Simple events emitter in JavaScript

EventsEmitter.js

export default class EventsEmitter{
    constructor(){
        this._events = {};
    }

    on(event, callback, once){
        if( !(this._events[event] instanceof Array) ){
            this._events[event] = [];
        }

        this._addListener(event, {
            once: Boolean(once),
            callback: callback
        });

        return this;
    }

    off(event, callback, once){
        if( !this._events[event] ){
            return null;
        }

        once = Boolean(once);

        for(let index = 0; index < this._events[event].length; index++){
            if( (this._events[event][index].once === once) && (this._events[event][index].callback.toString() === callback.toString()) ){
                this._removeListener(event, index);
                return true;
            }
        }

        return false;
    }

    once(event, callback){
        return this.on(event, callback, true);
    }

    emit(event, ...params){
        if( !this._events[event] ){
            return;
        }

        for( let index = 0; index < this._events[event].length; index++ ){
            let listener = this._events[event][index];

            if( !listener ){
                continue;
            }

            if( typeof listener.callback === 'function' ){
                listener.callback(...params);
            }

            if( listener.once ){
                this._removeListener(event, index);
            }
        }

        this._events[event] = this._events[event].filter((listener) => {
            return listener !== null;
        })
    }

    _addListener(event, listener){
        this._events[event].push(listener);
    }

    _removeListener(event, index){
        return this._events[event][index] = null;
    }
}