What is a closure? Let's talk about closures in JavaScript and see what functions they have?

What is closure? See what closures do? The following article will talk about closures in JavaScript. It has certain reference value. Friends in need can refer to it. I hope it will be helpful to everyone.

In the process of front-end learning, we will inevitably encounter many problems, so today we will talk about two problems from the perspective of a beginner:

What is a closure?

What are the functions of closures?

In fact, closures are everywhere when we learn javascript, you just need to be able to recognize and accept it. Closures are not a tool that requires learning a new syntax or pattern to use. Closures are a natural consequence of writing code based on lexical scope. We rarely need to intentionally create closures when writing code.

I believe that many friends are already muttering in their hearts, what is this lexical scope? Don’t panic, just listen to me slowly. In short, lexical scope is defined in lexical scope. scope of the stage. In other words, lexical scope is determined by where you place variables and block-level scopes when you write your code, so the scope remains unchanged when the lexical analyzer processes the code (most of the time) . ——"JavaScript Volume You Don't Know"

Let's take an example first

function test(){
   var arr = []
   for(var i=0;i8bad89ebe420e08e3e6c338863b7faac100; demo: undefined; a: fa(){} };
Then create an AO{ aaa: undefined--->123;b: fb(){} } for function a, and finally precompile function b in function a to create an AO{ b: undefined-- ->234};The order of the scope chain at this time is 1. AO object of function b; 2. AO object of function a; 3. Global GO object. When we print aaa in function b, we start from the top of the scope chain. If there is no aaa in the AO object of function b, we will search down along the scope chain to find the AO of the second-level function a. The object is to find the value of aaa as 123 and output the result. <p>如果我们没有从预编译的角度去分析就会认为此时的aaa应该会报错的，当var demo = a()执行时，当a函数执行结束，那么a对应的AO对象应该被销毁了，照常理分析当我们执行demo时作用域链此时应该会创建b的AO对象和GO对象，此时只有b的AO对象，没有a的AO对象，应该不能打印出aaa的值，但是此时aaa的值为123，则说明a的AO对象没有被销毁，那么为什么呢？原因就在于这里创建了闭包，当var demo = a()执行结束之后，垃圾回收机制会来问，a函数老兄，我看你都执行完毕了，你的运行内存是不是可以给我释放了，但是此时a函数只能无奈摇摇头说道，老哥，我也不确定我有没有执行完毕，我执行完创建了一个b，但是b又不归我管，所以我也不确定b有没有被调用，所以我也不确定我有没有执行完毕，垃圾回收机制想了想，既然你都不知道那我就不回收了，要是回收了还没执行完的就该报错了，所以此时a的AO对象就没有被回收。</p><h3 data-id="heading-4"><strong>补充全面一点就是：当一个函数内部的函数被保存到函数外部时，就会产生闭包。</strong></h3><p>相信通过这两个例子，你已经对闭包有了一个大概的了解，那接下来我们说一下闭包有哪些作用。</p><blockquote>
<p>闭包的作用</p>
<ul>
<li><ol><li>实现公有变量 例如：累加器（3.js）</li></ol></li>
<li><ol start="2"><li>做缓存</li></ol></li>
<li><ol start="3"><li>可以实现封装，属性私有化</li></ol></li>
<li><ol start="4"><li>模块化开发，防止污染全局变量</li></ol></li>
</ul>
</blockquote><p>我们对闭包的作用也来一个例子（3.js）<br></p><pre class="brush:js;toolbar:false"> var count = 0
 function add() {
   return count++
 }
 console.log(add());
 console.log(add());
 console.log(add());

 这是一段比较普通的累加的代码，但是如果我们在实习甚至是工作的时，公司要求你把累加器封装成一个模块化的代码，那么
 此时，为了模块化我们尽可能避免定义全局变量，但是不定义全局变量我们如何实现呢，此时我们就可以用到闭包了；

function add() {
    var count = 0
    function a() {
      ++count
      console.log(count);
    }
    return a
  }
  var res = add()
  res()
  res()
  
  //add函数结束之后，add的AO对象没有被销毁，因为add函数执行完了之后，返回的a不知道是否被调用就形成了闭包，这样
  就能使得不使用全局变量也能封装成一个模块化的累加器。

结语

那么关于闭包以及闭包的作用相关的一些个人见解就是这些，目前对于闭包也只是一些浅显的了解，后期学习之后完善过后会出后续关于闭包的相关文章，感谢您的观看，欢迎批评斧正，一起共同进步。

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