JS event bubbling and event capturing (graphic tutorial, simple violence)

This article mainly introduces the event bubbling and event capturing of JS. It describes the difference between the two in detail through code examples. Friends in need can refer to it

At school, listen to the teacher’s explanation The event bubbling and event capturing mechanisms are the same as Tingtianshu. I only vaguely remember that IE uses event bubbling, and other browsers use event capturing. At that time, I regarded it as an IE browser compatibility issue, so I did not delve into it (browsers below IE8 have basically been withdrawn from the market). Since my work, although I have encountered this type of problem many times, I have never investigated it in depth. I have always had a partial understanding of it and relied on guesswork (if A doesn’t work, just choose B). I made a demo today when I had nothing to do, and I finally understood this problem completely.

Let’s start with the conclusion: they are terms used to describe event triggering timing issues. Event capture refers to the process from the document to the node that triggers the event, that is, from top to bottom to trigger the event. In contrast, event bubbling triggers events from the bottom up. The third parameter of the bound event method is to control whether the event triggering sequence is event capture. true, event capture; false, event bubbling. The default is false, which means the event bubbles up. Jquery's e.stopPropagation will prevent bubbling, which means that as far as I am concerned, the events of my father and ancestors will not be triggered.

This is the HTML structure

  <p id="parent">
　　  <p id="child" class="child"></p>
  </p>

Now we bind events to them

  document.getElementById("parent").addEventListener("click",function(e){
  alert("parent事件被触发，"+this.id);
  })
  document.getElementById("child").addEventListener("click",function(e){
  alert("child事件被触发，"+this.id)
  })

Result:

child event is triggered, child
parent The event is triggered, parent

Conclusion: child first, then parent. Events are triggered in order from the inside out, which is called event bubbling.

Now change the value of the third parameter to true

　　 document.getElementById("parent").addEventListener("click",function(e){
  alert("parent事件被触发，"+e.target.id);
  },true)
  document.getElementById("child").addEventListener("click",function(e){
  alert("child事件被触发，"+e.target.id)
  },true)

Result:

parent event is triggered, parent
child event is triggered, child

Conclusion: First parent, then child. The order of event triggering is changed from outside to inside, which is event capture.

It seems to be of no use. The last case of using event bubbling is something I often use anyway.

　　<ul>
  <li>item1</li>
  <li>item2</li>
  <li>item3</li>
  <li>item4</li>
  <li>item5</li>
  <li>item6</li>
 </ul>

The requirement is as follows: the background of the corresponding li turns gray when the mouse is placed on it.

Use event bubbling to achieve:

　　$("ul").on("mouseover",function(e){
   $(e.target).css("background-color","#ddd").siblings().css("background-color","white");
  })

Some people may say that we can directly tie events to all li. It is not troublesome at all, as long as...

　　　$("li").on("mouseover",function(){
   $(this).css("background-color","#ddd").siblings().css("background-color","white");
  })

Yes, this is okay. And in terms of code simplicity, the two are similar. However, the former requires one less operation to traverse all li nodes, so it is definitely better in terms of performance.
Also, if we dynamically load some elements after the binding event is completed...

$("<li>item7</li>"). appendTo("ul");
At this time, in the second option, since item7 does not exist when the event is bound, so for the sake of effect, we have to bind an event to it again. The use of the bubbling plan is a matter tied to UL...
Let's judge!

The above is what I compiled for everyone. I hope it will be helpful to everyone in the future.

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