setTimeout中第二个参数是选填还是必填？

这种东西还是以规范为准吧 https://www.w3.org/TR/2011/WD-html5-20110525/timers.html

规范里的接口定义是这样的

[Supplemental, NoInterfaceObject]
interface WindowTimers {
  long setTimeout(in any handler, in optional any timeout, in any... args);
  void clearTimeout(in long handle);
  long setInterval(in any handler, in optional any timeout, in any... args);
  void clearInterval(in long handle);
};
Window implements WindowTimers;

看第一行，timeout是可选的