Encapsulation

Finally, let's see how to implement encapsulation in JavaScript. In the last article we discussed how we would like to make the year property of Student private, so we could change the rules about archery classes without breaking any code that uses the Student class.

Here's a declaration of the Student class that does just that:

class Student extends Person {
  #year;

  constructor(name, year) {
    super(name);
    this.#year = year;
  }

  introduceSelf() {
    console.log(`Hi! I'm ${this.name}, and I'm in year ${this.#year}.`);
  }

  canStudyArchery() {
    return this.#year > 1;
  }
}

In this class declaration, #year is a private data property. We can construct a Student object, and it can use #year internally, but if code outside the object tries to access #year the browser throws an error:

const summers = new Student("Summers", 2);

summers.introduceSelf(); // Hi! I'm Summers, and I'm in year 2.
summers.canStudyArchery(); // true

summers.#year; // SyntaxError

Note: Code run in the Chrome console can access private properties outside the class. This is a DevTools-only relaxation of the JavaScript syntax restriction.

Private data properties must be declared in the class declaration, and their names start with #.


Private methods

You can have private methods as well as private data properties. Just like private data properties, their names start with #, and they can only be called by the object's own methods:

class Example {
  somePublicMethod() {
    this.#somePrivateMethod();
  }

  #somePrivateMethod() {
    console.log("You called me?");
  }
}

const myExample = new Example();

myExample.somePublicMethod(); // 'You called me?'

myExample.#somePrivateMethod(); // SyntaxError