How DRY impacts JavaScript performance // Faster JavaScript execution for the lazy developer

How DRY impacts JavaScript performance //
Faster JavaScript execution for the lazy developer
Mathias Bynens – Velocity Europe, November 2011

JavaScript & performance

Rule #1: nothing to do with JS

JavaScript & performance

What about
the actual run-time performance
on the client side?

“DRY leads to readable,
maintainable code”

DRY JavaScript
improves
performance
…if you do it right

So, where to avoid
repetition?

What’s slow in JavaScript?
1. The DOM

1. The DOM

2. Function calls

1. The DOM

2. Function calls

3. Lookups

DOM manipulation
// Create the element in memory
var el = document.createElement('p');

// Insert the element into the DOM
document.body.appendChild(el);

DOM manipulation
<body>
…
<div>
<p></p>
</div>
</body>

DOM manipulation
var div = document.createElement('div'),
p = document.createElement('p');

// Bad
document.body.appendChild(div);
div.appendChild(p);

DOM manipulation
var div = document.createElement('div'),
p = document.createElement('p');

// Better
div.appendChild(p);
document.body.appendChild(div);

DOM manipulation
<body>
…
<p></p>
<p></p>
<p></p>
<p></p>
</body>

DOM manipulation
var p = document.createElement('p'),

i = 4;

while (i--) { // Add four <p> elements

document.body.appendChild(p.cloneNode(false));

}

DOM manipulation
var frag = document.createDocumentFragment(),
p = document.createElement('p'),
i = 4;

while (i--) { // Add four <p> elements
frag.appendChild(p.cloneNode(false));
}

document.body.appendChild(frag);

Function calls
// Function declaration

function foo(bar) {

return bar;

}

// Function call

foo('something');

Function calls
alert('foo');
document.getElementById('foo');
$('#foo');

Function calls
$('.foo').show();
// other stuff…
$('.foo').hide();

Function calls
var $foo = $('.foo');
$foo.show();
// other stuff…
$foo.hide();

Function calls
var $foo = $('.foo').show();
// other stuff…
$foo.hide();

Property lookups
var obj = {
'x': 42,
'y': {
'foo': 'bar'
}
};

obj.x; // 42
obj.y.foo; // 'bar'

Property lookups
document.title

dojo.query(…)

YAHOO.util.Dom.get(…)

Property lookups
var foo = YAHOO.util.Dom.get('foo'),
bar = YAHOO.util.Dom.get('bar'),
baz = YAHOO.util.Dom.get('baz'),
qux = YAHOO.util.Dom.get('qux');

Property lookups
var get = YAHOO.util.Dom.get,
foo = get('foo'),
bar = get('bar'),
baz = get('baz'),
qux = get('qux');

Array item lookups
var elems = document.getElementsByTagName('p'),
length = elems.length;

while (length--) {
if (elems[length].className == 'foo') {
// do something with elems[length]
elems[length].innerHTML = 'LOLWAT';
}
}

Array item lookups
var elems = document.getElementsByTagName('p'),
length = elems.length,
elem;

while (length--) {
elem = elems[length];
if (elem.className == 'foo') {
// do something with elem
elem.innerHTML = 'LOLWAT';
}
}

Scope lookups
var foo = 42;
foo; // no scope lookup

Scope lookups
var foo = 42;
(function() {
foo; // one scope lookup
}());
// IIFE – see http://mths.be/iife

Scope lookups
var foo = 42;
(function() {
(function() {
foo; // two scope lookups
}());
}());

Scope lookups
var foo = 42;
(function(foo) {
(function(foo) {
foo; // ZOMG, no scope lookups!!1
}(foo));
}(foo));

Scope lookups
(function() {
// every time you use `window`
// or `document` here
// that’s a scope lookup
}());

Scope lookups
(function() {
var doc = document,
win = window;
// lookup once, then cache
}());

Scope lookups
(function(win, doc) {
// use `win` and `doc` here
// no scope lookups
// no performance penalty!
}(this, document));

Recap: what’s slow in JavaScript?

1. The DOM

1. The DOM

2. Function calls

1. The DOM

2. Function calls

3. Lookups

Especially when used inside…

• Loops

• Loops

• Intervals

• Loops

• Intervals

• Handlers for events that ﬁre frequently

It happens to the best!
// Don’t do this:
$(window).scroll(function() {
$('.foo').something();
});

It happens to the best!
// Don’t do this:
$(window).scroll(function() {
$('.foo').something();
});

// See http://mths.be/azs

typeof performance != 'the whole story'

New objects
var obj = new Object();
obj.x = 42;
obj.y = 'foo';
obj.z = false;

New objects
var obj = {
'x': 42,
'y': 'foo',
'z': false
};

New arrays
var arr = new Array();
arr.push(42);
arr.push('foo');
arr.push(false);

New arrays
var arr = [
42,
'foo',
false
];

Avoid switch
switch(foo) {
case 'alpha':
// do X
break;
case 'beta':
// do Y
break;
default:
// do Z
break;
}

Avoid switch
var switchObj = {
'alpha': function() {
// do X
},
'beta': function() {
// do Y
},
'_default': function() {
// do Z
}
};
(switchObj.hasOwnProperty(foo) && switchObj[foo] || switchObj._default)(args);

Don’t use jQuery for everything
$('.foo').click(function() {
$(this).prop('id');
// same as this, before jQuery 1.6:
// $(this).attr('id');

// also `href`, `checked`, `value`…
});

Don’t use jQuery for everything
$('.foo').click(function() {
this.id;
this.href;
this.checked;
this.value;
// etc.
});

jQuery document ready
$(document).ready(function() {
// teh coads
});

$().ready(function() {
// heh
});

$.fn.ready(function() {
// not pretty, but fastest solution
});

$(function() {
// moar sexy, but slower
});

(function() {
// move <script>s to the bottom
// and just use an IIFE*
}());

// * unless you use .appendChild() / .innerHTML on document.documentElement or document.body: http://mths.be/ieoa

jQuery collection size
$('.foo').size(); // NO.

jQuery collection size
// jQuery source:
$.fn.size = function() {
return this.length;
};

// …so, just use:
$('.foo').length;

Use context
$('#foo .bar').addClass('baz');
$('#foo .qux').hide();
$('#foo input').removeClass('wut');

Use context
var $foo = $('#foo');
$('.bar', $foo).addClass('baz');
$('.qux', $foo).hide();
$('input', $foo).removeClass('wut');

this.location = 'http://jsperf.com/'

http://jsperf.com/browse/mathias-bynens

How DRY impacts JavaScript performance // Faster JavaScript execution for the lazy developer

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