EventEmiter es6 class intented to extend existing classes

EventEmitter.js

class EventEmitter {
  constructor() {
    this.cbs = {};
  }
  on(evt, cb) {
    if (evt && typeof cb === 'function') {
      if (!this.cbs[evt]) {
        this.cbs[evt] = [];
      }
      this.cbs[evt].push(cb);
    }
  }
  emit(evt) {
    const cbs = this.cbs[evt];
    if (cbs && cbs.length) {
      const args = Array.prototype.slice.call(arguments, 1);
      for (var i = 0, l = cbs.length; i < l; i++) {
        const cb = cbs[i];
        cb.apply(cb, args);
      }
    }
  }
  off(evt, cb) {
    if (evt) {
      if (typeof cb === 'function') {
        const i = this.cbs[evt].indexOf(cb);
        if (i !== -1) {
          this.cbs[evt].splice(i, 1);
        }
      } else {
        this.cbs[evt] = [];
      }
    }
  }
}