JS Essence

Goals
• JS popularization
• JS as engineering tool

Roadmap
• Embedded JS issues:
- bad parts, good parts
- imperative skin upon functional nature
- misunderstanding of prototype inheritance
- missing modules support
- performance issues

Bad parts
• global variables
• eval
• a++
• math
• with
• new (Number | String | Boolean)
• == vs ===

Good parts
• === : type safe vs (==)
• [] : new Array()
• {} : new Object()
• a && b : if (a) b else a
• a || b : if (a) a else b
• closures (~ lambda curring)

Java(??!)Script
• ..one more LISP dialect..
• Mocha
• LiveScript
• JavaScript

Functional nature
• functions are the 1-st class objects:
- assign to variables
- pass as an argument
- return as a result

OOP in JavaScript
• Объект_о-ориентированный
• Объектно-ориентированный

Classical OOP simulation
• Classical OOP inheritance can be simulated:
> I would recommend John Resig’s “Class”
object
http://ejohn.org/blog/simple-javascript-inheritance/

Classical OOP simulation
Class.extend({
init: function(a, b) {
// .ctor
this._super(a, b);
},
method1: function() {
// do something
}
});

Inheritance examples
• Some examples

var F = function(n) {
this.name = n;
}
var a = new F(“a”);
var b = new F(“b”);

this.name = n;
}
F.prototype = , root: “hello world!” -;
a.root // ???
b.root // ???

this.name = n;
}

a.root = “Prototype inheritance magic”;
b.root // ???

var F = function() {}

var a = new F();
a.constructor === F // ???

var F = function() {}
var a = new F();
a.constructor === F // ???

Dynamic inheritance
var F = function() {};
F.prototype = {
count: 0,
augment: function() {
++F.prototype.count;
F.prototype.test = function() { alert(this.count) }
}
};
var a = new F();
var b = new F();

a. augment();

a.test() // ???
b.test() // ???

Modules simulation
• No modules. Global variables RULEZZZ!!11

Modules simulation
• No modules. Global variables RULEZZZ!!11

BAD!

Modules simulation
• No named modules. BUT functional context

(function(global) { . . .})(window)

Modules simulation
• No named modules. BUT functional context

(function(global) { . . .})(window)

var myJQueryVar = $.noConflict()

Modules simulation
• Namespacing as global variables chains
newsite.common.utils
newsite.common.services

var newsite = newsite || {};
newsite.common = newsite.common || {};
newsite.common.utils = function() , … -;

Modules simulation
• Namespacing as global variables chains
newsite.common.utils
newsite.common.services
• $LAB
.script(“newsite.core.js").wait()
.script(“newsite.common.utils.js")
.script(“newsite.common.services.js“)
.wait(function() { /* ready */ })

Modules simulation

RequireJS
http://requirejs.org/

Modules simulation - RequireJS
<!DOCTYPE html>
<html>
<head>
<title>My Sample Project</title>
<script
src="path_to/require.js“
data-main="entry_points/main">
</script>
</head>
<body>
<h1>My Sample Project</h1>
</body>
</html>

entry_points/main.js
require(
*“dir/module1“, “dir/module2“+,
function(m1, m2) { /* to do: … */ }
);

dir/module1.js
define(
*“dependency-on-some-other-modules”+,
function () {
return {
color: "black",
clear: function() ,…-
};
}
);

Performance - prologue
• It’s still possible to write slow JavaScript on
the new fast JavaScript engines

• JavaScript performance directly affects user
experience

Performance
• Loading & execution
• DOM scripting

Loading and execution
• Most browsers use a single UI thread for UI
updates and JavaScript execution
• Appearance of a <script ..> tag cause page
download and rendering to stop and wait for
the script to complete before processing
• Even parallel script downloads block
downloading other resources (images, CSS)

Loading and execution
• Put <script> tags as close to the bottom of the
<body> as possible
• Load scripts in groups
(100 kb faster than 4 x 25kb)
• Minify your scripts
• Optimize your stylesheets

Non-blocking loading
• <script defer> (IE 4+, FF 3.5+)
• Dynamic <script> elements
– Parallel non-blocking loading
– Put into <head> to prevent “operation aborted”
– Remember of ordering (cross-browser variation)
• XMLHttpRequest injection
– Inline <script> vs eval()
– Downloading from CDNs impossible

DOM Scripting
• Live DOM collections
• Repaint and Reflow
• Handling DOM events

What is DOM?
• Document Object Model – language
independent application interface (API) for
working with XML and HTML documents
• Browsers keep DOM and JavaScript
implementations independent of each other

Toll bridge
• Touch the DOM lightly
• Stay within ECMAScript as much as possible

HTML collections
• Expensive live collections
• Use local variables when accessing collection
elements

Repaints and reflows
• DOM tree
• Render tree

Reflow process
When a DOM tree change affects element
geometry – browser recalculate geometry and
position of elements that could have been
affected by the change and reconstructs the
Render tree

Redraw process
Once the reflow is complete, the browser
redraws the affected parts of the screen

When does a reflow happen?
• Page renders initially
• Visible DOM elements are added or removed
• Elements change position
• Element change size
(margin, padding, border, width, height)
• Content is changed (text or image with
different size)
• Browser window is resized

Queuing and flushing reflows
• Browsers optimize reflow by queuing changes
and performing them in batches
• Never request layout information while it’s
being changed

Queuing and flushing reflows
• offsetX
• scrollX
• clientX
• getComputedStyle (currentStyle in IE)

* X – Top, Left, Width, Height

Minimizing repaints and reflows
• Combine multiple DOM and style changes into
a batch and apply them once

Batching DOM changes
• Take the element off of the document flow
• Apply multiply changes
• Bring the element back to the document

Ways to modify the DOM off the
document
• Hide, apply changes and show again
• Use document fragment to build subtree
outside of the live DOM and then copy it to
the document
• Copy the original element into an off-
document node, modify the copy and replace
original element when done

Take elements out of the flow for
animation
1. Use absolute positioning
2. Animate the element
3. When the animation is done, restore the
positioning

JQuery DO this job for you!

Event delegation
• A lot of event handlers affects
memory, performance and useless since user
clicks 1 button of 100 for example
• Set event handler for container element and
use event delegation

Performance essence
• http://jsperf.com/

• http://www.developer.nokia.com/Community
/Wiki/JavaScript_Performance_Best_Practices

Patterns
• http://www.addyosmani.com/resources/esse
ntialjsdesignpatterns/book/

How to move next
• Primary
• Advanced
• Meta-level

How to move next
• http://habrahabr.ru/post/117838/

JS Essence

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