the do keyword in coffeescript

generated_by_coffee.js

var filename, _fn, _i, _len;

_fn = function(filename) {
  return fs.readFile(filename, function(err, contents) {
    return compile(filename, contents.toString());
  });
};
for (_i = 0, _len = list.length; _i < _len; _i++) {
  filename = list[_i];
  _fn(filename);
}

sample.coffee

## coffeescript function in loop

# this not your expect

for filename in list
  fs.readFile filename, (err, contents) ->
    compile filename, contents.toString()

# bingo !!!
for filename in list
  do (filename) ->
    fs.readFile filename, (err, contents) ->
      compile filename, contents.toString()

sample.js

// javascript function in loop

for (var i = 0, n = elements.length; i<n; i++) {
  var el = elements[i];
  el.addEventListener('click', function() {
    doSomethingWith(i, el); // i, el are not what you expect!
  }, false);
}

for (var i = 0, n = elements.length; i<n; i++) {
  var el = elements[i];
  el.addEventListener('click', (function(i, el) { 
    return function() {
      doSomethingWith(i, el);
    }
  })(i, el), false);
}