There are several ways to handle javascript events

There are three ways to handle JavaScript events: 1. HTML event handler, add the event handler directly in the HTML code, the syntax is ""; 2. DOM0 Level event handler, adds event processing for the specified object, the syntax is "element object. event = function(){...}"; 3. DOM2 level event handler, also added to specific objects, is mainly used to handle specified and Action to delete an event handler.

The operating environment of this tutorial: Windows 10 system, JavaScript version 1.8.5, Dell G3 computer.

There are several ways to handle JavaScript events

3 ways of Javascript event handlers

When an event is generated, we have to handle it. The main Javascript event handlers are 3 ways:

1. HTML event handler

That is, we add the event handler directly in the HTML code, such as the following code:

<input id="btn" value="按钮" type="button" onclick="showmsg();">
　　<script>
　　 function showmsg(){
　　 alert("HTML添加事件处理");
　　 }
　　</script>

We can see from the above code that event processing is directly nested in the element. This has a problem: the coupling between html code and js is too strong. If one day you want to change showmsg in js, Then not only do you need to modify it in js, but you also need to modify it in html. We can accept one or two modifications, but when your code reaches the level of 10,000 lines, modification will require a lot of time and money, so we do not use this method. Recommended Use.

2. DOM0 level event handler

is to add event processing to the specified object. Look at the following code:

<input id="btn" value="按钮" type="button">
　　<script>
　　　　var btn= document.getElementById("btn");
　　 btn.onclick=function(){
　　　　　　alert("DOM级添加事件处理");
　　　　} 
　　　　btn.onclick=null;//如果想要删除btn的点击事件，将其置为null即可
　　</script>

From the above code , we can see that compared to HTML event handlers, DOM0-level events, the coupling between HTML code and js code has been greatly reduced. However, smart programmers are still not satisfied and hope to find a simpler way to deal with it. Let's look at the third way to deal with it.

3. DOM2-level event handler

DOM2 also adds event handlers to specific objects, but mainly involves two methods for handling designation and deletion. Event handler operations: addEventListener() and removeEventListener(). They all receive three parameters: the event name to be processed, the function as the event handler and a Boolean value (whether to handle the event in the capture phase), look at the following piece of code:

<input id="btn" value="按钮" type="button">
　　<script>
　　 var btn=document.getElementById("btn");
　　 btn.addEventListener("click",showmsg,false);//这里我们把最后一个值置为false，即不在捕获阶段处理，一般来说冒泡处
理在各浏览器中兼容性较好
　　 function showmsg(){
　　 alert("DOM级添加事件处理程序");
　　 }
　　 btn.removeEventListener("click",showmsg,false);//如果想要把这个事件删除，只需要传入同样的参数即可
　　</script>

Here we can see , when adding and deleting event processing, the last method is more direct and simplest. However, Ma Haixiang reminds everyone that when processing the deletion event, the parameters passed in must be consistent with the previous parameters, otherwise the deletion will be invalid!

Extended knowledge:

The event stream describes the order in which events are received on the page. In the early days of browser development, the two major browser manufacturers IE and Netscape competed with each other. , there is a cheating situation, that is, they have two completely opposite definitions of the event flow. That is what we are familiar with: IE's event bubbling, Netscape's event capture. Let’s take a picture first and briefly look at the structure:

1. Event bubbling

Event bubbling means that the event starts with the most specific element (the node with the deepest nesting level in the document) is received, and then propagated up to the least specific node (the document). Take the above picture to illustrate, when the text part is clicked, it is first received by the element at the text, and then propagated to the window step by step, that is, the process of 6-7-8-9-10 is executed.

2. Event capture

Event capture means that the event is received by the less specific node at the earliest, and the most specific node receives the event last. Similarly, in the above model, when the text part is clicked, it is first received by the window and then propagated to the text element step by step, that is, the process of 1-2-3-4-5 is executed.

【Related recommendations: javascript video tutorial, web front-end】

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