Refactoring to
Unobtrusive
Javascript
Federico Galassi
federico.galassi@gmail.com
http://federico.galassi.net/

Separation of Concerns
Keeping different
aspects of an
application separate

Separation of Concerns
So that everyone can
focus on one thing
at a time

Unobtrusive Javascript
Techniques to
enforce separation
of javascript from
other web
technologies

Web Technologies
Html Content
Css Presentation
Javascript Presentation Logic
Server side Business Logic

Rules of the Game
1 Javascript stays in its own files
2 Files are affected only by
changes directly related to
presentation logic

Game Strategy
How do we play?
Code refactoring
is improving quality of
existing code without
changing its functional
behavior

Game Strategy
No refactoring
without testing
• unit testing with jsTestDriver & friends
• minimal functional testing with selenium & friends
• mock the server by wrapping XMLHttpRequest

1 Round
Html
You see an
<script>
doSomething();
// ... more code ...
inline script
</script>

1 Bad Smell
Html
<script>
doSomething();
Hey, it’s
// ... more code ...
</script>
javascript in
html

1 Refactoring:
Externalize Inline Script
Html Js
<script>
doSomething();
// ... more code ...
</script>

1 Refactoring:
Externalize Inline Script
Html Js
<script src="myjavascript.js">
doSomething();
</script>
// ... more code ...

2 Round
Html
You see an
<button
onclick="refreshView();"/>
event handler
registration
Refresh
</button>
by element
attribute

2 Bad Smell
Html
<button
onclick="refreshView();"/>
Hey, it’s
Refresh
</button>
javascript in
html

2 Refactoring:
Attribute Event to Dom Event
Html Js
<button
onclick="refreshView();"/>
Refresh
</button>

2 Refactoring:
Attribute Event to Dom Event
Html Js
<!-- add id to locate it --> var btnrefresh =
<button id="btnRefresh"> document.getElementById(
Refresh "btnRefresh"
</button> );
btnrefresh.addEventListener(
"click",
refreshView,
false
);
<!-- loaded after DOM is built -->
<script src="myjavascript.js">
</script>
</body>
</html>

3 Round
Html
You see a
javascript link
<a
href="javascript:showCredits();">
Show Credits</a>

3 Bad Smell
Html
<a
href="javascript:showCredits();">
Hey, it’s
Show Credits</a>
javascript in
html

3 Refactoring:
Javascript Link to Click Event
Html Js
<a
href=""> javascript:showCredits();
Show Credits</a>

3 Refactoring:
Javascript Link to Click Event
Html Js
var showcredits =
<!-- add id to locate it -->
document.getElementById(
<a id="linkShowCredits"
"linkShowCredits"
href="#"> );
Show Credits</a>
// in refreshView you should
// event.preventDefault
showcredits.addEventListener(
"click",
refreshView,
false
);
<!-- loaded after DOM is built -->
<script src="myjavascript.js">
</script>
</body>
</html>

Game Break
Now HTML should
be Javascript free

4 Round
Js
You see
div.onclick = function(e) {
// div has been selected
presentation
set by
var clicked = this;
clicked.style.border =
"1px solid blue";
element.style
}
properties

4 Bad Smell
Js
div.onclick = function(e) {
// div has been selected
Hey, it’s
var clicked = this;
clicked.style.border =
"1px solid blue";
css in
}
javascript

4 Refactoring:
Dynamic Style to Css Class
Js Css
div.onclick = function(e) {
// div has been selected
var clicked = this;
clicked.style.border =
"1px solid blue";
}

4 Refactoring:
Dynamic Style to Css Class
Js Css
div.onclick = function(e) {
// div has been selected
var clicked = this;
.selected: {
// should be addClass
border: 1px solid blue;
clicked.setAttribute(
}
"class",
"selected"
);
}

5 Round
Js You test a
var account = JSON.parse(
complex
boolean
response
);
if (account.balance < 0) {
expression
show("can’t transfer money!");
}
which is not
presentation

5 Bad Smell
Js
var account = JSON.parse(
response
);
Hey, it’s
if (account.balance < 0) {
show("can’t transfer money!");
}
business logic
in javascript

5 Refactoring:
Business Logic Simple Test
Js Server
var account = JSON.parse(
response
);
if () { account.balance < 0
show("can’t transfer money!");
}

5 Refactoring:
Business Logic Simple Test
Js Server
var account = JSON.parse(
response
<?php
);
// use business logic to decide
if (account.canTransfer) { $account["canTransfer"] = false;
echo json_encode($account);
show("can’t transfer money!"); ?>
}

6 Round
Js
// add book to the list
var book = doc.createElement("li");
You see
complex
var title = doc.createElement("strong");
titletext = doc.createTextNode(name);
title.appendChild(titletext);
var cover = doc.createElement("img");
dom code to
cover.src = url;
book.appendChild(cover);
book.appendChild(title);
bookList.appendChild(book);
generate html

6 Bad Smell
Js
// add book to the list
var book = doc.createElement("li");
var title = doc.createElement("strong");
titletext = doc.createTextNode(name);
Hey, it’s
title.appendChild(titletext);
var cover = doc.createElement("img");
cover.src = coverurl;
book.appendChild(cover);
html in
book.appendChild(title);
bookList.appendChild(book);
javascript

6 Refactoring:
Dom Creation to Html Template
Js Html
// add book to the list
var book = doc.createElement("li");
var title = doc.createElement("strong");
titletext = doc.createTextNode(name);
title.appendChild(titletext);
var cover = doc.createElement("img");
cover.src = coverurl;
book.appendChild(cover);
book.appendChild(title);
bookList.appendChild(book);

6 Refactoring:
Dom Creation to Html Template
Js Html
// add book to the list
var tplBook = loadTemplate(
"book_tpl.html"
<li>
);
<strong>Title</strong>
var book = tplBook.substitute({
<img src="Cover" />
Title: name,
</li>
Cover: coverurl
});
bookList.appendChild(book);

Game Extra Time
Make javascript
play well with
other javascript

7 Round
Js
var counter = 0;
// ... more code ...
You see code
placed outside
a function

7 Bad Smell
Js
var counter = 0;
// ... more code ...
// ... later ...
// counter is 1
Hey, it’s a
Other Js
// redeclares previous
// counter !!
global variable
var counter = 1;

7 Refactoring:
Global Abatement
Js
var counter = 0;
// ... more code ...
// ... later ...
// counter is 1
Using the
Module pattern
Other Js
// redeclares previous
we can make it
private
// counter !!
var counter = 1;

7 Refactoring:
Global Abatement
Wrap code in
(function() {
Js
var counter = 0;
// ... more code ...
// ... later ...
// counter is still 0
})();
an anonymous
Other Js function which
is immediately
// don’t see previous
// counter
var counter = 1;
invoked

8 Round
Login Js
btnLogin.onclick = function(e){
login();
toolbar.update();
logger.log("login!");
You see an
event handler
}
function update() {
// ...
Toolbar Js
which calls
many unrelated
}
Logger Js
function log(msg) {
}
// ...
modules

8 Bad Smell
Login Js
btnLogin.onclick = function(e){
login();
toolbar.update();
logger.log("login!");
}
Toolbar Js
function update() {
Hey, it’s
// ...
}
function log(msg) {
Logger Js
other modules
javascript
// ...
}

8 Impact
Login Js
btnLogin.onclick = function(e){
login();
toolbar.update();
logger.log("login!");
}
FAIL
Toolbar Js
function update() {
Time coupling
// ...
}
function log(msg) {
Logger Js
issues
t initialized
// ...
No
}

8 Impact
Login Js
btnLogin.onclick = function(e){
login();
toolbar.update();
logger.log("login!");
hange
friends.notify("login");
eds C
}
Ne
Toolbar Js
function update() {
// ... Friends Js
Divergent
}
function notify(event) {
// ...
dded
}
A
Logger Js
function log(msg) {
}
// ...
change

8 Refactoring:
Custom Events
Login Js
Custom events
btnLogin.onclick = function(e){
login();
invert
}
function update() {
toolbar.update();
Toolbar Js
dependency
and make
}
code readable
Logger Js
function log(msg) {
logger.log("login!");
}

8 Refactoring:
Custom Events
Login Js
btnLogin.onclick = function(e){
login();
event.fire("login"); Fire an high
level custom
}
Toolbar Js
function update() {...}
event.listen("login", function(e) {
update();
event and make
other modules
});
Logger Js
listen to it
function log(msg) {...}
event.listen("login", function(e) {
log("login attempt");
});

9 Round
Js
// home getElementById "tabHome"
// news getElementById "tabNews"
// about getElementById "tabAbout"
You see many
home.onclick = function() {
}
showPage("pageHome");
news.onclick = function() {
similar event
handlers
showPage("pageNews");
}
about.onclick = function() {
showPage("pageAbout");
}

9 Bad Smell
Js
// home getElementById "tabHome"
// news getElementById "tabNews"
// about getElementById "tabAbout"
home.onclick = function() {
showPage("pageHome");
}
Hey, it’s
news.onclick = function() {
showPage("pageNews");
}
about.onclick = function() {
duplicated
showPage("pageAbout");
}
javascript

9 Impact
Js
// home getElementById "tabHome"
// news getElementById "tabNews"
// about getElementById "tabAbout"
home.onclick = function() {
}
showPage("pageHome"); More tabs
more code
news.onclick = function() {
showPage("pageNews");
}
about.onclick = function() {
more handlers
showPage("pageAbout");
}
contact.onclick = function() {
showPage("pageContact");
}
Needs Change more memory
usage

9 Impact
Js
// home getElementById "tabHome"
Need to track
// news getElementById "tabNews"
// about getElementById "tabAbout"
home.onclick = function() {
showPage("pageHome");
if new elements
}
news.onclick = function() {
showPage("pageNews");
}
about.onclick = function() {
are added to
showPage("pageAbout");
}
Contact Js
tabContainer.addChild(
"tabContact"
register handlers
);
Missing click handler

9 Refactoring:
Events Delegation
Js
Event delegation
makes code
// home getElementById "tabHome"
// news getElementById "tabNews"
// about getElementById "tabAbout"
home.onclick = function() {
more compact
showPage("pageHome");
}
news.onclick = function() {
showPage("pageNews");
}
and
about.onclick = function() {
showPage("pageAbout");
}
maintainable

9 Refactoring:
Events Delegation
Handle the event
Js
in an elements
// container getElementById
// "tabContainer"
container.onclick = function(e) {
ancestor.
var id = e.target.id
var page = id.replace(
"tab", "page"
);
showPage(page);
Bubbling makes
}
it work

Game Over
Separation of concerns