3. What are private member variables. in js

Private Member Variables in JavaScript

Private member variables in JavaScript are variables that are only accessible within the scope of a specific class or object. They are not directly accessible from outside the class or object, providing encapsulation and data hiding.

To define private member variables in JavaScript, you can use the concept of closures or the new private class fields syntax introduced in ECMAScript 2022.

Using Closures

One common way to create private member variables in JavaScript is by using closures. In this approach, you define variables within the constructor function of a class, and they are accessible only within the scope of that function. Other methods defined within the constructor function can access these variables, but they are not directly accessible from outside the class.

Here's an example that demonstrates the use of closures to create private member variables in JavaScript:

function MyClass() {
  var privateVariable = 42;

  this.publicMethod = function() {
    console.log(privateVariable);
  };
}

var obj = new MyClass();
obj.publicMethod(); // Output: 42
console.log(obj.privateVariable); // Output: undefined

In the above example, privateVariable is a private member variable that can only be accessed within the MyClass constructor function. The publicMethod is a public method that can access the private variable and log its value.

Using Private Class Fields

Starting from ECMAScript 2022, JavaScript introduced a new syntax for defining private member variables using private class fields. This syntax allows you to define private variables directly within the class definition using the # symbol.

Here's an example that demonstrates the use of private class fields to create private member variables in JavaScript:

class MyClass {
  #privateVariable = 42;

  publicMethod() {
    console.log(this.#privateVariable);
  }
}

const obj = new MyClass();
obj.publicMethod(); // Output: 42
console.log(obj.#privateVariable); // SyntaxError: Private field '#privateVariable' must be declared in an enclosing class

In the above example, #privateVariable is a private member variable that can only be accessed within the MyClass class. The publicMethod is a public method that can access the private variable and log its value. However, attempting to access the private variable directly from outside the class will result in a SyntaxError.

Please note that private class fields are a relatively new feature in JavaScript and may not be supported in all environments or browsers. It's important to check the compatibility of your target environment before using this syntax.