Variable shaddowing in coffee is to a problem!

variable-shaddowing-compiled.js

(function() {
  foo(function() {
    var a1, a2;
    a1 = void 0;
    a2 = null;
    return bar(function(value) {
      a1 = value;
      a2 = value;
      return value;
    });
  });
}).call(this);

variable-shaddowing.coffee

# variable shadowing is not really a problem in coffee,
# you just need to be explicit, that you don't want to shadow it! 
foo ->
    a1 = undefined   # long version
    a2 = null        # shorter (that I use personally) but not exactly same
    bar (value)->
        a1 = value
        a2 = value
        value