_isFunction.js

// known issues:
// 1) not cross frame (_fn.toString === Function.prototype.toString)  
var _isFunction = function(_fn)
{
  return typeof(_fn) == 'function' || 
    (
      typeof(_fn) != 'string' && 
      !(_fn instanceof String) &&
      String(_fn).replace(/^\s+/, '').slice(0, 8) == 'function' &&
      (
        !('toString' in _fn) ||
        _fn.toString === Function.prototype.toString
      )
    );
};