expected.js

(function(){
  var _a, _b, _c, _d, evens, num;
  evens = (function() {
    _a = []; _c = [1, 2, 3, 4, 5, 6];
    for (_b = 0, _d = _c.length; _b < _d; _b++) {
      num = _c[_b];
      num % 2 === 0 ? _a.push((function() {
        num *= -1;
        num -= 2;
        return num * -1;
      })()) : null;
    }
    return _a;
  })();
})();

mine.js

(function(){
  var _a, _b, _c, _d, evens, num;
  evens = (function() {
    _a = []; _c = [1, 2, 3, 4, 5, 6];
    for (_b = 0, _d = _c.length; _b < _d; _b++) {
      num = _c[_b];
      num % 2 === 0 ? _a.push(      num *= -1;
      num -= 2;
      return num * -1;) : null;
    }
    return _a;
  })();
})();

// note the lack of enclosing function

source.coffee

evens: for num in [1, 2, 3, 4, 5, 6] when num % 2 is 0
	num *= -1
	num -= 2
	num * -1

(note the missing function(){} wrapper around the expression being pushed into the accumulator array _a)