That's a really smart hack, I didn't realize comments were included in the output of Function#toString. You could do other cool things, e.g. an annotation for type checking:

  function checkType(f) {
    return function(a) {
      var type = f.toString().match(/\/\/(.*)\n/)[1].trim();
      if(type !== typeof(a)) throw new Error('Invalid type');
      return f(a);
    }
  }
  
  var halve = checkType(function(n) {
    // number
    return n / 2;
  });
  
  halve(4); // returns 2
  halve('string'); // emits error