向监听器传递参数和 this

【Passing arguments and this to listeners】

eventEmitter.emit() 方法允许将任意一组参数传递给监听器函数。请注意，当调用普通的监听器函数时，标准的 this 关键字会被故意设置为引用附加了该监听器的 EventEmitter 实例。

【The eventEmitter.emit() method allows an arbitrary set of arguments to be passed to the listener functions. Keep in mind that when an ordinary listener function is called, the standard this keyword is intentionally set to reference the EventEmitter instance to which the listener is attached.】

import { EventEmitter } from 'node:events';
class MyEmitter extends EventEmitter {}
const myEmitter = new MyEmitter();
myEmitter.on('event', function(a, b) {
  console.log(a, b, this, this === myEmitter);
  // Prints:
  //   a b MyEmitter {
  //     _events: [Object: null prototype] { event: [Function (anonymous)] },
  //     _eventsCount: 1,
  //     _maxListeners: undefined,
  //     [Symbol(shapeMode)]: false,
  //     [Symbol(kCapture)]: false
  //   } true
});
myEmitter.emit('event', 'a', 'b');const EventEmitter = require('node:events');
class MyEmitter extends EventEmitter {}
const myEmitter = new MyEmitter();
myEmitter.on('event', function(a, b) {
  console.log(a, b, this, this === myEmitter);
  // Prints:
  //   a b MyEmitter {
  //     _events: [Object: null prototype] { event: [Function (anonymous)] },
  //     _eventsCount: 1,
  //     _maxListeners: undefined,
  //     [Symbol(shapeMode)]: false,
  //     [Symbol(kCapture)]: false
  //   } true
});
myEmitter.emit('event', 'a', 'b');拷贝

可以将 ES6 箭头函数用作监听器，但是这样做时，this 关键字将不再引用 EventEmitter 实例：

【It is possible to use ES6 Arrow Functions as listeners, however, when doing so, the this keyword will no longer reference the EventEmitter instance:】

import { EventEmitter } from 'node:events';
class MyEmitter extends EventEmitter {}
const myEmitter = new MyEmitter();
myEmitter.on('event', (a, b) => {
  console.log(a, b, this);
  // Prints: a b undefined
});
myEmitter.emit('event', 'a', 'b');const EventEmitter = require('node:events');
class MyEmitter extends EventEmitter {}
const myEmitter = new MyEmitter();
myEmitter.on('event', (a, b) => {
  console.log(a, b, this);
  // Prints: a b {}
});
myEmitter.emit('event', 'a', 'b');拷贝