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class EventEmitter {  constructor() {    // This is an Object containing Maps:    //    // { [event: string]: Map<listener: function, numTimesAdded: number> }    //    // We use a Map for O(1) insertion/deletion and because it can have functions as keys.    //    // We keep track of numTimesAdded (the number of times it was added) because if you attach the same listener twice,    // we should actually call it twice for each emitted event.    this.observers = {};  }
  on(events, listener) {    events.split(' ').forEach((event) => {      if (!this.observers[event]) this.observers[event] = new Map();      const numListeners = this.observers[event].get(listener) || 0;      this.observers[event].set(listener, numListeners + 1);    });    return this;  }
  off(event, listener) {    if (!this.observers[event]) return;    if (!listener) {      delete this.observers[event];      return;    }
    this.observers[event].delete(listener);  }
  emit(event, ...args) {    if (this.observers[event]) {      const cloned = Array.from(this.observers[event].entries());      cloned.forEach(([observer, numTimesAdded]) => {        for (let i = 0; i < numTimesAdded; i++) {          observer(...args);        }      });    }
    if (this.observers['*']) {      const cloned = Array.from(this.observers['*'].entries());      cloned.forEach(([observer, numTimesAdded]) => {        for (let i = 0; i < numTimesAdded; i++) {          observer.apply(observer, [event, ...args]);        }      });    }  }}
export default EventEmitter;