获取已注册的侦听器数量以侦听特定事件

函数 Emitter.listenerCount(eventName) 将返回当前正在侦听作为参数提供的事件的侦听器数

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