Javascript - 箭头在事件处理程序中发挥作用? [英] Javascript - arrow functions this in event handler?

问题描述

我是 ES6 的新手，无法让它正常工作:

$(this) 在点击时返回 undefined?

dom.videoLinks.click((e) => {e.preventDefault();console.log($(this));var self = $(this),url = self.attr(configuration.attribute);eventHandlers.showVideo(url);//停用任何活动的视频缩略图dom.videoLinks.filter('.video-selected').removeClass('video-selected');//激活选定的视频缩略图self.addClass('视频选择');});

但是，如果我改变它而不是像这样的箭头功能，它会按预期工作吗?:

dom.videoLinks.click(function(e) {e.preventDefault();控制台日志(这个)；console.log($(this));var self = e.this,url = self.attr(configuration.attribute);eventHandlers.showVideo(url);//停用任何活动的视频缩略图dom.videoLinks.filter('.video-selected').removeClass('video-selected');//激活选定的视频缩略图self.addClass('视频选择');});

如果我在回调中使用箭头函数，我将如何处理?

解决方案

使用箭头函数作为回调，而不是使用this来获取处理程序所指向的元素已绑定，您应该使用 event.currentTarget.
this 在箭头函数中的值取决于箭头函数的定义位置，而不是使用位置.
所以从现在开始请记住，event.currentTarget 总是指 DOM 当前正在处理其事件监听器的元素.

<小时>

.currentTarget 与 .target

使用 event.currentTarget 而不是 event.target 因为事件冒泡/捕获:

• event.currentTarget- 是附加了事件侦听器的元素.
• event.target- 是触发事件的元素.
  

来自文档:

<块引用>

currentTarget 类型 EventTarget, readonly 用于指示EventTarget 当前正在处理其 EventListeners.这个在捕获和冒泡期间特别有用.

检查下面代码段

中的基本示例

var parent = document.getElementById('parent');parent.addEventListener('click', function(e) {document.getElementById('msg').innerHTML = "this:" + this.id +"<br> currentTarget: " + e.currentTarget.id +"<br>目标:" + e.target.id;});$('#parent').on('click', function(e) {$('#jQmsg').html("*jQuery<br>this:" + $(this).prop('id')+ "<br>currenTarget: " + $(e.currentTarget).prop('id')+ "<br>目标:" + $(e.target).prop('id'));});$('#parent').on('click', e => $('#arrmsg').html('*Arrow function <br> currentTarget: ' + e.currentTarget.id));

#parent {background-color:red;宽度:250px；高度:220px;}#child {background-color:yellow;height:120px;width:120px;margin:0 auto;}#grand-child {background-color:blue;height:50px;width:50px;margin:0 auto;}#msg, #jQmsg, #arrmsg {font-size:16px;font-weight:600;background-color:#eee;font-family:sans-serif;color:navy;}

<script src="https://ajax.googleapis.com/ajax/libs/jquery/2.1.1/jquery.min.js"></script><div id="parent">Parent-(附加事件处理程序)<br><br><div id="孩子">儿童<br><br><p id="grand-child">孙子</p>

<div id="msg"></div><br><div id="jQmsg"></div><br><div id="arrmsg"></div>

I'm new to ES6, and can't quite get this to work:

$(this) returns undefined on click?

dom.videoLinks.click((e) => {
            e.preventDefault();
            console.log($(this));
            var self = $(this),
                url = self.attr(configuration.attribute);

            eventHandlers.showVideo(url);

            // Deactivate any active video thumbs
            dom.videoLinks.filter('.video-selected').removeClass('video-selected');

            // Activate selected video thumb
            self.addClass('video-selected');
        });

However if I change it so not be an arrow function like so, it works as expected?:

dom.videoLinks.click(function(e) {
            e.preventDefault();
            console.log(this);
            console.log($(this));
            var self = e.this,
                url = self.attr(configuration.attribute);

            eventHandlers.showVideo(url);

            // Deactivate any active video thumbs
            dom.videoLinks.filter('.video-selected').removeClass('video-selected');

            // Activate selected video thumb
            self.addClass('video-selected');
        });

So how would I go about it if I use an arrow function in the callback?

解决方案

With arrow function as a callback, instead of using this to get the element to which the handler is bound, you should use event.currentTarget.
Value of this inside an arrow function is determined by where the arrow function is defined, not where it is used.
So from now on, keep in mind that event.currentTarget always refers to the DOM element whose EventListeners are currently being processed.

---

.currentTarget vs .target

Use event.currentTarget instead of event.target because of event bubbling/capturing:

• event.currentTarget- is the element that has the event listener attached to.
• event.target- is the element that triggered the event.
  

From the documentation:

currentTarget of type EventTarget, readonly Used to indicate the EventTarget whose EventListeners are currently being processed. This is particularly useful during capturing and bubbling.

Check the basic example in the below snippet

var parent = document.getElementById('parent');
parent.addEventListener('click', function(e) {
  
  document.getElementById('msg').innerHTML = "this: " + this.id +
    "<br> currentTarget: " + e.currentTarget.id +
    "<br>target: " + e.target.id;
});

$('#parent').on('click', function(e) {

  $('#jQmsg').html("*jQuery<br>this: " + $(this).prop('id')
                   + "<br>currenTarget: " + $(e.currentTarget).prop('id') 
                   + "<br>target: " + $(e.target).prop('id'));
});

$('#parent').on('click', e => $('#arrmsg').html('*Arrow function <br> currentTarget: ' + e.currentTarget.id));

#parent {background-color:red; width:250px; height:220px;}
#child {background-color:yellow;height:120px;width:120px;margin:0 auto;}
#grand-child {background-color:blue;height:50px;width:50px;margin:0 auto;}
#msg, #jQmsg, #arrmsg {font-size:16px;font-weight:600;background-color:#eee;font-family:sans-serif;color:navy;}

<script src="https://ajax.googleapis.com/ajax/libs/jquery/2.1.1/jquery.min.js"></script>

  <div id="parent">Parent-(attached event handler)<br><br>
    <div id="child"> Child<br><br>
      <p id="grand-child">Grand Child</p>
    </div>
  </div>
 
  <div id="msg"></div><br>
  <div id="jQmsg"></div><br>
  <div id="arrmsg"></div>

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