variable_shadowing.coffee

foo = 10

bar = ->
  ((foo)-> 
    foo = 20
    console.log foo # => 20
  )()
  console.log foo # => 10

  foo = 30

bar()
console.log foo # => 30

You could use the do keyword rather than wrapping your function in parentheses:

foo = 10

bar = ->
  do (foo) -> 
    foo = 20
    console.log foo # => 20
  console.log foo # => 10

  foo = 30

bar()
console.log foo # => 30

But generally speaking, the use of shadowing is discouraged in CoffeeScript. There's been talk of adding a compiler warning in cases like this where a function argument shadows a variable declared in a surrounding scope. The exception is that shadowing is highly useful in a loop, which is why the do keyword was added to the language:

for i in [1..3]
  do (i) ->
    setTimeout (-> console.log i), 1 # 1, 2, 3

(Without the do, you'd get 4, 4, 4, since the function passed to setTimeout is only invoked after the loop has completed.)