So I watched Eric Leads talk at Fluent 2013...I really liked the composition he implemented...I'm going to blatantly copy it.

gistfile1.coffee

#
# Base abilities mixin
abilitiesPrototype = (() ->
  str = 9
  strAdj = 0
  dex = 9
  dexAdj = 0
  int = 9
  intAdj = 0
  wis = 9
  wisAdj = 0
  con = 9
  conAdj = 0
  luk = 9

  return {
    constitution: -> con + conAdj
    strength: -> str + strAdj
  }
)()

module.exports = abilitiesPrototype
# 
# Health mixin
_ = require 'lodash'

abilities = require './abilities'

healthPrototype = (() ->
  wounds = 0

  return {
    isDead: -> wounds >= @totalHitPoints()
    hitPoints: -> @totalHitPoints() - wounds
    totalHitPoints: -> @constitution() + @strength()
  }
)()

module.exports = _.extend abilities, healthPrototype
#
# Test
_ = require 'lodash'
util = require 'util'
require('chai').should()

health = require '../src/health'

describe 'Health', ->
  describe 'mixin', ->
    before ->
      @character = _.extend {}, health

    it 'provides hitpoints', ->
      @character.should.respondTo 'hitPoints'

    it 'provides total hitpoints', ->
      @character.should.respondTo 'totalHitPoints'

    it 'indicates if the character is dead', ->
      @character.should.respondTo('isDead')

    it 'is based on basic constitution', ->
      @character.hitPoints().should.equal(18)