'unhandledRejection' 事件

• reason <Error> | <any> 此对象包含了 promise 被拒绝的相关信息（通常是一个 Error 对象）。
• promise 被拒绝的 promise 对象。

如果在事件循环的一次轮询中，一个 Promise 被拒绝，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，异常会以被拒绝的 promise 的形式封装。 拒绝可以被 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 被拒绝，并且拒绝未被处理的场景中是很有用的。

process.on('unhandledRejection', (reason, promise) => {
  console.log('未处理的拒绝：', promise, '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 的状态设置为一个拒绝的 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拒绝。 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。

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

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.

process.on('unhandledRejection', (reason, promise) => {
  console.log('Unhandled Rejection at:', promise, '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.