Javascript - arrow在事件处理程序中执行这个操作? [英] Javascript - arrow functions this in event handler?
问题描述
$(this)
返回未定义点击?
dom.videoLinks.click((e)=> {
e.preventDefault();
console.log($(this));
var self = $(this),
url = self.attr(configuration.attribute);
eventHandlers.showVideo url);
//停用任何活动的视频缩略图
dom.videoLinks.filter('。video-selected')。removeClass('video-selected');
//激活所选的视频缩略图
self.addClass('video-selected');
});
但是,如果我更改它不是一个箭头函数它按预期工作?:
dom.videoLinks.click(function(e){
e.preventDefault() ;
console.log(this);
console.log($(this));
var self = e.this,
url = self.attr(configuration.attribute );
eventHandlers.showVideo(url);
//停用任何活动的视频缩略图
dom.videoLinks.filter('。video-selected')。 removeClass('video-selected');
//激活所选的视频缩略图
self.addClass('video-selected');
});
所以如果我使用箭头函数,我该怎么办回调?
使用 arrow function 作为回调,而不是使用 / code>获取处理程序所绑定的元素,您应该使用
event.currentTarget
。
这个
在箭头函数内部由箭头函数定义的位置确定,而不是使用 。
所以从现在开始请注意,
event.currentTarget
始终指向 DOM
元素,其EventListeners正在处理。
.currentTarget vs .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。
currentTarget
类型为EventTarget
,readonly用于指示
EventTarget
其EventListeners
正在处理中。这个
在捕获和冒泡期间特别有用。
检查
data-hide =true> = 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 ')的.html(' *箭头函数< br> currentTarget:'+ e.currentTarget.id));
#parent {background-color:red;宽度:250像素; 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/库/ jquery的/ 2.1.1 / jquery.min.js>< /脚本> < div id =parent> Parent-(附加的事件处理程序)< br>< br> < div id =child>子< BR><峰; br> < p id =grand-child> Grand Child< / p> < / DIV> < / DIV> < 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?
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.
currentTarget
of typeEventTarget
, readonly Used to indicate theEventTarget
whoseEventListeners
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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