Sexy method argument overloading in CoffeeScript.

gistfile1.coffee

#
# Overloads:
# - fetch(url, callback)
# - fetch(url, opts, callback)
#
fetch = (args...) ->
    [arg0, arg1, arg2] = args
    [url, opts, callback] = switch args.length
        when 0, 1 then throw new Error "Usage: fetch(url[, opts], callback)"
        when 2 then [arg0, null, arg1]
        else [arg0, arg1, arg2]

    # ...