Private instance field of JavaScript of a class example.

private-instance-field.js

/**
 * Not precisely a "read-only" property but it can (in the near future) behave as one with 
 * ES2019 spec private instance fields. With a single get method and a "private instance field" 
 * it is possible to simulate the effect, like the examples above, with the added bonus of having 
 * the private property truly hidden.
 *
 * Although MDN is not the ultimate authority, their examples don't show the necessity to have a `setter`
 * with a `getter`. Making having only a `getter` already a case of having a "read-only" value. 
 *
 * @link https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Classes/Class_fields
 */
class Counter {

   #count = 0;

   increment() {
      this.#count++;
   }

   decrement() {
      this.#count--;
   }

   get currentCount() {
      return this.#count;
   }

}

const counter = new Counter();
counter.increment();
counter.increment();
counter.decrement();
counter.currentCount; // 1
counter.currentCount = 3; // 3
counter.currentCount = 'everything'; // 'everything'
counter.currentCount; // 1
counter.count; // undefined;
counter.#count // Uncaught SyntaxError: Private field '#count' must be declared in an enclosing class