笔试题：EventEmitter 实现 ​

class EventEmitter {
  constructor() {
    this.events = {};
  }

  on(event, callback) {
    let callbacks = this.events[event] || [];
    callbacks.push(callback);
    this.events[event] = callbacks;

    return this;
  }

  off(event, callback) {
    let callbacks = this.events[event];
    this.events[event] = callbacks && callbacks.filter((fn) => fn !== callback);

    return this;
  }

  emit(event, ...args) {
    let callbacks = this.events[event];
    callbacks.forEach((fn) => {
      fn(...args);
    });

    return this;
  }

  once(event, callback) {
    let wrapFun = (...args) => {
      callback(...args);

      this.off(event, wrapFun);
    };
    this.on(event, wrapFun);

    return this;
  }
}