原文:https://medium.com/@florenceliang/javascript-event-delegation-and-event-target-vs-event-currenttarget-c9680c3a46d1

In this case, at the time you call console.log(e), there's a DOM element in the currentTarget property. But sometime later, that property is reset to null for some reason. When you expand the event object, that's what you see.
你的情况是，当调用console.log(e)时，currentTarget属性是有值的，但是过后这个值就被重置为null了。所以当你展开事件对象，看到的就是null。

<!DOCTYPE html>

<html lang="en">

<head>

    <meta charset="UTF-8">

    <title>Document</title>

    <style type="text/css">

        #wrapDiv,

        #innerP,

        #textSpan {

            margin: 5px;

            padding: 5px;

            box-sizing: border-box;

            cursor: default;

        }

        #wrapDiv {

            width: 300px;

            height: 300px;

            border: indianred 3px solid;

        }

        #innerP {

            width: 200px;

            height: 200px;

            border: hotpink 3px solid;

        }

        #textSpan {

            display: block;

            width: 100px;

            height: 100px;

            border: orange 3px solid;

        }

    </style>

</head>

<body>

    <div id="wrapDiv">wrapDiv

        <p id="innerP">innerP

            <span id="textSpan">textSpan</span>

        </p>

    </div>

    <script>

        var div = document.getElementById('wrapDiv');

        var p = document.getElementById('innerP');

        var span = document.getElementById('textSpan');

        div.onclick = function(ev){

            console.log(ev); //

            console.log("target:", ev.target);

            console.log("currentTarget:", ev.currentTarget);

        }

    </script>

</body>

</html>

　　

-----------------------------------------------

Event delegation is a popular methodology in JavaScript. It allows us to add an event listener to one parent, and avoid to add many event listeners to specific nodes. We can demonstrate this technique with simple example.

Let’s say we have a list with thousands of items:

<body>
  <div id="container">
   <ul id="list">
     <li><a href="#">Item 1</a></li>
     <li><a href="#">Item 2</a></li>
     <li><a href="#">Item 3</a></li> 
     <li><a href="#">Item 4</a></li>
     ..................................
     <li><a href="#">Item 1000</a></li>
   </ul>
  </div>
</body>

With such number of items, it would be a nightmare to loop through every <a> element on the page, adding an event listener one after one. Moreover, it may “freeze” the page when JavaScript is trying to create them all.

So here comes the event delegation: When the event bubbles up to the body element, we can check the element that triggered the event, using the event object’s target property.

document.addEventListener("click", function(e) {
  if(e.target && e.target.nodeName == "A") {
  console.log("List item ", e.target.textContent, " was clicked!");
  }
});
// When we click the 2nd item, the page prints out:
"List item Item 2 was clicked!"

target vs. currentTarget

Since we already talked about the event.target property, there is another property called event.currentTarget in JavaScript event. It can be very confused by just reading about them on JavaScript documentation.

As we’ve seen from the last example, when we clicked the a element, clickevent bubbles up to <body> node of the document like below:

<a> → <li> → <ul> → <div> → <body>

Let’s add one more line of code and prints out what the e.currentTarget is from the example we used above:

document.addEventListener(“click”, function(e) {
  if(e.target && e.target.nodeName == “A”) {
    console.log(“List item “, e.target.textContent, “ was clicked!”);  // "List item Item 2 was clicked!"
  }
  console.log(e.currentTarget);  // #document
});

It prints out “document” since we attached current event listener to the document while e.target refers to <a> which we clicked.

We can also look at one more example to see the differences between target and currentTarget. This time, we add the event listener to the <ul>:

document.getElementById(“list”).addEventListener(“click”, function(e) {
  console.log(e.currentTarget);  //<ul><li>...</li><ul>
  console.log(e.target);         //<a href="#">Item 2</a>
);

Again, the currentTarget refers to the element that the event listener directly attached to while the target still refers to the specific <a> we clicked.

With these two properties target and currentTarget, we can easily manipulate the node when the event gets triggered, as well as the node the event is attached to.

JavaScript Event Delegation, and event.target vs. event.currentTarget的更多相关文章

  1. Event对象中的target属性和currentTarget属性的区别

    先上结论: Event.target:触发事件的元素: Event.currentTarget:事件绑定的元素: 通过下面的例子来理解这两个属性的区别: 使用Event.target属性的例子:(我在 ...

  2. JavaScript Interview Questions: Event Delegation and This

    David Posin helps you land that next programming position by understanding important JavaScript fund ...

  3. javascript 事件委托 event delegation

    事件委托 event delegation 一.概念: 假设我们有很多个子元素,每个元素被点击时都会触发相应事件,普通的做法是给每个子元素添加一个事件监听. 而,事件委托则是给它们的父元素添加一个事件 ...

  4. javascript事件代理(Event Delegation)

    看了几篇文章,放上来供参考 司徒正美的文章,Event Delegation Made Easy --------------------------------------------------- ...

  5. JavaScript------事件委托（event delegation）

    简单的说,事件委托(event delegation)是在DOM上层(也就是在触发事件的元素的父元素上)定义事件的处理程序,而不是定义在触发事件的元素本身上. 首先我们来举这样一个例子:我有N个li元 ...

  6. window.event.srcElement与window.event.target 触发事件的元素 触发事件对象的获取，window.event与时间函数参数的event是同一个 事件对象

    判断事件触发的元素:     var tag = window.event.target || window.event.srcElement;    if (tag.tagName.toLowerC ...

  7. JS:event对象下的target属性和取消冒泡事件

    1.target 通过获取DOM元素 var box = document.getElementById("box"); document.box.onclick = functi ...

  8. 【前端】event.target 和 event.currentTarget 的区别

    event.target 和 event.currentTarget 的区别 举例说明: <!DOCTYPE html> <html> <head> <tit ...

  9. javascript运行模式：并发模型 与Event Loop

    看了阮一峰老师的JavaScript 运行机制详解:再谈Event Loop和[朴灵评注]的文章,查阅网上相关资料,把自己对javascript运行模式和EVENT loop的理解整理下,不一定对,日 ...

随机推荐

  1. Sass 基础教程

    0. Sass 文件后缀名 sass 有两种后缀名文件:一种后缀名为 sass,不使用大括号和分号:另一种就是我们这里使用的 scss 文件,这种和我们平时写的 css 文件格式差不多,使用大括号和分 ...

  2. 阿里云无法远程连接数据库MySQL错误码10060解决办法

    使用图形界面管理工具Navicat for MySQL连接Mysql数据库时提示错误:Can't connect to MySQL server (10060) 导致些问题可能有以下几个原因: 1.网 ...

  3. android 实现 view 滑动

    韩梦飞沙  韩亚飞  313134555@qq.com  yue31313  han_meng_fei_sha 1,通过view 的  滑动到 方法 或者 通过什么滑动  方法 实现.  适合 视图 ...

  4. 【洛谷】1477：[NOI2008]假面舞会【图论】

    P1477 [NOI2008]假面舞会 题目描述 一年一度的假面舞会又开始了,栋栋也兴致勃勃的参加了今年的舞会. 今年的面具都是主办方特别定制的.每个参加舞会的人都可以在入场时选择一 个自己喜欢的面具 ...

  5. 【BZOJ】1864: [Zjoi2006]三色二叉树

    1864: [Zjoi2006]三色二叉树 Time Limit: 1 Sec  Memory Limit: 64 MBSubmit: 1295  Solved: 961[Submit][Status ...

  6. Windows Phone background Audio 后台音频

    Windows Phone 后台音频的确不是什么新鲜的话题了,但发现目前在WP平台的音频播放应用多多少少会有一些瑕疵,所以在此给大家在此介绍下这个功能给有需要的朋友们. 首先介绍下我们的应用在后台播放 ...

  7. Codeforces Round #254 (Div. 1) A. DZY Loves Physics 智力题

    A. DZY Loves Physics 题目连接: http://codeforces.com/contest/444/problem/A Description DZY loves Physics ...

  8. opencv hog算子

    梯度直方图特征(HOG) 是一种对图像局部重叠区域的密集型描述符, 它通过计算局部区域的梯度方向直方图来构成特征.Hog特征结合SVM分类器已经被广泛应用于图像识别中,尤其在行人检测中获得了极大的成功 ...

  9. Git_Feature分支

    软件开发中,总有无穷无尽的新的功能要不断添加进来. 添加一个新功能时,你肯定不希望因为一些实验性质的代码,把主分支搞乱了,所以,每添加一个新功能,最好新建一个feature分支,在上面开发,完成后,合 ...

  10. ARM JTAG 20

    http://infocenter.arm.com/help/index.jsp?topic=/com.arm.doc.dui0499b/BEHEIHCE.html he ARM JTAG 20 co ...