javascript - setTimeout 后事件 currentTarget 发生变化

Question

考虑:

let sel=document.getElementById('mys');

sel.onchange=function(e) {
  console.log(e.currentTarget===null); // false
  setTimeout(e => {
     console.log(e.currentTarget===null); // true
  }, 0, e);
 }

<select id="mys">
  <option value="volvo">Volvo</option>
  <option value="saab">Saab</option>
  <option value="mercedes">Mercedes</option>
  <option value="audi">Audi</option>
</select>

• 为什么 e.currentTarget 在超时后会发生变化？这是浏览器 (chrome) 错误吗？

• 如何将事件的精确克隆传输到超时函数？我尝试了简单的克隆，但 currentTarget 不可写且不能被覆盖..

Answer 1

灵感来自 Joseph Marikle's comment ，我找到了真正的原因:

DOM Standard § 2.9. Dispatching events描述了我在此处部分复制的算法:

To **dispatch** an event to a target […] run these steps:
[…]
5. If target is not relatedTarget or target is event’s relatedTarget, then:
    […]
    9. While parent is non-null:
        […]
        8. Otherwise, set target to parent and then:
            […]
        9. If parent is non-null, then set parent to the result of invoking parent’s get the parent with event.
        […]
    […]
    12. Set event’s eventPhase attribute to CAPTURING_PHASE.
    […]
    14. For each tuple in event’s path, in reverse order:
        1. […] invoke […]
    15. For each tuple in event’s path, in order:
        1. If tuple’s target is non-null, then set event’s eventPhase attribute to AT_TARGET.
        2. Otherwise, set event’s eventPhase attribute to BUBBLING_PHASE.
        3. […] invoke […]
[…]
6. Set event’s eventPhase attribute to NONE.
7. Set event’s currentTarget attribute to null.
[…]

To **invoke** […] run these steps:
[…]
5. Initialize event’s currentTarget attribute to struct’s invocation target.
[…]

因此，在事件处理完所有阶段(捕获、at_target、冒泡)后，currentTarget 将被分配为 null。