Revealing Constructor

Revealing constructor is used in JavaScript, for example, to implement Promise class. The constructor of Promise class takes function with two parameters (resolve and reject) into constructor. It is perfect way to hide implementation details and there is no way somebody can mess around with resolve and reject after the instance if created.

Example - Read-only event emitter

The emit function of ReadOnlyEmitter can be called only in function that is passed into the constructor.

const EventEmitter = require('events');

class ReadOnlyEmitter extends EventEmitter {
  constructor (executor) {
    super();
    const emit = this.emit.bind(this);
    this.emit = undefined; // hide emit function
    executor(emit);
  }
};

const ticker = new ReadOnlyEmitter((emit) => {
  let tickCount = 0;
  setInterval(() => emit('tick', tickCount++), 1000);
});

ticker.on('tick', (count) => console.log('Tick', count));