'unhandledRejection' 事件

如果在事件循环的一次轮询中，一个 Promise 被 rejected，并且此 Promise 没有绑定错误处理器，'unhandledRejection 事件会被触发。 当使用 Promise 进行编程时，异常会以 "rejected promises" 的形式封装。Rejection 可以被 promise.catch() 捕获并处理，并且在 Promise 链中传播。'unhandledRejection 事件在探测和跟踪 promise 被 rejected，并且 rejection 未被处理的场景中是很有用的。

此事件监听器的回调函数被传递如下参数:

• reason <Error> | <any> 此对象包含了 promise 被 rejected 的相关信息（通常是一个 Error 对象）。
• p 被 rejected 的 promise 对象。

process.on('unhandledRejection', (reason, p) => {
  console.log('未处理的 rejection：', p, '原因：', reason);
  // 记录日志、抛出错误、或其他逻辑。
});

somePromise.then((res) => {
  return reportToUser(JSON.pasre(res)); // 故意输错 `pasre`。
}); // 没有 `.catch` 或 `.then`。

如下代码也会触发'unhandledRejection'事件

function SomeResource() {
  // 将 loaded 的状态设置为一个 rejected promise。
  this.loaded = Promise.reject(new Error('错误信息'));
}

const resource = new SomeResource();
// resource.loaded 上没有 .catch 或 .then。

在例子中，可以像在其他 'unhandledRejection' 事件的典型场景中一样，跟踪开发者错误导致的 rejection。 为了解决这样的错误，resource.loaded 中可以加一个不做任何操作的 .catch(() => { }) 处理器， 这样可以阻止 'unhandledRejection' 事件的触发。或者也可以使用 'rejectionHandled' 事件来解决。

The 'unhandledRejection' event is emitted whenever a Promise is rejected and no error handler is attached to the promise within a turn of the event loop. When programming with Promises, exceptions are encapsulated as "rejected promises". Rejections can be caught and handled using promise.catch() and are propagated through a Promise chain. The 'unhandledRejection' event is useful for detecting and keeping track of promises that were rejected whose rejections have not yet been handled.

The listener function is called with the following arguments:

• reason <Error> | <any> The object with which the promise was rejected (typically an Error object).
• p the Promise that was rejected.

process.on('unhandledRejection', (reason, p) => {
  console.log('Unhandled Rejection at:', p, 'reason:', reason);
  // application specific logging, throwing an error, or other logic here
});

somePromise.then((res) => {
  return reportToUser(JSON.pasre(res)); // note the typo (`pasre`)
}); // no `.catch()` or `.then()`

The following will also trigger the 'unhandledRejection' event to be emitted:

function SomeResource() {
  // Initially set the loaded status to a rejected promise
  this.loaded = Promise.reject(new Error('Resource not yet loaded!'));
}

const resource = new SomeResource();
// no .catch or .then on resource.loaded for at least a turn

In this example case, it is possible to track the rejection as a developer error as would typically be the case for other 'unhandledRejection' events. To address such failures, a non-operational .catch(() => { }) handler may be attached to resource.loaded, which would prevent the 'unhandledRejection' event from being emitted. Alternatively, the 'rejectionHandled' event may be used.