Mixins in CoffeeScript

Posted on May 17, 2012 by Tomasz Nazar

So, we’ve finally achieved a small utility function that implements mixins in Coffee. In Javascript too :-)

Implementation might not yet be super clean, but it allows us to:

mix all methods of mixin class into a base class
mix many classes into a base class
a mixin has a place for constructing its data if necessary
a mixin can read&modify “base” class’ data
a base class can read&modify mixin data (awkward, but works)

In short:

<pre>
class Nothing

constructor: (@name=”nothing”) ->
class Presenter extends Mixin

presentYourself: =>

“My name is #{@name}”
something = new Nothing()

ObjectHelper.addRole(something, Presenter)
something.presentYourself() == “My name is nothing”
</pre>

and implementation is bit hacky, but works. Uses underscore.js and modified hashmal’s Mixin.
One small requirement of this solution is all roles must extend Mixin class.

<pre>
class ObjectHelper

@addRole: (base, role) =>

if not role::augment?

throw “RoleMustHave.augment()”

role::augment(base)

_.bindAll(base)
</pre>

A longer example of what’s possible along with part of our Jasmine test:

<pre>
class Nothing

constructor: (@name=”nothing”) ->
doSomething: =>

“I cannot do anything”
class Presenter extends Mixin

presentYourself: =>

“My name is #{@name}”
doIPlay: =>

“I play with many #{@players}”
class Game extends Mixin

setup: =>

@name = ‘Monopoly’

@players = []

@players.push “David”
sayHello: =>

@presentYourself()
something = new Nothing()

expect(something.name).toBe(‘nothing’)
ObjectHelper.addRole(something, Presenter)

expect(something.presentYourself()).toBe(‘My name is nothing’)

ObjectHelper.addRole(something, Game)

expect(something.sayHello()).toBe(‘My name is Monopoly’)

expect(something.doIPlay()).toBe(‘I play with many David’)
something.name = ‘something’

expect(something.presentYourself()).toBe(‘My name is something’)
something.players.push “John”

expect(something.doIPlay()).toBe(‘I play with many David,John’)
</pre>

And stripped down whole Mixin.coffee

<pre>
Swappable Mixins in CoffeeScript

# ================================

# Many thanks to Hashmal, who wrote this to start.

# https://gist.github.com/803816/aceed8fc57188c3a19ce2eccdb25acb64f2be94e

class Mixin

augment: (t) ->

(t[n] = m unless n == ‘augment’ or !this[n].prototype?) for n, m of this

t.setup()

eject: (mixin) ->

(delete this[n] if m in (p for o, p of mixin::)) for n, m of this

setup: ->
</pre>

So, happy mixxxxing to you :-)

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Mixins in CoffeeScript