Further understanding of js closures-JS Tutorial-php.cn

Home

Web Front-end

JS Tutorial

Further understanding of js closures

不言

Jul 18, 2018 pm 05:16 PM

js closure

The content shared with you in this article is about further understanding of js closures. The content is very good. Friends in need can refer to it.

The concept of closure has been confusing to me since I started learning JS a few months ago

I understood it before, but then I didn’t use it for a long time Just forgot

Closure: In layman’s terms, what most people accept is that a function has the right to use local variables in another function

I see a lot of differences

Use the simplest code to express

<span style="font-size: 14px;">function out(){<br/><br/>var age=21;<br/><br/>function inner(){<br/>  <br/>  console.log(age);<br/><br/>}<br/><br/>return inner;<br/><br/>}<br/><br/>var fn=out();<br/> fn();  //22</span>

Very consistent with the concept

I think closure is reflecting the scope

The inner function is defined inside the out function

So console(age);

will use the variable search mechanism. First, search within the scope of your own (inner) function. If not found, go to Find

in the out function scope and then output. If not found in out, it will look for

in a larger scope. Until the scope of the window, the lower-level scope can be accessed upwards, but the upper-level scope cannot be accessed downwards

Scope refers to

{ }

And JS does not have block-level scope

for(var i=0;i

console.log(i);// 1 2 3 4 5

}

cosole.log(i );//5

i will not be destroyed just because the for loop is out.

This should be noted

Okay, I talked about a little bit about scope. Now back to closures

The core of closures is return. Just look at the code and you will know.

My understanding is that return returns the function body of inner and the scope that inner can access!

So inner can be accessed anywhere age

Example:

<span style="font-size: 14px;">function  test(){<br/>               var age=23;<br/>                var fn=out();<br/>                 fn();  //21<br/>             <br/>             }<br/>    <br/>    test();//21</span>

It gets 21 instead of 22 because the function body and scope are returned together Then the nearest scope is the out function scope.

Even if age is defined in the test function, it cannot be overwritten because the existing scopes are different

It returns the scope, so it accesses all the variables in that scope and has nothing to do with the scope where your function is now.

Closure is actually a This phenomenon is that everyone who plays DNF is painting pictures and selling materials to make money. This phenomenon is called moving bricks

To sum up: it has to do with the scope of the function you define, and the role of the function you execute. Domain-independent

Contrary to this, this has nothing to do with definition time and is related to execution time. Compare memory

So if you don’t understand it well Closure

Then you can understand it like me

What is returned is the function itself and the scope that the function can access

Give one commonly used one

Closure Tab bar switching

<span style="font-size: 14px;"><!doctype html>
<html lang="en">
 <head>
  <meta charset="UTF-8">
  <meta name="Generator" content="EditPlus®">
  <meta name="Author" content="">
  <meta name="Keywords" content="">
  <meta name="Description" content="">
  <title>Document</title>
  <style type="text/css">
  *{margin:0;padding:0<span style="font-size: 12px; color: rgb(0, 0, 0);">;}
   .box{
     width:140px;
     height:18px;
     position:relative;
     padding:6px 20px;
     margin:50px auto;
     background:#ff6666;
   }

   .box ul{
     list</span>-<span style="font-size: 12px; color: rgb(0, 0, 0);">style:none;
   } 

   .box li{
      width:18px;
      height:18px;
      background:#ff3300;
      line</span>-<span style="font-size: 12px; color: rgb(0, 0, 0);">height:18px;
      text</span>-<span style="font-size: 12px; color: rgb(0, 0, 0);">align:center;      </span><span style="font-size: 12px; color: rgb(0, 0, 255);">float</span><span style="font-size: 12px; color: rgb(0, 0, 0);">:left;
      margin</span>-<span style="font-size: 12px; color: rgb(0, 0, 0);">right:5px;
      cursor:pointer;
   }

   .current{
     background:#ffccff</span>!<span style="font-size: 12px; color: rgb(0, 0, 0);">important;
   }  </span></style>
 </head>
 <body>

  
  <p class="box">
    <ul>
        <li>1</li>
        <li>2</li>
        <li>3</li>
        <li>4</li>
        <li>5</li>
        <li>6</li>
    </ul>
  </p>
 
<script type="text/javascript">     <br/><span style="font-size: 12px; color: rgb(0, 0, 255);">function</span><span style="font-size: 12px; color: rgb(0, 0, 0);"> $(name){          <br/> <br/></span><span style="font-size: 12px; color: rgb(0, 0, 255);">return</span><span style="font-size: 12px; color: rgb(0, 0, 0);"> document.querySelectorAll(name);
       }        <br/> <br/></span><span style="font-size: 12px; color: rgb(0, 0, 255);">var</span>  list=$(".box ul li"<span style="font-size: 12px; color: rgb(0, 0, 0);">);        <br/><br/></span><span style="font-size: 12px; color: rgb(0, 0, 255);">var</span> len=<span style="font-size: 12px; color: rgb(0, 0, 0);">list.length;      <br/> </span><span style="font-size: 12px; color: rgb(0, 0, 255);">for</span>(<span style="font-size: 12px; color: rgb(0, 0, 255);">var</span> i=0;i<len;i++<span style="font-size: 12px; color: rgb(0, 0, 0);">){
           list[i].onmouseover</span>=(<span style="font-size: 12px; color: rgb(0, 0, 255);">function</span><span style="font-size: 12px; color: rgb(0, 0, 0);">(n){               
               </span><span style="font-size: 12px; color: rgb(0, 0, 255);">return</span> <span style="font-size: 12px; color: rgb(0, 0, 255);">function</span><span style="font-size: 12px; color: rgb(0, 0, 0);">(){                  <br/><br/></span><span style="font-size: 12px; color: rgb(0, 0, 255);">for</span>(<span style="font-size: 12px; color: rgb(0, 0, 255);">var</span> j=0;j<len;j++<span style="font-size: 12px; color: rgb(0, 0, 0);">){
                
                      list[j].className</span>=""<span style="font-size: 12px; color: rgb(0, 0, 0);">;
                
                  }
                list[n].className</span>="current"<span style="font-size: 12px; color: rgb(0, 0, 0);">;
               
               }
           })(i);


       }<br/><br/><br/><br/></span></script>



 </body>
</html></span>

Whenever the for loop executes list[i].onmouseover, the function will be executed immediately and the current variable i is passed in

Return a function. This forms a closure. Return the function and the scope that the function can access.

Whenever onmouseover is triggered, the returned function will be executed.

Then execute the for loop in the generation function to clear all li's className

This sentence is the most important when executing list[n]. The n here is the i ## passed in when defining onmouseover

# Because the function is executed immediately when it is defined and i is passed to the anonymous function. This i is within the scope of the anonymous function

Each onmouseover saves its own i

, so when triggered Onmouseover allows li to access the i

that was previously saved in the scope, thus realizing the need to change the background color of someone

Related recommendations:

Understanding of actual parameters, formal parameters and closures of js functions

The above is the detailed content of Further understanding of js closures. For more information, please follow other related articles on the PHP Chinese website!

Statement

The content of this article is voluntarily contributed by netizens, and the copyright belongs to the original author. This site does not assume corresponding legal responsibility. If you find any content suspected of plagiarism or infringement, please contact admin@php.cn

Python vs. JavaScript: A Comparative Analysis for DevelopersMay 09, 2025 am 12:22 AM

The main difference between Python and JavaScript is the type system and application scenarios. 1. Python uses dynamic types, suitable for scientific computing and data analysis. 2. JavaScript adopts weak types and is widely used in front-end and full-stack development. The two have their own advantages in asynchronous programming and performance optimization, and should be decided according to project requirements when choosing.

Python vs. JavaScript: Choosing the Right Tool for the JobMay 08, 2025 am 12:10 AM

Whether to choose Python or JavaScript depends on the project type: 1) Choose Python for data science and automation tasks; 2) Choose JavaScript for front-end and full-stack development. Python is favored for its powerful library in data processing and automation, while JavaScript is indispensable for its advantages in web interaction and full-stack development.

Python and JavaScript: Understanding the Strengths of EachMay 06, 2025 am 12:15 AM

Python and JavaScript each have their own advantages, and the choice depends on project needs and personal preferences. 1. Python is easy to learn, with concise syntax, suitable for data science and back-end development, but has a slow execution speed. 2. JavaScript is everywhere in front-end development and has strong asynchronous programming capabilities. Node.js makes it suitable for full-stack development, but the syntax may be complex and error-prone.

JavaScript's Core: Is It Built on C or C ?May 05, 2025 am 12:07 AM

JavaScriptisnotbuiltonCorC ;it'saninterpretedlanguagethatrunsonenginesoftenwritteninC .1)JavaScriptwasdesignedasalightweight,interpretedlanguageforwebbrowsers.2)EnginesevolvedfromsimpleinterpreterstoJITcompilers,typicallyinC ,improvingperformance.

JavaScript Applications: From Front-End to Back-EndMay 04, 2025 am 12:12 AM

JavaScript can be used for front-end and back-end development. The front-end enhances the user experience through DOM operations, and the back-end handles server tasks through Node.js. 1. Front-end example: Change the content of the web page text. 2. Backend example: Create a Node.js server.

Python vs. JavaScript: Which Language Should You Learn?May 03, 2025 am 12:10 AM

Choosing Python or JavaScript should be based on career development, learning curve and ecosystem: 1) Career development: Python is suitable for data science and back-end development, while JavaScript is suitable for front-end and full-stack development. 2) Learning curve: Python syntax is concise and suitable for beginners; JavaScript syntax is flexible. 3) Ecosystem: Python has rich scientific computing libraries, and JavaScript has a powerful front-end framework.

JavaScript Frameworks: Powering Modern Web DevelopmentMay 02, 2025 am 12:04 AM

The power of the JavaScript framework lies in simplifying development, improving user experience and application performance. When choosing a framework, consider: 1. Project size and complexity, 2. Team experience, 3. Ecosystem and community support.

The Relationship Between JavaScript, C , and BrowsersMay 01, 2025 am 12:06 AM

Introduction I know you may find it strange, what exactly does JavaScript, C and browser have to do? They seem to be unrelated, but in fact, they play a very important role in modern web development. Today we will discuss the close connection between these three. Through this article, you will learn how JavaScript runs in the browser, the role of C in the browser engine, and how they work together to drive rendering and interaction of web pages. We all know the relationship between JavaScript and browser. JavaScript is the core language of front-end development. It runs directly in the browser, making web pages vivid and interesting. Have you ever wondered why JavaScr

See all articles