LazyEventListeners.js

function LazyEventListeners(){
    this._listeners = {};
}

LazyEventListeners.prototype = {
    constructor: LazyEventListeners,
    has: function has(propertyKey){
        return this._listeners.hasOwnProperty(propertyKey);
    },
    _createListener: function _createListener(propertyKey, eventTarget,
                                              eventType, eventHandler){
        var listener = {
            key: propertyKey,
            target: eventTarget,
            type: eventType,
            handler: eventHandler,
            value: undefined,
            lastEvent: null,
            listener: function(e){
                listener.lastEvent = e;
                listener.ready = false;
            }.bind(this),
            ready: false
        };
        eventTarget.addEventListener(eventType, listener.listener);
        return listener;
    },
    set: function set(propertyKey, eventTarget, eventType, eventHandler){
        if(this.has(propertyKey)){
            throw new Error(propertyKey + ' has already been assigned');
        }
        this._listeners[propertyKey] = this._createListener(
            propertyKey, eventTarget, eventType, eventHandler);
    },
    get: function get(propertyKey){
        if(!this.has(propertyKey)) return;
        var listener = this._listeners[propertyKey];
        if(!listener.ready){
            listener.value = listener.handler(
                listener.value, listener.lastEvent);
            listener.ready = true;
        }
        return listener.value;
    },
    remove: function remove(propertyKey){
        if(!this.has(propertyKey)) return;
        var listener = this._listeners[propertyKey];
        listener.target.removeEventListener(listener.type, listener.listener);
        this._listeners[propertyKey] = null;
        delete this._listeners[propertyKey];
    }
};