Writing modern web applications requires a ton of JS, and somewhere in that JS lies some application logic (we're not just talking DOM manipulations here). If you require that same logic on the server-side for say, generating reports, what do you do? I'll show you how ValuationUP.com pushes the single responsibility principle to the max by "embedding" V8 into our report generation code so the same JS that powers our Backbone.js frontend powers our PDF's generated by Prawn. Thin wrappers, no duplication, practical IoC, ultimate SRP.

1. Reusing your frontend JS
on the server with V8/Rhino
Pushing SRP & DI/IoC to whole new level

2. Kenneth Kalmer
Chief Rocket Scientist @ ValuationUP.com
@kennethkalmer
github/kennethkalmer
www.opensourcery.co.za
kennethkalmer

3. ValuationUP.com
Help entrepreneurs make REAL sense of their
management accounts
Comparative analysis within industry
Refactoring financials by getting from red to green
Take actions to increase their valuations
Valuation is the ultimate metric

4. The challenge
We need a very responsive UI
Realtime updates as the user inputs information
The RTT to the server is not worth the wait for such a
simple formula

6. Done!
Repeat that for 50+ financial models
Build fully buzzword compliant frontend

7. Panic!
Let’s offer a PDF with all the information
A PDF...
...WITH ALL THE INFORMATION

9. We did it!
But how?

10. Dilemma
Duplicate?
Reuse?
wkhtmltopdf? (a.k.a. wtftopdf)

11. Reuse!
Single Responsibility Principle
Dependency Injection/Inversion on Control
Use as much JS on the server as possible, where
possible!

12. By using
therubyracer (github.com/cowboyd/rubyracer)
therubyrhino (github.com/cowboyd/rubyrhino)
coffee-script
1 gem 'therubyracer', platforms: :ruby
2 gem 'therubyrhino', platforms: :jruby
3
4 gem 'coffee-script'

13. That’s WACC!

14. With coffee
1 # Determine the WACC of the business (weighted average cost of capital)
2 class Demo.WACC
3
4 constructor: (statement) ->
5 @statement = statement
6
7 @cost_of_equity = statement.cost_of_equity
8 unless _.isFinite( @cost_of_equity )
9 @cost_of_equity = 0.30
10
11 calculate: ->
12
13 e = ( @statement.equity / ( @statement.debt + @statement.equity ) ) * @cost_of_equity
14 d = ( @statement.debt / ( @statement.debt + @statement.equity ) ) * @statement.costOfDebt
15 t = ( 1 - @statement.taxRate )
16
17 e + d * t
18
19 # Return the calculated WACC
20 value: ->
21 @wacc or= @calculate()

15. Present it
1 class Demo.WaccLayout
2
3 # The view that will render our information
4 view: null
5
6 initialize: (data, view = null) ->
7 @data = data
8 @view = view
9
10 render: ->
11 wacc = new Demo.WACC( @data.statement )
12
13 @view.render( wacc: wacc.value() )

16. Client-side view & usage
1 # Simple backbone view
2 class Demo.WaccView extends Backbone.View
3
4 render: (options) ->
5 wacc = options.wacc
6
7 @$el.empty().append( "<h1>#{wacc}</h1>" )
8
9
10
11
12
13 # Initialize a view
14 view = new Demo.WaccView( el: '#wacc' )
15
16 # Load and render our layout
17 layout = new Demo.WaccLayout( view: view, data: data )
18 layout.render()

17. Reuse on the server...

18. JavaScript Context
1 require 'v8'
2 # require 'rhino'
3
4 # This service wraps a JS context and allows us to interact with our Javascript
5 # directly in Ruby.
6 class Javascript
7
8 # Our V8 context object
9 attr_reader :context
10
11 # Some delegations
12 delegate :load, :eval, :[], :[]=, to: :context
13
14 def initialize()
15 @context = V8::Context.new
16 #@context = Rhino::Context.new
17
18 # Setup a fake 'window' object
19 @context['window'] = FakeWindow.new( @context )
20 @context['console'] = FakeConsole.new
21
22 # Load our global setup
23 load_coffee Rails.root.join( 'app/assets/javascripts/setup.js.coffee' )
24 end
25
26 # truncated, lots more detracting stuff down below...
27
28 end

19. CoffeeScript sprinkles
1 # Compile and load a CoffeeScript file into the current context
2 def load_coffee( path )
3 compiled_coffeescript = CoffeeScript.compile( File.read( path ) )
4 context.eval compiled_coffeescript
5 end

20. Pause
We have a JavaScript context (V8) at our finger tips
Have some basic CoffeeScript loading abilities
And some additional plumbing
Far from a DOM

21. Server-side wrapper
1 class WaccLayout
2
3 def initialize( view, context = Javascript.new )
4 @context = context
5
6 load_dependencies
7
8 # Pass our Ruby view into the context
9 @context["view"] = view
10
11 # Setup
12 @context.eval <<-EOJS
13 var layout = new Demo.WaccLayout({
14 view: view, data: data
15 });
16 EOJS
17 end
18
19 def render
20 context['layout']['render'].methodcall( context['layout'] )
21 end
22
23 def load_dependencies
24 @context.load_coffee Rails.root.join("app/assets/javascripts/wacc.js.coffee")
25 @context.load_coffee Rails.root.join("app/assets/javascripts/wacc_layout.js.coffee")
26 end
27 end

22. Server-side view & usage
1 class WaccPdf
2
3 def initialize
4 @pdf = Prawn::Document.new
5 end
6
7 def render( options )
8 wacc = options.wacc
9
10 @pdf.text "WACC: #{wacc}"
11 end
12
13 end
14
15
16 # Setup a view
17 view = WaccPdf.new
18
19 # Setup a layout and render
20 layout = WaccLayout.new( view )
21 layout.render

23. 1,000 ft view

24. Reminder why we did it?

26. Yes please!
https://github.com/kennethkalmer/rubyfuza-2013
https://rubyfuza-2013-wacc.herokuapp.com
Later this weekend, you’re all getting drunk tonight
anyway!!

27. Thank you!
It’s been wild!