JavaScript Best Pratices

JavaScript Best Practices
@GregWeng
NCU 2015/10/02

I'm
Working for Mozilla as a Software Engineer; 2yr+
Responsible for FirefoxOS screen locker (LockScreen)
ECMAScript experience: 6yr+
Free Software and Open Source are important to me
And I'm also a language enthusiast
Like to research programming language themselves
Haskell, Ruby, JavaScript
Java, PHP, Erlang, Python, C/C++

Find me at
Mail: gweng@mozilla.com
IRC: snowmantw
Twitter: @GregWeng

No I don't use Facebook and LINE

The Lesson
Some best practices from our daily work

In this lesson you shall learn how to...
Bind this in Event handlers in the proper way
Deal with object properties w/ constructor & prototype
Extend instantiable object properly
Manage asynchronous flows with Promise
Write your first test case for a pure function

In this lesson you may also try to...
Avoid using closure when it shouldn't appear
Implement functions with closure when it's necessary
Write meaningful comments in JSDoc format
Make your code more expressive with map/forEach

About quizzes and the homework
We will refactor an example from scratch
Quizzes help you to think and learn things
They require you to add comments on several PR pages
Or, raise your hand
If you fail at homework they may save you, too
Homework will be marked strictly according to the public
marking criteria
That means, you either get passed or failed, no other
adjustments except the quizzes

Since I suppose you know this
class is not JavaScript tutorial

Questionnaire
Before the class begins, I would like to ask you if you know:
How to write constructor + prototype (instantiable object)
How to extend an object to add new methods
How to receive and dispatch events
What the 'this' may refer to?
What's a 'closure'?
What's the better way to control asynchronous flow

Let's start from the
embarrassing past...

Some anti-patterns
You put all states in local variables
And then bind events with anonymous callbacks
And then encapsulate them as a 1K+ lines closure
And then try to create a "singleton" via a function that will be
invoked automatically when bootstrapping
And then make it as a Window Manager as Metacity of
Gnome or KWin of KDE
And then you shoot yourself in the foot

Beware of these terms
You put all states in local variables
And then bind events with anonymous callbacks
And then encapsulate them as a 1K+ lines closure
And then try to create a "singleton" via a function that will be
invoked automatically when bootstrapping
And then make it as a Window Manager as Metacity of
Gnome or KWin of KDE
And then you shoot yourself in the foot
class, private, flags, global objects, temporary variables

Usually, bad things happen
when they appear

Example #1
http://bit.
ly/1LeY5lv

"The real badass Todo list"
"You want to write a Todo list,
and you feel it is a simple quest"

"So you think, you only need to write several functions"

I have three little functions,
one is to draw the list,
one is to fetch the data,
and the last one is to save the data

http://bit.ly/1LeY5lv

Quiz#1
(5 minutes)http://bit.ly/1NVdc2y

Things become worse when
the project grows up

Issues
What if TodoItem and TodoInput are written by different
people?
And what will happen if they both write a function called
"draw"?

Since we put all things globally
window
drawList fetchData saveData ...

Names and states may collide
window
window
drawInput fetchData saveData ...
Working for the List
Working for the Input

Encapsulation
"Why don't we solve collisions with closure, which is the
most natural mechanism to do that in JavaScript?"

Closure
It's about capture
http://bit.ly/1LINo9t

Encapsulation by closures
window
window
drawInput fetchData saveData ...
The "List" closure
The "Input" closure
(insulated)

It seems a successful solution to the issues...
Now we have two isolated parts of the program
People can work on that individually
There are no more tangled names and states
To those things like "singletons", closure looks a good
solution to disallow others create the instance twice

It seems a successful solution to the issues...
Now we have two isolated parts of the program
People can work on that individually
There are no more tangled names and states
To those things like "singletons", closure looks a good
solution to disallow others create the instance twice
Really?

Always think about these when you are coding
How do I test the function or instance?
Are these duplicated code necessary?
Could people reuse my code later on?
Is my solution the most expressive one?
Did I express the intention clearly?

Quiz#2
(5 minutes)http://bit.ly/1FCwhF9

Don't be intrigued by closure in such cases
You don't have any references for unit tests
If you really have some re-usable data or functions across
different parts, closures can't use them
That means, it's uneviable to write lots of duplicated
functions do the almost same thing

Don't be intrigued by closure in such cases
You don't have any references for unit tests
If you really have some re-usable data or functions across
different parts, closures can't use them
That means, it's uneviable to write lots of duplicated
functions do the almost same thing
However...

Using closure carefully can still benefit you

"Managers"
"You put all states in an object, and listen to all events and
hope everything will be fine"
TodoListManager
drawList fetchData saveData ...onItemAdded
user event
used by
The only "public" interface

Public interfaces of JavaScript program components
JavaScript components usually use handlers as public
interfaces among the user, server and other components
like to draw or to fire
new events
private helper
methods...
event handlers
server gateway
local data keeper
component
usernative events
server
sync
componentscustom events

So now we have
Managers that can keep queried elements and local data as
their states
TodoListManager
user event
used by

And we're finally able to test our code
Unlike closure, we now have some references the test can
manipulate

About unit test
Right now, you don't need to figure out how it works
Just follow the structure and copy it

However, to test them
could be painful soon

The globally "singleton" instance mess up tests
Consider what will happen when we load the files to test
such global object pattern
unit test for 'saveData'
save some dummy data
TodoListManager
do some tests
_listTodoItem (null)
_listTodoItem' (dummy)
unit test for 'drawList'
do some tests
(wrong)

Quiz #3
(5 minutes)http://bit.ly/1FM4KkF

With such patterns, we need to clear the dummy data and
other states made by previous tests
TodoListManager
do some tests
do some tests
(correct)
_listTodoItem' (null)
*reset it*

With such patterns, we need to clear the dummy data and
other states made by previous tests
And you barely know how to reset them properly if the
component is 1K+ lines long, and written by lots of people
In Gaia, this has happened countless times
And worse, people sometimes wrote tests rely on that: they
do some changes at the first test, and then use the
contaminated data to perform the next test

"What's the
matter?
I don't write
any test"

Test is good to you because...
There are lots of commits may change your code

People will always screw you and your
code if you don't test them

People will always screw you and your
code if you don't test them
We call these issues regressions

To test your code and kill possible regressions
Instantiable Objects

Instantiable objects
TodoListManager
(constructor) start saveData ...onItemAddednew

Test with instantiable objects
Create new instance when new test is performing
TodoListManager
do some tests
do some tests
TodoListManager

The pattern
Design a constructor + prototype and then create instances
(constructor)
function
TodoListManager
TodoListManager.prototype
function start()
{...}
function drawList()
{...}
...

Constructor
(constructor)
function
TodoListManager
Is just a function

Constructor
(constructor)
function
TodoListManager
It should set up all data members and do nothing
(constructor)
function TodoListManager() {
this._wrapper = document.query...
this._listTodoItems = []
...
}

Don't do anything with effects
inside the constructor

Constructor with effects
(constructor)
function
TodoListManager
We prefer to hold the instance before use it
(constructor)
function TodoListManager() {
this._wrapper = document.query...
this._listTodoItems = []
this.fetchData(...);
}
(create the instance & test it)
var instance = new TodoListManager();
// It will do fetching immediately,
// which is BAD
instance.drawList(...)
// Although we only want to test this

Constructor
(constructor)
function
TodoListManager
Put all initial effects in another function
function start()
{
this.fetchData(...)
}
...
(create the instance & test it)
var instance = new TodoListManager();
// It will do noththing now
instance.drawList(...)
// We can test this without deal with
// the effect we don't need

It means the shareable methods and data among instances
Prototype
(constructor)
function
TodoListManager(){
this.foo = [];
}
function start()
{
this.fetchData(...)
}
foo: [ ]
start() {}
foo: [ ]
start() {}
foo: [ ]
start() {}
foo: [ ]
start() {}
foo: [ ]
start() {}
(new instances)

So it's better to put data ONLY IN SETUP FUNCTIONS
Prototype
(constructor)
function
TodoListManager(){
}
function start()
{
this.fetchData(...)
}
start() {} start() {} start() {} start() {}
(wrong!)
foo: [ ]
foo: [ ]
start() {}

Every test can now modify the instance by their own needs
For tests
(constructor)
function
TodoListManager(){
this.foo = [];
}
function start()
{
this.fetchData(...)
}
foo: [1]
start() {}
foo: [ ]
start() {}
foo: [3,2]
start() {}
foo: null
start() {}
foo: {}
start() {}
(new instances)test #1 test #2 test #3 test #4

"this"
the necessary evil to be instantiable

Trolls you when you are not
familiar with it

Quiz#4
5 mins
discuss & search it freely
comment it on the page please
http://bit.ly/1iQdb3V

this
the owner
the window
the bound one

this
the owner
var SomeObject = {
foo: 3,
bar() {
console.log('Foo: ', this.foo);
}
}
SomeObject.bar();
// Will print "Foo:3"

this
the window
var SomeObject = {
foo: 3,
bar() {
}
}
var bar = SomeObject.bar;
// Will get reference to bar
bar();
// Will print "Foo:undefined"
// without binding and owner,
// the 'this' will become 'window'

this
the bound one
var SomeObject = {
foo: 3,
bar: (function() {
}).bind(SomeObject)
}
var bar = SomeObject.bar;
// Will get reference to bar
bar();
//
// since it's bound, the this will
// always be 'SomeObject'

You need to bind the 'this' if you use that
It is important since every time you put a callback...

Event handlers should follow the EventListener interface to
make a centralized dispatcher with the bound 'this'
It is important since every time you put a callback...

Example #4
That's why there are lots of
binding code in the

or face the Callback Hell
I know, I know, it's actually from the Abyss, but the chaos fits the case better better than the order

If you think it's tolerable,
think about this

That's why Promise is so
important

new Promise(function(resolve, reject) {
// Do some async things.
// Then resolve it after done.
resolve();
// Or reject it due to the error.
reject();
}).then(function() {
// This function only executes after the
// previous step.
return [1,2,3];
// If return another Promise,
// the flow will stop here and only
// continue when the promise is resolved
return new Promise(function(r, c) {
setTimeout(r, 1000);
});
// This will not execute within the second
}).catch(function(error) {
// !IMPORTANT!
// Remember to catch Promise error in a
// function like this.
});
The Flow

|dispatchEvent| as the public interface to send message
|handleEvent| as the public interface to receive message
|Promise| to concat all steps of event handling
handleEvent(event) {
switch(event.type) {
case 'todo-item-created':
var data = event.detail;
this.saveData(data)
.then(this.tidyUp.bind(this))
.then(...)
.then(...)
...
.catch(this.onError.bind(this))
}
}

Every asynchronous method should return a Promise to
allow we concating it, so we can eliminate all the callbacks
saveData() {
var promise = new Promise(function() {
...
};
return promise;
}
fetchData() {
var promise = new Promise(function() {
...
};
return promise;
}

Solve things in advance but native JavaScript ways
"class" (not ES6 class)
Don't try to copy them
Use "instantiable objects"
prototype + constructor
private
encapsulate
closure Use them very carefully and
wiselyanonymous functions
callback ** PROMISE ** (or beyond)
local variables As immutable as possible
global object Usually it means that you're
doing something really bad
Stop it and re-think the
whole case again
flags, temporary variables
singleton

Homework
1. Fork the Homework repo here
2. Fix all the issues in the code to refactor it
3. After that, make a git branch, commit, and then push it
4. Fire a Pull Request and set review to TAs and me
Remember: you can always ask any question at anytime
Don't struggle with any part you don't really understand

Homework: How to get points
You need at least address all the Tier 1 issues to get 4 points
You will get the extra 3 points from addressing Tier 2 issues
However, you will NOT get any points from the Tier 2,
if you fail to address ANY single issue of Tier 1
You need at least the 4 points from Tier 1 to pass this class
The extra points will benefit you in the following lessons
You can add or remove some code from the example.
However, make sure every change you make is with clear
intentions

Homework: Requirements (Tier 1)
Bind this in Event handlers
Deal with object properties w/ constructor & prototype
Deal with asynchronous flows with Promise
Write your first test case for a pure function (as the last test
of |test-list.js| file shows; for example, to test if |Math.sin|
works well as your assumption)

Homework: Requirements (Tier 2)
Implement functions with closure when it's necessary
Avoid using closure when it shouldn't appear (follow what
we taught in the class)
Write meaningful comments in JSDoc format
(reference: https://en.wikipedia.org/wiki/JSDoc#Example)

Update: people who already commented on PRs
Quiz#1
rockwyc992 : 吳易璋
CaeserNieh : 聶順成
rockwyc992 : 吳易璋
Quiz#2
jason1122g : 邱義傑
bdsword : 蔣彥亭
c910335 : 陳達仁
Davisanity : 林唐正
amm040341 : 蘇聖雅
rueian : 黃瑞安
w181496 : 黃詩凱
Peter654q :莊侑穎
bdsword : 蔣彥亭
Quiz#3
rueian : 黃瑞安
c910335 : 陳達仁
CrashedBboy : 吳承霖
Sharknevercries : 李政遠
jason1122g : 邱義傑
Good work, buddies.

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