EventEmitter.js

// Class

class Emitter {
  constructor (events = []) {
    this.events = new Map(events)
  }

  on (name, cb) {
    this.events.set(name, [...(this.events.get(name) || []), cb])
    return () => this.events.set(name, this.events.get(name).filter(fn => fn !== cb))
  }
    
  once (name, cb) {
    let unsubscribe;
    const onceCb = (...args) => {
       cb(...args)
       unsubscribe()
    }
    unsubscribe = this.on( name, onceCb )
    return unsubscribe
  }

  trigger (name, ...args) {
    return this.events.has(name) && this.events.get(name).map(fn => fn(...args))
  }
}

// func

const pubSub = () => {
    const events = new Map();
    const sub = (name,cb) => {
        events.set(name, (events.get(name) || new Set()).add(cb));
        return () => events.get(name).delete(cb);
    };

    return {
        sub,
        emit: (name,...args) => {
            events.has(name) && events.get(name).forEach(cb => cb(...args));
        },
        once: (name,cb) => {
            let unsub;
            const newcb = (...args) => {
                unsub();
                cb(...args);
            }
            return unsub = sub(name, newcb);
        }
    };
}
  
const ps = pubSub();

const fn1 = () => console.log(1)
const fn2 = (msg) => console.log(msg)
const fn3 = (msg, msg2) => console.log(msg + msg2)
const fn4 = () => console.log('Only once')

const offSubChangeFn1 = ps.sub('change', fn1)
const offSubChangeFn2 = ps.sub('change', fn2)
const offSubChangeFn3 = ps.sub('change', fn3)
const offSubceChangeFn4 = ps.once('change', fn4)

offSubChangeFn3()
offSubceChangeFn4()

ps.emit('change', 'hello', ' world')

const ee = new Emitter();

const offOnChangeFn1 = ee.on('change', fn1)
const offOnChangeFn2 = ee.on('change', fn2)
const offOnChangeFn3 = ee.on('change', fn3)
const offOnceChangeFn4 = ee.once('change', fn4)

offOnChangeFn3()

ee.trigger('change', 'hello', ' world')




/*
class EventEmitter {
    
    constructor() {
        this.events = {};
    }

    on(eventName, callback) {
        this.events[eventName] = this.events[eventName] || [];
        this.events[eventName].push(callback)        
    }

    off(eventName, callback) {
        if (this.events[eventName]) {
            for (let i = 0; i < this.events[eventName].length; i++) {
                if (this.events[eventName][i] === callback) {
                    this.events[eventName].splice(i,1);
                    break; 
                }
            }
        }
    }

    trigger(eventName, ...rest) {
        this.events[eventName].forEach(fn => fn(...rest))
    }
}
*/