Lazy Loading Because I'm Lazy

Lazy Loading
“do the least work possible”

Evolution of javascript application
optimization

<head> <script> code </script> </head>

Many separate script ﬁle as separate
resources (cacheable)

Far futures expires headers, e-tags

Reduce requests: miniﬁcation and
concatenation

Serve from static, “cookieless” domains, CDNs

We’re still slow....what’s next?

The problem: the ﬁle
keeps growing!
Not scalable for large applications

Complexity of production support increases,
more code = more liability (one syntax error
and boom!)

Big ball of mud

Hard to maintain development vs. production
systems (dev mode with separate ﬁles, build
process, versioning, etc)

The solution: lazy load
Always load core javascript framework
(jquery + your framework/bootstrap) You don’t have a
framework???

Lazy load models and controllers based upon
interaction

Don’t have an MVC framework? coremvc,
JavascriptMVC, many others

Avoid dependency hell: keep it one-level/
controller based. Don’t “abuse” prototype

Fine Lines
Understand
DOMContentLoaded and load events
Absolute final “End User” experience. DOMContentLoaded = DOM is
ready, loaded. load = all page components have finished loading

It matters even for a primed cache, big/many javascript
components delay final onload event, causing loading/blocking

It takes time to parse & evaluate big javascript files. Caching can’t
fix this, users see it

Possible improvements:
why invest the time?
Increase simplicity

“Snappiness”, reduce memory footprint --
less code in memory

Hide latency

Manage user expectations (users expect to
wait when they do some action the ﬁrst
time, but hate waiting on every page load)

Evaluate your site metrics (passers-by vs.
heavy users), cache rate percentage for

Where do I start?
Today (probably yesterday)

Evaluate your application, especially your users

Y-SLOW & Firebug Net Panel. Google Page Speed Plugin

Look at existing solutions

jquery plugins available

yuiloader, requirejs, many others as well

dojo loader & dep management

(maybe) do your own - simple bootstrap with simple API

test in all target browsers for loading indications. hacks will be
needed

<html>
A maybe-contrived example
<head>
<title>LazyLoading Test</title>
<script>
var AsyncLoader = function(src, cb) {
var s = document.createElement('script');
s.type = 'text/javascript';
s.async = true;
s.src = src + '?=' + Math.random(); // make sure we are loading this once
if(typeof(cb) == 'function') {
this.onLoad(cb);
}

https://gist.github.com/904251
var onLoad = this.onLoad();

s.onreadystatechange = function() {
if(this.readyState == 'loaded' ||
this.readyState == 'complete') {
onLoad();
}
};
s.onload = onLoad;
s.onerror = onLoad;

var asyncLoad = function() {
setTimeout(function() {
document.getElementById('msg').innerHTML = document.getElementById('msg').innerHTML + 'Loading...';
var x = document.getElementsByTagName('script')[0];
var head = document.getElementsByTagName("head")[0];
x.parentNode.insertBefore(s, x);
}, 1000);
};

if (window.attachEvent) {
window.attachEvent('onload', asyncLoad);
} else if(window.addEventListener) {
window.addEventListener('load', asyncLoad, false);
}
};

AsyncLoader.prototype.onLoad = function(cb) {
if(typeof(cb) == 'function') {
this._cb = cb;
} else if(typeof(this._cb) == 'function') {
var that = this;
return function() {
that._cb();
}
}
};

var loader = new AsyncLoader('http://www.mersenneforum.org/txt/43.txt', function() {
document.getElementById('msg').innerHTML = document.getElementById('msg').innerHTML + 'done';
});

</script>
</head>
<body>
<h1>Javascript Lazy Loading Test</h1>
<div id='msg'></div>
<div style="height:1000px; width: 50px; background: #aaaaaa;"></div>
</body>
</html>

Desired Outcome
Minimal loading indication across most
browsers. “Defer” load until after load or
your custom event

Good for third party javascripts which may
be slow or trigger more blocking requests
themselves which continue page loading

Die silently, but handle it. Don’t go on like
nothing happened; don’t try to use code that
didn’t load!

Example Results

http://www.letseehere.com/examples/lazy_loading/

thanks
Johnathan Leppert
Developer, Manta.com
twitter: @iamleppert
johnathan.leppert@gmail.com
Let me google that for you
(further reading):
https://gist.github.com/904251
http://velocityconf.com/velocity2010/public/schedule/detail/15412
http://ajaxpatterns.org/On-Demand_Javascript
http://jasonleveille.com/blog/2008/05/javascript-lazy-loading
http://developer.yahoo.com/yui/3/yui/
http://requirejs.org
http://wonko.com/post/painless_javascript_lazy_loading_with_lazyload
http://ajaxian.com/archives/a-technique-for-lazy-script-loading
http://www.digital-web.com/articles/improve_page_performance_with_lazy_loading/

Lazy Loading Because I'm Lazy

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