In-depth understanding of JavaScript event mechanism

This article introduces an in-depth understanding of the event mechanism of JavaScript and has a certain reference value. Now I share it with you. Friends in need can refer to it

What is an event

First of all, let’s use a person as an analogy. People can sleep, but only when people feel sleepy will they go to sleep.
In program terms, the object human has a sleep method. This method will only be executed when the event mental fatigue is triggered:

function sleep(){    do sleep
}

man.ontired=function(){
    sleep();
}

Of course, some Because a lot of work has not been completed, people still have to work overtime even if they are tired at 12 o'clock in the night. At this time, people may execute a method to continue working after triggering MENTAL FATIGUE (such as my QAQ. . .

man.ontired=function(){
    work()
}

• So events are widely present around us, that is, a trigger point, just like a switch, the light turns on when you press it. The light goes out. This is an event.

• ##Put the event on the web. To be precise, it should be in the browser. What is the event?
  The answer is of course the user's response to all actions performed by the browser, such as the simplest clicks, keyboard presses, mouse movements, closing web pages, loading web pages, etc., as long as the browser can. All those that respond to user behavior are web events. For example, if a user smashes the computer angrily, the browser cannot monitor or respond, so it is not considered a web event.

• ##So, an event is a response of the browser to user behavior

• js is event-driven

And it is Programmers and users can relate to the existence of these events. Programmers don't know what users do on the web page, but programmers know how to deal with it after users do those actions. You can imagine our front-end programmers. They are all landmines. The page is covered with "event mines" of all sizes. As long as the user does not step on them, the code we wrote will lie in the js file and never trigger. But once the user clicks and drags, the code will not be triggered. If we press a button and step on an "event mine", the code we write will start executing with a "boom" like a mine, for example:

    document.body.onclick=function(){
        alert('boom')
    }

• 如果用户只是打开了网页，根本没有点击页面，那么这个"boom"永远不会弹出来。

• 因为js是事件驱动的。

事件驱动是什么？

• 上面那几个例子已经很好说明了什么是事件驱动。事件驱动就是，事件没发生，代码不执行，事件是触发代码执行的直接原因。

• 一般来讲，可能你有遇到，在一个点击事件里面写了一个小小或者大大的逻辑错误，js页面加载的时候并不会给你报错，它只是匆匆看了一眼onclick里面的内容，但不会去执行它，自然不会去发现里面的逻辑错误，除非你里面写了一个语法错误，js看了一眼发现，卧槽这里面的语句不通顺读不通啊，它才会提示错误。只有你点击事件触发的时候，js执行了下代码，然后发现里面的逻辑狗屁不通，然后才会报错。

• 通常我们都会写个window.onload，页面加载完成触发的事件，把要执行的代码放里面，这样资源加载好了js好操作。但是，页面还存在一个隐性的事件，就是read事件，js引擎阅读代码事件。如果读到了function 函数名(){...}活着var 函数名=function(){...}这样的结构，它会匆匆扫一眼，不执行里面的代码，然后继续读下去，其他的语句一旦遇到了，js就会去执行它。比如var 变量名或着函数名()这样。这个是read事件，其他事件也类似，只有当前浏览器对用户行为的响应时，js才会执行里面的代码。

其他语言的事件驱动

• 说到事件驱动，还有一个语言也是事件驱动的，那就是php(我所知道的。。。)。

• php的事件驱动在于，它同样为服务器划分了事件，最简单的，一个请求进来，这是一个事件；服务器做出一个响应，这也是一个事件。然后后台服务器围绕这几个事件，一旦触发哪个就执行相关代码。

• js的后台版node也是一样的

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