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Merge pull request #21053 from wxy/20210215-A-practical-guide-to-JavaScript-closures
TSL:sources/tech/20210215 A practical guide to JavaScript closures.md
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[#]: collector: (lujun9972)
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[#]: translator: (wxy)
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[#]: reviewer: ( )
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[#]: publisher: ( )
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[#]: url: ( )
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[#]: subject: (A practical guide to JavaScript closures)
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[#]: via: (https://opensource.com/article/21/2/javascript-closures)
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[#]: author: (Nimisha Mukherjee https://opensource.com/users/nimisha)
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A practical guide to JavaScript closures
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======
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Get a better understanding of how JavaScript code works and executes by
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diving into one of the advanced concept: closures.
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![Woman programming][1]
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In [_4 reasons why JavaScript is so popular_][2], I touched on a few advanced JavaScript concepts. In this article, I will dive into one of them: closures.
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According to [Mozilla Developer Network][3] (MDN), "A closure is the combination of a function bundled together (enclosed) with references to its surrounding state (the lexical environment)." Simplified, this means that a function inside another function can access the variables from the outer (parent) function.
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To better understand closures, take a look at scopes and their execution context.
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Here is a simple code snippet:
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```
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var hello = "Hello";
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function sayHelloWorld() {
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var world = "World";
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function wish() {
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var year = "2021";
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console.log(hello + " " + world + " "+ year);
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}
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wish();
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}
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sayHelloWorld();
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```
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Here's the execution context for this code:
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![Execution context for JS code][4]
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(Nimisha Mukherjee, [CC BY-SA 4.0][5])
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Closures are created every time a function is created (at function-creation time). Every closure has three scopes:
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* Local scope (own scope)
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* Outer functions scope
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* Global scope
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I'll modify the above code slightly to demonstrate closure:
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```
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var hello = "Hello";
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var sayHelloWorld = function() {
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var world = "World";
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function wish() {
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var year = "2021";
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console.log(hello + " " + world + " "+ year);
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}
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return wish;
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}
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var callFunc = sayHelloWorld();
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callFunc();
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```
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The inner function `wish()` is returned from the outer function before it's executed. This happens because functions in JavaScript form **closures**.
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* `callFunc` holds a reference to the function `wish` when `sayHelloWorld` runs
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* `wish` maintains a reference to its surrounding (lexical) environment where the variable `world` exists.
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### Private variables and methods
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Natively, JavaScript does not support the creation of private variables and methods. A common and practical use of closure is to emulate private variables and methods and allow data privacy. Methods defined within the closure scope are privileged.
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This code snippet captures how closures are commonly written and used in JavaScript:
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```
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var resourceRecord = function(myName, myAddress) {
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var resourceName = myName;
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var resourceAddress = myAddress;
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var accessRight = "HR";
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return {
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changeName: function(updateName, privilege) {
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//only HR can change the name
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if(privilege === accessRight ) {
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resourceName = updateName;
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return true;
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} else {
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return false;
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}
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},
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changeAddress: function(newAddress) {
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//any associate can change the address
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resourceAddress = newAddress;
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},
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showResourceDetail: function() {
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console.log ("Name:" + resourceName + " ; Address:" + resourceAddress);
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}
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}
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}
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//Create first record
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var resourceRecord1 = resourceRecord("Perry","Office");
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//Create second record
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var resourceRecord2 = resourceRecord("Emma","Office");
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//Change the address on the first record
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resourceRecord1.changeAddress("Home");
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resourceRecord1.changeName("Perry Berry", "Associate"); //Output is false as only an HR can change the name
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resourceRecord2.changeName("Emma Freeman", "HR"); //Output is true as HR changes the name
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resourceRecord1.showResourceDetail(); //Output - Name:Perry ; Address:Home
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resourceRecord2.showResourceDetail(); //Output - Name:Emma Freeman ; Address:Office
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```
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The resource records (`resourceRecord1` and `resourceRecord2`) are independent of one another. Each closure references a different version of the `resourceName` and `resourceAddress` variable through its own closure. You can also apply specific rules to how private variables need to be handled—I added a check on who can modify `resourceName`.
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### Use closures
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Understanding closure is important, as it enables deeper knowledge of how variables and functions relate to one another and how JavaScript code works and executes.
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--------------------------------------------------------------------------------
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via: https://opensource.com/article/21/2/javascript-closures
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作者：[Nimisha Mukherjee][a]
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选题：[lujun9972][b]
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译者：[译者ID](https://github.com/译者ID)
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校对：[校对者ID](https://github.com/校对者ID)
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本文由 [LCTT](https://github.com/LCTT/TranslateProject) 原创编译，[Linux中国](https://linux.cn/) 荣誉推出
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[a]: https://opensource.com/users/nimisha
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[b]: https://github.com/lujun9972
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[1]: https://opensource.com/sites/default/files/styles/image-full-size/public/lead-images/programming-code-keyboard-laptop-music-headphones.png?itok=EQZ2WKzy (Woman programming)
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[2]: https://opensource.com/article/20/11/javascript-popular
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[3]: https://developer.mozilla.org/en-US/docs/Web/JavaScript/Closures
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[4]: https://opensource.com/sites/default/files/uploads/execution-context.png (Execution context for JS code)
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[5]: https://creativecommons.org/licenses/by-sa/4.0/
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@ -0,0 +1,136 @@
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[#]: collector: (lujun9972)
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[#]: translator: (wxy)
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[#]: reviewer: ( )
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[#]: publisher: ( )
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[#]: url: ( )
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[#]: subject: (A practical guide to JavaScript closures)
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[#]: via: (https://opensource.com/article/21/2/javascript-closures)
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[#]: author: (Nimisha Mukherjee https://opensource.com/users/nimisha)
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JavaScript 闭包实践
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======
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> 通过深入了解 JavaScript 的高级概念之一：闭包，更好地理解 JavaScript 代码的工作和执行方式。
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![女性编程][1]
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在《[JavaScript 如此受欢迎的 4 个原因][2]》中，我提到了一些高级 JavaScript 概念。在本文中，我将深入探讨其中的一个概念：<ruby>闭包<rt>closure</rt></ruby>。
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根据 [Mozilla 开发者网络][3]（MDN），“闭包是将一个函数和对其周围的状态（词法环境）的引用捆绑在一起（封闭）的组合。”简而言之，这意味着在一个函数内部的函数可以访问其外部（父）函数的变量。
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为了更好地理解闭包，可以看看作用域及其执行上下文。
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下面是一个简单的代码片段：
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```
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var hello = "Hello";
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function sayHelloWorld() {
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var world = "World";
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function wish() {
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var year = "2021";
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console.log(hello + " " + world + " "+ year);
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}
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wish();
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}
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sayHelloWorld();
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```
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下面是这段代码的执行上下文：
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![JS 代码的执行上下文][4]
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每次创建函数时（在函数创建阶段）都会创建闭包。每个闭包有三个作用域。
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* 本地作用域（自己的作用域）
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* 外部函数范围
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* 全局范围
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我稍微修改一下上面的代码来演示一下闭包：
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```
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var hello = "Hello";
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var sayHelloWorld = function() {
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var world = "World";
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function wish() {
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var year = "2021";
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console.log(hello + " " + world + " "+ year);
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}
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return wish;
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}
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var callFunc = sayHelloWorld();
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callFunc();
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```
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内部函数 `wish()` 在执行之前就从外部函数返回。这是因为 JavaScript 中的函数形成了**闭包**。
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* 当 `sayHelloWorld` 运行时，`callFunc` 持有对函数 `wish` 的引用。
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* `wish` 保持对其周围（词法）环境的引用，其中存在变量 `world`。
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### 私有变量和方法
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本身，JavaScript 不支持创建私有变量和方法。闭包的一个常见和实用的用途是模拟私有变量和方法，并允许数据隐私。在闭包范围内定义的方法是有特权的。
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这个代码片段捕捉了 JavaScript 中闭包的常用编写和使用方式：
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```
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var resourceRecord = function(myName, myAddress) {
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var resourceName = myName;
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var resourceAddress = myAddress;
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var accessRight = "HR";
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return {
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changeName: function(updateName, privilege) {
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// only HR can change the name
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if (privilege === accessRight ) {
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resourceName = updateName;
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return true;
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} else {
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return false;
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}
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},
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changeAddress: function(newAddress) {
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// any associate can change the address
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resourceAddress = newAddress;
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},
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showResourceDetail: function() {
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console.log ("Name:" + resourceName + " ; Address:" + resourceAddress);
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}
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}
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}
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// Create first record
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var resourceRecord1 = resourceRecord("Perry","Office");
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// Create second record
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var resourceRecord2 = resourceRecord("Emma","Office");
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// Change the address on the first record
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resourceRecord1.changeAddress("Home");
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resourceRecord1.changeName("Perry Berry", "Associate"); // Output is false as only an HR can change the name
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resourceRecord2.changeName("Emma Freeman", "HR"); // Output is true as HR changes the name
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resourceRecord1.showResourceDetail(); // Output - Name:Perry ; Address:Home
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resourceRecord2.showResourceDetail(); // Output - Name:Emma Freeman ; Address:Office
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```
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资源记录（`resourceRecord1` 和 `resourceRecord2`）相互独立。每个闭包通过自己的闭包引用不同版本的 `resourceName` 和 `resourceAddress` 变量。你也可以应用特定的规则来处理私有变量，我添加了一个谁可以修改 `resourceName` 的检查。
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### 使用闭包
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理解闭包是很重要的，因为它可以更深入地了解变量和函数之间的关系，以及 JavaScript 代码如何工作和执行。
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--------------------------------------------------------------------------------
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via: https://opensource.com/article/21/2/javascript-closures
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作者：[Nimisha Mukherjee][a]
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选题：[lujun9972][b]
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译者：[wxy](https://github.com/wxy)
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校对：[校对者ID](https://github.com/校对者ID)
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本文由 [LCTT](https://github.com/LCTT/TranslateProject) 原创编译，[Linux中国](https://linux.cn/) 荣誉推出
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[a]: https://opensource.com/users/nimisha
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[b]: https://github.com/lujun9972
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[1]: https://opensource.com/sites/default/files/styles/image-full-size/public/lead-images/programming-code-keyboard-laptop-music-headphones.png?itok=EQZ2WKzy (Woman programming)
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[2]: https://linux.cn/article-12830-1.html
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[3]: https://developer.mozilla.org/en-US/docs/Web/JavaScript/Closures
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[4]: https://opensource.com/sites/default/files/uploads/execution-context.png (Execution context for JS code)
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[5]: https://creativecommons.org/licenses/by-sa/4.0/
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