section11.4.md

Section 11.4: Get the number of listeners registered to listen for a specific event

The function Emitter.listenerCount(eventName) will return the number of listeners that are currently listening for the event provided as argument

let EventEmitter = require("events");
class MyEmitter extends EventEmitter{};

let emitter = new MyEmitter();

emitter
  .on("data", ()=>{ // add listener for data event
    console.log("data event emitter");
  });

console.log(emitter.listenerCount("data")) // => 1
console.log(emitter.listenerCount("message")) // => 0


emitter
  .on("message", function mListener(){ //add listener for message event
    console.log("message event emitted");
  });

console.log(emitter.listenerCount("data")) // => 1
console.log(emitter.listenerCount("message")) // => 1


emitter
  .once("data", (stuff)=>{ //add another listener for data event
    console.log(`Tell me my ${stuff}`);
  });

console.log(emitter.listenerCount("data")) // => 2
console.log(emitter.listenerCount("message"))// => 1
``