module.exports = isFunction

var toString = Object.prototype.toString

function isFunction (fn) {
  if (!fn) {
    return false
  }
  var string = toString.call(fn)
  return string === '[object Function]' ||
    (typeof fn === 'function' && string !== '[object RegExp]') ||
    (typeof window !== 'undefined' &&
     // IE8 and below
     (fn === window.setTimeout ||
      fn === window.alert ||
      fn === window.confirm ||
      fn === window.prompt))
};