捕捉 Promise 拒绝

将 async 函数与事件句柄一起使用是有问题的，因为它会在抛出异常的情况下导致未处理的拒绝：

const ee = new EventEmitter();
ee.on('something', async (value) => {
  throw new Error('kaboom');
});

EventEmitter 构造函数中的 captureRejections 选项或全局的设置可以改变这种行为，在 Promise 上安装 .then(undefined, handler) 句柄。 此句柄将异常异步地路由到 Symbol.for('nodejs.rejection') 方法（如果有）或 'error' 事件句柄（如果没有）。

const ee1 = new EventEmitter({ captureRejections: true });
ee1.on('something', async (value) => {
  throw new Error('kaboom');
});

ee1.on('error', console.log);

const ee2 = new EventEmitter({ captureRejections: true });
ee2.on('something', async (value) => {
  throw new Error('kaboom');
});

ee2[Symbol.for('nodejs.rejection')] = console.log;

设置 events.captureRejections = true 将更改 EventEmitter 的所有新实例的默认值。

const events = require('events');
events.captureRejections = true;
const ee1 = new events.EventEmitter();
ee1.on('something', async (value) => {
  throw new Error('kaboom');
});

ee1.on('error', console.log);

由 captureRejections 行为生成的 'error' 事件没有捕获句柄以避免无限的错误循环：建议不要使用 async 函数作为 'error' 事件句柄。

Using async functions with event handlers is problematic, because it can lead to an unhandled rejection in case of a thrown exception:

const ee = new EventEmitter();
ee.on('something', async (value) => {
  throw new Error('kaboom');
});

The captureRejections option in the EventEmitter constructor or the global setting change this behavior, installing a .then(undefined, handler) handler on the Promise. This handler routes the exception asynchronously to the Symbol.for('nodejs.rejection') method if there is one, or to 'error' event handler if there is none.

const ee1 = new EventEmitter({ captureRejections: true });
ee1.on('something', async (value) => {
  throw new Error('kaboom');
});

ee1.on('error', console.log);

const ee2 = new EventEmitter({ captureRejections: true });
ee2.on('something', async (value) => {
  throw new Error('kaboom');
});

ee2[Symbol.for('nodejs.rejection')] = console.log;

Setting events.captureRejections = true will change the default for all new instances of EventEmitter.

const events = require('events');
events.captureRejections = true;
const ee1 = new events.EventEmitter();
ee1.on('something', async (value) => {
  throw new Error('kaboom');
});

ee1.on('error', console.log);

The 'error' events that are generated by the captureRejections behavior do not have a catch handler to avoid infinite error loops: the recommendation is to not use async functions as 'error' event handlers.