node-js-event-emitter.js

function Emitter() {
  this.events = {};
}

Emitter.prototype.addListener = function (type, listener) {
  // check if the listener is a function and throw error if it is not
  if (typeof listener !== "function") {
    throw new Error("Listener must be a function!");
  }
  // create the event listener property (array) if it does not exist.
  this.events[type] = this.events[type] || [];
  // adds listners to the events array.
  this.events[type].push(listener);
};

Emitter.prototype.on = function (type, listener) {
  return this.addListener(type, listener);
};

Emitter.prototype.listenerCount = function (type) {
  let listenersCount = 0;
  let listeners = this.events[type] || [];
  listenersCount = listeners.length;
  console.log("listenersCount", listenersCount);
  return listenersCount;
};

Emitter.prototype.emit = function (type) {
  if (this.events[type]) {
    // checks if event is a property on Emitter
    this.events[type].forEach(function (listener) {
      // loop through that events array and invoke all the listeners inside it.
      listener();
    });
  }
};

const eventEmitter = new Emitter();

eventEmitter.on("greet", () => {
  console.log("Hello World!");
});

eventEmitter.on("greet", () => {
  console.log("Hello from LogRocket!");
});

eventEmitter.on("greet", () => {
  console.log("Hello from Eagles!");
});

eventEmitter.listenerCount("greet");

eventEmitter.emit("greet"); // manually emit an event