOAD

Practical Example

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Javascript High Performance

DATA STORAGE

Where should we store our data?
Variables and objects

Local variables are the fastest!
Avoid very long dot notations

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Javascript High Performance

DATA STORAGE

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Javascript High Performance

Data Storage

Scope

Scope chain
avoid with, try-catch and eval()

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Javascript High Performance

DOM SCRIPTING

DOM what?

DOM = Document Object Model
The main library you're going to use to navigate through and modify elements of the page.
A separate library from javascript itself.

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Javascript High Performance

DOM SCRIPTING

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Javascript High Performance

DOM SCRIPTING

HTML collections can be expensive to use.

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Javascript High Performance

DOM SCRIPTING

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Javascript High Performance

DOM SCRIPTING

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Javascript High Performance

DOM SCripting

Query Selectors

Can save up a lot of time on complex queries
Slower than simple getElementById

Let's say you want all divs with 'warning' and 'notice' classes.

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Javascript High Performance

DOM SCRIPTING

QuerySelectors provide a simpler and faster way

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Javascript High Performance

DOM Scripting

Repaint / Reflow

Repaint = Drawing pieces of the page that changed
Reflow = Recalculation of the render tree

Both are very expensive operations
How to avoid?

Use CSS classes instead of inline styles
Avoid getting/setting layout informations multiple times. Same reason you should avoid length on an HTML Collection.

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Javascript High Performance

DOM SCRIPTING

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Javascript High Performance

DOM SCRIPTING

Event Delegation
How bad can this be?

You could have hundreds or events in one page
Or just 1.

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Javascript High Performance

DOM SCRIPTiNG

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Javascript High Performance

Algorithms and flow control

Loops

regular loops are the fastest ones

for
while
do-while

for-in loops can be 7 times slower than the regular ones
function based iterations like forEach can be useful but should also be avoided

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Javascript High Performance

Algorithms and flow control

How to optimize loops?

reverse the iterator
cache the length

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Javascript High Performance

Algorithms and flow control

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Javascript High Performance

Algorithms and flow control

if-else vs switch

if-else when there are only a few conditions
for large number of conditions use switch

how to optimize ifs?

Put the most likely scenario first.

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Javascript High Performance

Algorithms and flow control

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Javascript High Performance

Algorithms and flow control

But if you can use lookup tables...

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Javascript High Performance

Strings

What's the best way to do String operations?
Keep it simple and just use + and +=
Very optimized by browsers but still expensive

Try to do as few as possible.
forget about String.prototype.concat
forget about array joining.

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Javascript High Performance

TOOLS

Some of the ones I liked the most

http://jsperf.com/if-else-vs-switch-test
chrome profiler
YSlow

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Javascript High Performance

Conclusion

Knowing how to optimize your code can make a big difference.
My advice?

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Javascript High Performance

Learn more

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Javascript High Performance

CHALLENGE

https://github.com/gtrindade/animations

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Javascript High Performance

By Avenue Code

Javascript High Performance

Learn to make your web application faster with a better response time.

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