Memoization fibonacci in coffeescript

Memoization_fibonacci.coffee

fib = (n)->
  arr = {}
  $ = (_) ->
    return _ if _ < 2
    return arr[_] if arr[_]    
    arr[_] = $(_-1) + $(_-2)
    arr[_]
  $(n)

console.time('a')
console.log alert fib(500)
console.timeEnd('a')