'unhandledRejection' 事件

• reason <Error> | <any> Promise 被拒绝的信息（通常是 Error 对象）。
• promise <Promise> 被拒绝的 promise。

当 Promise 被拒绝且在事件循环的此次轮询中没有绑定错误句柄时，则会触发 'unhandledRejection 事件。 当使用 Promise 进行编程时，异常会被封装为“被拒绝的 promise”。 拒绝可以被 promise.catch() 捕获并处理，并且在 Promise 链中传播。 'unhandledRejection 事件在检测和跟踪被拒绝的（且拒绝还未被处理）promise 时很有用。

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()`。

以下代码也会触发 'unhandledRejection' 事件：

function SomeResource() {
  // 设置 loaded 的状态为被拒绝的 promise。
  this.loaded = Promise.reject(new Error('错误信息'));
}

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

在此示例中，可以像其他 'unhandledRejection' 事件一样，将拒绝作为开发者错误进行跟踪。 为了解决此类故障，可以在 resource.loaded 上绑定一个无操作的 .catch(() => { }) 句柄，这样就可以阻止 '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.