Simple EventEmitter for TS

EventEmitter.ts

interface EventEmitterHandlers {
    [s: string]: IAction1<any>[];
}

class EventEmitter {
    private events: EventEmitterHandlers = {};

    on(event: string, listener: IAction1<any>) {
        !(event in this.events) && this.events[event] = [];
        this.events[event].push(listener);
    }

    once(event: string, listener: IAction1<any>) {
        this.on(event, function _handler (data) {
            this.removeListener(event, _handler);
            listener(data);
        });
    }

    removeListener(event: string, listener?: IAction1<any>) {
        let idx;
        if (event in this.events) {
            if (listener) {
                idx = this.events[event].indexOf(listener);
                if (idx > -1) {
                    this.events[event].splice(idx, 1);
                }
            } else {
                this.events[event] = [];
            }
        }
    }

    emit(event: string, data: any) {
        this.events[event] && this.events[event].forEach((listener) => listener(data));
    }
}