一个简单的EventEmitter模式的实现

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

  on(key, cb) {
    if (!this.listeners[key]) {
      this.listeners[key] = [];
    }
    this.listeners[key].push(cb);
  }

  emit(key, args) {
    const cbs = this.listeners[key] || [];
    cbs.forEach((cb) => {
      cb.apply(this, args);
    });
  }

  removeListener(key, listener) {
    const cbs = this.listeners[key] || [];
    const idx = cbs.indexOf(listener);
    if (idx === -1) {
      return;
    }
    cbs.splice(idx, 1);
  }
}

/****************************************分割线****************************************/
// 用例1:
const emitter = new EventEmitter();
emitter.on("a", () => {
  console.log("1");
});
emitter.on("a", () => {
  console.log("2");
});
emitter.on("b", () => {
  console.log("12");
});
console.log("我是一个断点打印, 时间：15:16:12", emitter, emitter.listeners);
emitter.emit("a");