js 事件处理函数和事件默认行为的执行先后问题

[lyhper]

请问哪个先执行呢？如果是事件默认行为先执行，那为什么可以使用preventDefault（），returnValue=false等方式阻止默认行为呢？

[Septembers]

事件冒泡模型
see http://www.quirksmode.org/js/events_order.html

[lyhper]

@Septembers 谢谢你，事件冒泡我知道，但是和这个好像没关系呀？比如点击一个checkbox，是先勾选它还是先执行事件处理程序？

[FuryBean]

先上规范：
https://w3c.github.io/uievents/#event-flow-default-cancel

大部分情况下EventListener先执行，规范上有写：
Default actions should be performed after the event dispatch has been completed, but in exceptional cases may also be performed immediately before the event is dispatched.

接着举了一个例子，就是checkbox的例子：
The default action associated with the click event on <input type="checkbox"> elements toggles the checked IDL attribute value of that element. If the click event's default action is cancelled, then the value is restored to its former state.

[lyhper]

@FuryBean 十分感谢！我试了一下，同样存在这个问题的还有单选按钮