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How to create private variables in JavaScript?

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如何在 JavaScript 中创建私有变量？

Closures, symbols, WeakMap, class private fields, and proxies are some of the techniques that can be used to create private variables in JavaScript. Each technology has advantages and disadvantages, so it is crucial to choose the one that best suits your requirements.

Like many other programming languages, JavaScript also has the concept of private variables and public variables. Private variables can only be accessed and changed by code belonging to the same scope, but public variables can be accessed and changed by any code. Let’s look at the different techniques for creating private variables in JavaScript.

Use closures

The closure method is a way to create private variables in JavaScript. A function is a closure if it can access variables defined in the scope of its parent function even after the parent function has completed and returned. We can define a variable inside a function so that it becomes private and can only be accessed by code inside that function.

Example

<html>
<body>
   <div id="demo"></div>
   <script>
      function createPrivateVariable() {
         let privateVariable = "This is a private variable";
         return {
            getValue: function () {
               return privateVariable;
            },
            setValue: function (value) {
               privateVariable = value;
            },
         };
      }
      let privateVar = createPrivateVariable();
      document.getElementById("demo").innerHTML = privateVar.getValue();
   </script>
</body>
</html>

The function createPrivateVariable in the above example returns an object with methods getValue and setValue. These methods can retrieve or change the value of the privateVariable declared in the parent function because they have access to it. If you try to access privateVariable from outside a function, a reference error occurs.

Use symbolic data types

Using symbolic data types is the second way to create private variables. Symbols can be used as property keys because they are distinct non-string identifiers. Because they are unique, external programs cannot easily access or change them.

let privateVariable = Symbol();
let obj = {
   [privateVariable]: "This is a private variable"
};
console.log(obj[privateVariable]);

Example

We can use the above code as follows -

<html>
<body>
   <div id="demo"></div>
   <script>
      let privateVar = Symbol();
      let obj = {
         [privateVar]: "This is a private variable"
      };
      Object.defineProperty(obj, 'getValue', {
         get: function() {
            return obj[privateVar];
         }
      });
      document.getElementById("demo").innerHTML = obj.getValue;
   </script>
</body>
</html>

In this example, a symbol named privateVariable has been defined and used as the object's property key. Because it is a Symbol, you cannot get the value of the property using dot notation, but you can access it through the object using square bracket notation.

Use WeakMap

WeakMaps can be used as a third way to construct private variables. The JavaScript engine only weakly references key-value pairs in a WeakMap, which allows you to link objects with keys. This makes it harder to incorrectly maintain a reference to a private variable, since the garbage collector will destroy the key-value pair if there are no other references to the key.

Example

<html>
<body>
   <div id="demo"></div>
   <script>
      let privateVariables = new WeakMap();
      let obj = {};
      privateVariables.set(obj, "This is a private variable");
      document.getElementById("demo").innerHTML = privateVariables.get(obj);
   </script>
</body>
</html>

In this example, we create a WeakMap named privateVariables to save the private variables of the object. The get() method is used to obtain a private variable after linking it to an object using the set() method. However, a private variable cannot be accessed from outside the scope formed by the object because you can access it only when you have a reference to the object.

Use object-oriented class syntax

Using object-oriented class syntax can also be used to create private variables. JavaScript’s class keyword enables us to define classes that act as templates for objects. Classes can define variables and add the # symbol before the variable name to generate private variables; this is an experimental feature and represents private variables. It is not recommended to use this feature in production code as it is not yet widely supported.

Example

<html>
<body>
   <p id="output"></p>
   <script>
      class MyClass {
         #privateVariable = "This is a private variable";
         getValue() {
            return this.#privateVariable;
         }
         setValue(value) {
            this.#privateVariable = value;
         }
      }
      let obj = new MyClass();
      document.getElementById("output").innerHTML = obj.getValue(); // "This is a private variable"
      obj.setValue("New value");
      document.getElementById("output").innerHTML = obj.getValue(); // "New value"
   </script>
</body>
</html>

In this example, a class named MyClass is constructed using private variables #privateVariable, getValue, and setValue. A reference error occurs if a method outside the class attempts to access a private variable that can only be accessed by methods inside the class.

Use proxy objects

Finally, using a Proxy object is another option for constructing private variables. Objects that can be used to intercept or change the behavior of other objects are called proxies. You can create private variables that can only be accessed by code with a proxy by encapsulating the object in a proxy.

Example

<html>
<body>
   <div id="demo"></div>
   <script>
      let privateVariable = "This is a private variable-proxy obj.";
      let handler = {
         get: function (target, name) {
            if (name === "privateVariable") {
               return privateVariable;
            }
         },
         set: function (target, name, value) {
            if (name === "privateVariable") {
               privateVariable = value;
            }
         },
      };
      let obj = new Proxy({}, handler);
      document.getElementById("demo").innerHTML = obj.privateVariable;
   </script>
</body>
</html>

In this example, a proxy with handlers with getter and setter methods has been built. These methods can retrieve or change the value of a private variable defined outside the proxy and access it. However, due to a reference error, the private variable will not be accessible from outside the proxy.

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See more

How to create private variables in JavaScript?

Use closures

Example

Use symbolic data types

Example

Use WeakMap

Example

Use object-oriented class syntax

Example

Use proxy objects

Example

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