CoffeeScriptワンポイント - 値を返さない関数にはreturnを付ける ref: http://qiita.com/higuma/items/ff329775f3a91a6c6fb3

file0.coffee

a = for x in [0..5]
  x * x
console.log a   # => [ 0, 1, 4, 9, 16, 25 ]

b = while a.length > 0
  a.pop()
console.log b   # => [ 25, 16, 9, 4, 1, 0 ]

file1.coffee

a = (x * x for x in [0..5])         # (...)が必要
console.log a   # => [ 0, 1, 4, 9, 16, 25 ]

b = (a.pop() while a.length > 0)    # これも(...)が必要
console.log b   # => [ 25, 16, 9, 4, 1, 0 ]

file2.coffee

# sprintf("%#{w}.#{f}f", n) のCoffeeScript版
sprintf = (n, w, f) ->
  buf = n.toFixed f
  buf = ' ' + buf while w > buf.length
  buf

printNumbers = (numbers, w, f) ->   # CoffeeScriptならたった一行(なのに...)
  console.log sprintf n, w, f for n in numbers

printNumbers [1.5, 7.3, -4.1], 5, 1
### 出力は次の通り
  1.5
  7.3
 -4.1
###

file3.txt

// Generated by CoffeeScript 1.7.1
(function() {
  var printNumbers, sprintf;

  // 中略

  // 何でこんなに長くなるのか? 実は理由がある
  printNumbers = function(numbers, w, f) {
    var n, _i, _len, _results;
    _results = [];      // 値を返すための(無駄な)配列
    for (_i = 0, _len = numbers.length; _i < _len; _i++) {
      n = numbers[_i];
      _results.push(console.log(sprintf(n, w, f)));
      // console.log(...)の戻り値(undefined)を追加
    }
    return _results;    // => [ undefined, undefined, ... ]
  };

  // 以下略

}).call(this);

file4.coffee

printNumbers = (numbers, w, f) ->
  console.log sprintf n, w, f for n in numbers
  return        # <= !!! 追加 !!!

file5.txt

// Generated by CoffeeScript 1.7.1
(function() {

  // 中略

  // この出力ならリーズナブル(修正不要)
  printNumbers = function(numbers, w, f) {
    var n, _i, _len;
    for (_i = 0, _len = numbers.length; _i < _len; _i++) {
      n = numbers[_i];
      console.log(sprintf(n, w, f));
    }
  };

  // 以下略

}).call(this);

file6.coffee

log = (x) ->    # console.logの短縮形
  console.log x

file7.txt

// Generated by CoffeeScript 1.7.1
(function() {
  var log;

  log = function(x) {
    return console.log(x);      // このreturnは不要
  };

}).call(this);

file8.coffee

log = (x) ->
  console.log x
  return        # <= 追加

file9.txt

// Generated by CoffeeScript 1.7.1
(function() {
  var log;

  log = function(x) {
    console.log(x);     // returnは付かない
  };

}).call(this);

printNumbers.coffee

# sprintf("%#{w}.#{f}f", n) のCoffeeScript版
sprintf = (n, w, f) ->
  buf = n.toFixed f
  buf = ' ' + buf while w > buf.length
  buf

printNumbers = (numbers, w, f) ->   # CoffeeScriptならたった一行(なのに...)
  console.log sprintf n, w, f for n in numbers

printNumbers [1.5, 7.3, -4.1], 5, 1
### 出力は次の通り
  1.5
  7.3
 -4.1
###

printNumbers.js

// Generated by CoffeeScript 1.7.1
(function() {
  var printNumbers, sprintf;

  // 中略

  // 何でこんなに長くなるのか? 実は理由がある
  printNumbers = function(numbers, w, f) {
    var n, _i, _len, _results;
    _results = [];      // 値を返すための(無駄な)配列
    for (_i = 0, _len = numbers.length; _i < _len; _i++) {
      n = numbers[_i];
      _results.push(console.log(sprintf(n, w, f)));
      // console.log(...)の戻り値(undefined)を追加
    }
    return _results;    // => [ undefined, undefined, ... ]
  };

  // 以下略

}).call(this);