事件：'newListener'

【Event: 'newListener'】

• eventName <string> | <symbol> 正在监听的事件名称
• listener <Function> 事件处理函数

EventEmitter 实例将在监听器被添加到其内部监听器数组之前 发出 自身的 'newListener' 事件。

【The EventEmitter instance will emit its own 'newListener' event before a listener is added to its internal array of listeners.】

注册了 'newListener' 事件的监听器会接收到事件名称以及正在添加的监听器的引用。

【Listeners registered for the 'newListener' event are passed the event name and a reference to the listener being added.】

事件在添加监听器之前触发这一事实有一个微妙但重要的副作用：在 'newListener' 回调中注册到同一 name 的任何 额外 监听器都会被插入到正在添加的监听器之前。

【The fact that the event is triggered before adding the listener has a subtle but important side effect: any additional listeners registered to the same name within the 'newListener' callback are inserted before the listener that is in the process of being added.】

import { EventEmitter } from 'node:events';
class MyEmitter extends EventEmitter {}

const myEmitter = new MyEmitter();
// Only do this once so we don't loop forever
myEmitter.once('newListener', (event, listener) => {
  if (event === 'event') {
    // Insert a new listener in front
    myEmitter.on('event', () => {
      console.log('B');
    });
  }
});
myEmitter.on('event', () => {
  console.log('A');
});
myEmitter.emit('event');
// Prints:
//   B
//   Aconst EventEmitter = require('node:events');
class MyEmitter extends EventEmitter {}

const myEmitter = new MyEmitter();
// Only do this once so we don't loop forever
myEmitter.once('newListener', (event, listener) => {
  if (event === 'event') {
    // Insert a new listener in front
    myEmitter.on('event', () => {
      console.log('B');
    });
  }
});
myEmitter.on('event', () => {
  console.log('A');
});
myEmitter.emit('event');
// Prints:
//   B
//   A拷贝