Simple! but necessary, Simple JS Event Handler

index.js

const SBNEventHandler = function(){
    let listeners = {};
    let onceListeners = {};

    const $on = function(event, func){
        if(!listeners[event]){
            listeners[event] = [];
        }
        listeners[event].push(func)
    };

    // Remove the whole listener array

    const $off = function(event=undefined){
        if(event === undefined){
            listeners = {};
            onceListeners = {};
        }
        // Delete the event listeners

        if(listeners[event] !== undefined){
            delete listeners[event];
        }
        if(onceListeners[event] !== undefined){
            delete onceListeners[event];
        }
    };

    const $once = function(event, callback){
        if(!onceListeners[event]){
            onceListeners[event] = [];
        }

        // Push the event
        onceListeners[event].push(callback);
    };

    const $emit = function(event, ...args){
        // If the event is registered in listeners

        if(listeners[event]){
            for(let i in listeners[event]){
                listeners[event][i](...args);
            }
        }
        if(onceListeners[event]){
            for(let i in onceListeners[event]){
                onceListeners[event][i](...args);

            }
            // Delete the event functions
            delete onceListeners[event];
        }
    };

    return {
        $on,
        $once,
        $off,
        $emit
    }

};

// Testing
function hurt(name){
    console.log(name + ' Said: "Ouch, It hurts!"');
}

function injured(name){
    console.log(name + ' Must be injured!');
}

var eventHandler = SBNEventHandler();
eventHandler.$on('fall', hurt);

// Must only call hurt
console.log('\nhurt() is called:');
eventHandler.$emit('fall', 'John Doe');
// Add Once Event:
eventHandler.$once('fall', injured);

// Must call both
console.log('\nhurt() and injured() are called:');
eventHandler.$emit('fall', 'John Doe');
// Must call hurt() only
console.log('\nOnly hurt() is called (injured is removed automatically):');
eventHandler.$emit('fall', 'John Doe');

eventHandler.$off('fall');
console.log('\nNothing will happen:');
eventHandler.$emit('fall', 'John Doe');