class vs instance vars in CoffeeScript

gistfile1.coffee

class Blah

  @counter = 0

  @incr: ->
    return @counter += 1

  constructor:
    @counter = 0

  incr: ->
    @counter += 1

b1 = new Blah
b2 = new Blah

b1.incr()
console.log(Blah.counter)
console.log(b1.counter)
console.log(b2.counter)


b2.incr()
console.log(Blah.counter)
console.log(b1.counter)
console.log(b2.counter)

Blah.incr()
console.log(Blah.counter)
console.log(b1.counter)
console.log(b2.counter)