const {toString} = Object.prototype

export function isFunction(fn: any): fn is (...args: any[]) => any {
  const 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))
}