This lesson's interactive features are locked, please to keep using them

Private Properties

By default, all properties of a class are public, meaning they can be accessed and modified from outside the class. Here's an example:

class Movie {
  constructor(title, rating) {
    this.title = title;
    this.rating = rating;
  }
}

const matrixMovie = new Movie("The Matrix", 9.5);
console.log(matrixMovie.title);
// The Matrix
matrixMovie.title = "The Matrix Reloaded";
console.log(matrixMovie.title);
// The Matrix Reloaded

Maybe we don't want our title to be able to be changed anywhere in our code. We can make it private by prefixing it with a hash # and declaring it at the top of the class:

class Movie {
  #title;
  constructor(title, rating) {
    this.#title = title;
    this.rating = rating;
  }
}

const matrixMovie = new Movie("The Matrix", 9.5);
console.log(matrixMovie.#title);
// Uncaught SyntaxError: Private field '#title' must be declared in an enclosing class

Private properties can still be used from within the class:

class Movie {
  #title;
  constructor(title, rating) {
    this.#title = title;
    this.rating = rating;
  }

  getTitleAllCaps() {
    const allCaps = this.#title.toUpperCase();
    return allCaps;
  }
}

const matrixMovie = new Movie("The Matrix", 9.5);
console.log(matrixMovie.getTitleAllCaps());
// THE MATRIX

Encapsulation in JavaScript is typically enforced at two levels:

The class level: Public/private methods using # for private fields
The module level: Exporting only what you want to be public (we'll talk about modules later)

Assignment

Textio messages have a createdAt field that stores the date the message was created. In order to avoid accidentally altering the creation date, change the createdAt field from a public field to a private field.