Online/offline emitter in vanilla JS

vanilla-online-offline.js

function createOnlineEmitter() {
  let cbs = []; //array of registered callbacks for the event
  let unsub; //function for removing the main event listener

  //this is the main event listener that gets registered with window.online/offline event
  const mainListener = (isOnline) => {
    //call all the subscribed callbacks
    cbs.forEach(cb => cb(isOnline));
  };

  const registerMainListener = () => {
    const boundOnline = mainListener.bind(null, true);
    const boundOffline = mainListener.bind(null, false);
    window.addEventListener('online', boundOnline);
    window.addEventListener('offline', boundOffline);
    //return unsubcribe functionality in a closure
    return function unsubscribe() {
      window.removeEventListener('online', boundOnline);
      window.removeEventListener('offline', boundOffline);
    };
  };

  const addCb = (cb) => {
    cbs.push(cb);
    //register main listener only once
    //use existence of `unsub` as indicator if main event listener is added or not
    if(!unsub) {
      unsub = registerMainListener();
    }
  };
  
  const removeCb = (cb) => {
    const index = cbs.indexOf(cb);
    if(index > -1) {
      cbs.splice(index, 1);
    }
    //if no callbacks left, remove main event listener
    if(cbs.length === 0 && unsub) {
      unsub();
      unsub = null;
    }
  };

  return function initOnlineEmitter(cb) {
    addCb(cb);
    //call it async with the initial val
    setTimeout(() => {
      cb(navigator.onLine);
    });
    //return unsubscribe function to caller
    return removeCb.bind(null, cb);
  };
}

// implement it 
const onlineEmitter = createOnlineEmitter();
let unsub = onlineEmitter(isOnline => console.log(isOnline));
unsub();