在 JavaScript 中创建真正的私有方法有什么缺点？

根据 c++、 Java等其他编程语言，私有是不能与其他类共享的东西，不能直接使用对象访问。同样的规则也适用于 JavaScript。

所以首先我们必须看看如何使用 JavaScript 生成私有方法？主要有四个关键字使用我们可以在类中创建一个私有方法。

1. 变量
2. 让
3. 常量
4. #（哈希）

例子：

Javascript

// class Geek
  var Geek = function (article, published) {
      // public member
      this.programmingLang = "Js";
  
      // private member
      var topic = "private_method";
      var status = published;
  
      // private member function
      var publish = () => {
          console.log(article);
          return status;
      }
  
      // calling private member 
      // function inside same class
      console.log(publish());
  };
  
  // Geek1 is object of Geek class
  var Geek1 = new Geek('ABC', 'YES');
  
  // calling public member outside the class
  console.log(Geek1.programmingLang);
<script/></code></pre></div><br><hr><h5 class="code">Javascript</h5><div class="hcb_wrap"><pre class="prism undefined-numbers lang-js" data-lang="JavaScript"><code class="replace"><script>
    var Geek = function (article, published) {
        this.programmingLang = "Js";
        var topic = "private_method";
        var status = published;
        var publish = () => {
            console.log(article);
            return status;
        }
        console.log(topic);
    };
    var Geek1 = new Geek('ABC', 'YES');
    console.log(Geek1.programmingLang);
    console.log(publish());
<script /></code></pre></div><br><hr><h5 class="code">Javascript</h5><div class="hcb_wrap"><pre class="prism undefined-numbers lang-js" data-lang="JavaScript"><code class="replace"><script>  
  class A {
      // Private field
      #foo;
      constructor(foo) {
          this.#foo = "ABC";
      }
      #privateFun(prefix) {
          return prefix + this.#foo;
      }
      publicFun() {
          return this.#privateFun(">>");
      }
  }
  let a = new A();
  console.log(a.publicFun());
<script/></code></pre></div><br><hr><h5 class="code">Javascript</h5><div class="hcb_wrap"><pre class="prism undefined-numbers lang-js" data-lang="JavaScript"><code class="replace"><script>
    var Employee = function (name, comp, sal) {
        this.name = name;
        this.company = comp;
        this.salary = sal;
  
        // Private method
        var Sal_increase = () => {
            this.salary = this.salary + 45000;
        };
  
        // Public method
        this.dispalySal = () => {
            Sal_increase();
            console.log(this.salary);
        };
    };
  
    const A = new Employee("A", "dream", 45000);
    const B = new Employee("B", "young", 6000);
    const C = new Employee("C", "inspire", 9000);
  
    A.dispalySal()
    B.dispalySal()
    C.dispalySal()

输出：

ABC
YES
Js

在 JavaScript 中创建真正的私有方法有两个主要缺点。

1. 不能在类外调用私有方法。
2. 当为同一个类创建不同的对象时，会造成内存效率低下，因为将为每个实例创建方法的新副本。

示例：如果我们在类外部调用私有成员函数，那么在这种情况下会出错。

Javascript

    var Geek = function (article, published) {
        this.programmingLang = "Js";
        var topic = "private_method";
        var status = published;
        var publish = () => {
            console.log(article);
            return status;
        }
        console.log(topic);
    };
    var Geek1 = new Geek('ABC', 'YES');
    console.log(Geek1.programmingLang);
    console.log(publish());
<script />
</code></pre></div></div></div><p><strong>输出：</strong> </p><p><img src="https://mangodoc.oss-cn-beijing.aliyuncs.com/geek8geeks/What_is_the_drawback_of_creating_true_private_methods_in_JavaScript_?_0.jpg" srcset="https://media.geeksforgeeks.org/wp-content/uploads/20220224012413/Annotation20220224012113.png," sizes="100vw" width="300" class="aligncenter"></p><p>但下面的例子给出了正确的输出：</p><div class="responsive-tabs"><h2 class="tabtitle"> Javascript </h2><div class="tabcontent"><div class="hcb_wrap"><pre class="prism undefined-numbers lang-js" data-lang="JavaScript"><code class="replace"><script>  
  class A {
      // Private field
      #foo;
      constructor(foo) {
          this.#foo = "ABC";
      }
      #privateFun(prefix) {
          return prefix + this.#foo;
      }
      publicFun() {
          return this.#privateFun(">>");
      }
  }
  let a = new A();
  console.log(a.publicFun());
<script/>
</code></pre></div></div></div><p><strong>输出：</strong> </p><div class="hcb_wrap"><pre class="prism undefined-numbers lang-js" data-lang="JavaScript"><code class="replace">>>ABC                </code></pre></div><p><strong>示例：</strong>如果我们为同一个类创建不同的对象，那么在这种情况下，每个对象都会创建自己的实例，或者说是自己的函数副本。那么在这种情况下，就会出现内存效率低下的问题</p><div class="responsive-tabs"><h2 class="tabtitle">Javascript </h2><div class="tabcontent"><div class="hcb_wrap"><pre class="prism undefined-numbers lang-js" data-lang="JavaScript"><code class="replace"><script>
    var Employee = function (name, comp, sal) {
        this.name = name;
        this.company = comp;
        this.salary = sal;
  
        // Private method
        var Sal_increase = () => {
            this.salary = this.salary + 45000;
        };
  
        // Public method
        this.dispalySal = () => {
            Sal_increase();
            console.log(this.salary);
        };
    };
  
    const A = new Employee("A", "dream", 45000);
    const B = new Employee("B", "young", 6000);
    const C = new Employee("C", "inspire", 9000);
  
    A.dispalySal()
    B.dispalySal()
    C.dispalySal()

输出：

90000
51000
54000

注意：这里提到的代码可以在 HTML 或控制台中运行。