js event handlers often encountered in JavaScript learning

There are three main methods for specifying event handlers for events. 1. html event handler: First of all, this method is outdated; 2. DOM level 0 event handler: This method is simple and cross-browser, but it can only add one to an element. Event handling function; 3. DOM2-level event handler: DOM2-level event handler can add multiple event handlers for an element.

1. Event handler

As mentioned earlier, an event is a certain action performed by the user or the browser itself, such as click, load and mouseover are all event names. The function that responds to an event is called an event handler (also called an event handler, event handler). The name of the event handler starts with "on", so the event handler for the click event is onclick, and the event handler for the load event is onload.

There are three main ways to specify event handlers for events.

1. HTML event handler

First of all, this method is outdated. Because action (javascript code) and content (html code) are tightly coupled. But it can still be used when writing a small demo.

There are also two methods in this method, both of which are very simple:

The first one: directly define the event handler and the included actions in the html.

The code is as follows:

Second type: define event handlers in html, Actions performed call scripts defined elsewhere.

The code is as follows:

<script>function showMessage(){ alert("clicked!"); }</script>

##note:

1) The event itself can be directly accessed through the event variable, such as onclick="alert(event .type)" will pop up a click event.

2) This value is equal to the target element of the event, where the target element is input. For example, onclick="alert(this.value)" can get the value of the input element.

2. DOM0-level event handler

This method is simple and cross-browser, but it can only add one event handler to one element.

Because this method adds multiple event handlers to elements, the later ones will overwrite the previous ones.

Add event handler:

<input type="button" value="click me!" onclick="showMessage()"/>
<script>
function showMessage(){
 alert("clicked!");
}
</script>

Delete event handler:

The code is as follows:

myBtn.onclick=null;

3. DOM2-level event handler

DOM2-level event handler can add multiple event handlers for an element. It defines two methods for adding and removing event handlers: addEventListener() and removeEventListener().

Both methods require 3 parameters: event name, event processing function, and Boolean value.

This Boolean value is true, the event is processed in the capture phase, false, and the event is processed in the bubbling phase, the default is false.

Add event handlers: Now add two event handlers for the button, one pops up "hello" and the other pops up "world".

<input id="myBtn" type="button" value="click me!"/>
<script>
 var myBtn=document.getElementById("myBtn");
 myBtn.addEventListener("click",function(){
  alert("hello");
 },false);
 myBtn.addEventListener("click",function(){
  alert("world");
 },false);
</script>

Delete event handlers: Event handlers added through addEventListener must be deleted through removeEventListener, and the parameters must be consistent.

note: Anonymous functions added through addEventListener cannot be deleted. The code below will not work!

The code is as follows:

myBtn.removeEventListener("click",function(){ alert("world"); },false);

It seems that the removeEventListener parameter is consistent with the addEventListener parameter above, but in fact the anonymous function in the second parameter is completely different.

So in order to delete the event handler, the code can be written like this:

<input id="myBtn" type="button" value="click me!"/>
<script>
 var myBtn=document.getElementById("myBtn");
 var handler=function(){
  alert("hello");
 }
 myBtn.addEventListener("click",handler,false);
 myBtn.removeEventListener("click",handler,false);
</script>

2. IE event handler

1. Actual application scenarios

IE8 and below browsers do not support addEventListener. In actual development, if you want to be compatible with IE8 and below browsers. If you use native binding events and need to do compatibility processing, you can use jquery's bind instead.

2. IE8 event binding

IE8 and below browsers implement two methods similar to those in DOM: attachEvent() and detachEvent().

Both methods require two parameters: the event handler name and the event handler function.

note:

IE11 only supports addEventListener!

IE9 and IE10 support both attachEvent and addEventListener!

TE8 and below versions only support attachEvent!

You can use the following code to test in various versions of IE browsers.

<input id="myBtn" type="button" value="click me!"/>
<script>
 var myBtn=document.getElementById("myBtn");
 var handlerIE=function(){
  alert("helloIE");
 }
 var handlerDOM= function () {
  alert("helloDOM");
 }
 myBtn.addEventListener("click",handlerDOM,false);
 myBtn.attachEvent("onclick",handlerIE);
</script>

添加事件处理程序：现在为按钮添加两个事件处理函数，一个弹出“hello”,一个弹出“world

<script>
 var myBtn=document.getElementById("myBtn");
 myBtn.attachEvent("onclick",function(){
  alert("hello");
 });
 myBtn.attachEvent("onclick",function(){
  alert("world");
 });
</script>

note:这里运行效果值得注意一下：

IE8以下浏览器中先弹出“world”，再弹出“hello”。和DOM中事件触发顺序相反。

IE9及以上浏览器先弹出“hello”，再弹出“world”。和DOM中事件触发顺序相同了。

可见IE浏览器慢慢也走上正轨了。。。

删除事件处理程序:通过attachEvent添加的事件处理程序必须通过detachEvent方法删除，且参数一致。

和DOM事件一样，添加的匿名函数将无法删除。

所以为了能删除事件处理程序，代码可以这样写：

<input id="myBtn" type="button" value="click me!"/>
<script>
 var myBtn=document.getElementById("myBtn");
 var handler= function () {
  alert("hello");
 }
 myBtn.attachEvent("onclick",handler);
 myBtn.detachEvent("onclick",handler);
</script>

note:IE事件处理程序中还有一个地方需要注意：作用域。

使用attachEvent()方法，事件处理程序会在全局作用域中运行，因此this等于window。

而dom2或dom0级的方法作用域都是在元素内部，this值为目标元素。

下面例子会弹出true。

<input id="myBtn" type="button" value="click me!"/>
<script>
 var myBtn=document.getElementById("myBtn");
 myBtn.attachEvent("onclick",function(){
  alert(this===window);
 });
</script>

在编写跨浏览器的代码时，需牢记这点。

IE7\8检测：

//判断IE7\8 兼容性检测
   var isIE=!!window.ActiveXObject;
   var isIE6=isIE&&!window.XMLHttpRequest;
   var isIE8=isIE&&!!document.documentMode;
   var isIE7=isIE&&!isIE6&&!isIE8;
   if(isIE8 || isIE7){
    li.attachEvent("onclick",function(){
     _marker.openInfoWindow(_iw);
    }) 
   }else{
    li.addEventListener("click",function(){
     _marker.openInfoWindow(_iw);
    })
   }

以上所述是小编给大家介绍的JavaScript事件学习小结(二)js事件处理程序的相关知识，希望对大家有所帮助！

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