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 html event handler: First of all, this method is outdated; 2.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First of all, this method is outdated.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:
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.
The code is as follows:
<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！
相关推荐：
The above is the detailed content of js event handlers often encountered in JavaScript learning. For more information, please follow other related articles on the PHP Chinese website!
Hot AI Tools
Undresser.AI Undress
AI-powered app for creating realistic nude photos
AI Clothes Remover
Online AI tool for removing clothes from photos.
Undress AI Tool
Undress images for free
Clothoff.io
AI clothes remover
Video Face Swap
Swap faces in any video effortlessly with our completely free AI face swap tool!
Hot Article
Hot Tools
Notepad++7.3.1
Easy-to-use and free code editor
SublimeText3 Chinese version
Chinese version, very easy to use
Zend Studio 13.0.1
Powerful PHP integrated development environment
Dreamweaver CS6
Visual web development tools
SublimeText3 Mac version
God-level code editing software (SublimeText3)
Hot Topics
1677
14
1429
52
1333
25
1278
29
1257
24
Python vs. JavaScript: The Learning Curve and Ease of Use
Apr 16, 2025 am 12:12 AM
Python is more suitable for beginners, with a smooth learning curve and concise syntax; JavaScript is suitable for front-end development, with a steep learning curve and flexible syntax. 1. Python syntax is intuitive and suitable for data science and back-end development. 2. JavaScript is flexible and widely used in front-end and server-side programming.
JavaScript and the Web: Core Functionality and Use Cases
Apr 18, 2025 am 12:19 AM
The main uses of JavaScript in web development include client interaction, form verification and asynchronous communication. 1) Dynamic content update and user interaction through DOM operations; 2) Client verification is carried out before the user submits data to improve the user experience; 3) Refreshless communication with the server is achieved through AJAX technology.
JavaScript in Action: Real-World Examples and Projects
Apr 19, 2025 am 12:13 AM
JavaScript's application in the real world includes front-end and back-end development. 1) Display front-end applications by building a TODO list application, involving DOM operations and event processing. 2) Build RESTfulAPI through Node.js and Express to demonstrate back-end applications.
Understanding the JavaScript Engine: Implementation Details
Apr 17, 2025 am 12:05 AM
Understanding how JavaScript engine works internally is important to developers because it helps write more efficient code and understand performance bottlenecks and optimization strategies. 1) The engine's workflow includes three stages: parsing, compiling and execution; 2) During the execution process, the engine will perform dynamic optimization, such as inline cache and hidden classes; 3) Best practices include avoiding global variables, optimizing loops, using const and lets, and avoiding excessive use of closures.
Python vs. JavaScript: Development Environments and Tools
Apr 26, 2025 am 12:09 AM
Both Python and JavaScript's choices in development environments are important. 1) Python's development environment includes PyCharm, JupyterNotebook and Anaconda, which are suitable for data science and rapid prototyping. 2) The development environment of JavaScript includes Node.js, VSCode and Webpack, which are suitable for front-end and back-end development. Choosing the right tools according to project needs can improve development efficiency and project success rate.
The Role of C/C in JavaScript Interpreters and Compilers
Apr 20, 2025 am 12:01 AM
C and C play a vital role in the JavaScript engine, mainly used to implement interpreters and JIT compilers. 1) C is used to parse JavaScript source code and generate an abstract syntax tree. 2) C is responsible for generating and executing bytecode. 3) C implements the JIT compiler, optimizes and compiles hot-spot code at runtime, and significantly improves the execution efficiency of JavaScript.
Python vs. JavaScript: Use Cases and Applications Compared
Apr 21, 2025 am 12:01 AM
Python is more suitable for data science and automation, while JavaScript is more suitable for front-end and full-stack development. 1. Python performs well in data science and machine learning, using libraries such as NumPy and Pandas for data processing and modeling. 2. Python is concise and efficient in automation and scripting. 3. JavaScript is indispensable in front-end development and is used to build dynamic web pages and single-page applications. 4. JavaScript plays a role in back-end development through Node.js and supports full-stack development.
From Websites to Apps: The Diverse Applications of JavaScript
Apr 22, 2025 am 12:02 AM
JavaScript is widely used in websites, mobile applications, desktop applications and server-side programming. 1) In website development, JavaScript operates DOM together with HTML and CSS to achieve dynamic effects and supports frameworks such as jQuery and React. 2) Through ReactNative and Ionic, JavaScript is used to develop cross-platform mobile applications. 3) The Electron framework enables JavaScript to build desktop applications. 4) Node.js allows JavaScript to run on the server side and supports high concurrent requests.
