> ## Documentation Index
> Fetch the complete documentation index at: https://docs.rebellapp.com/llms.txt
> Use this file to discover all available pages before exploring further.

# my.onUnhandledRejection

> Listen for unhandled Promise rejections in the Mini Program.

Use this API to listen for unhandled Promise rejections. The callback triggers when a Promise without a rejection handler is rejected.

## Parameters

| Property | Type     | Required | Description                                         |
| -------- | -------- | -------- | --------------------------------------------------- |
| callback | Function | Yes      | Handler executed when an unhandled rejection occurs |

## Callback Parameters

| Property | Type    | Description                                      |
| -------- | ------- | ------------------------------------------------ |
| reason   | String  | The rejection reason (typically an error object) |
| promise  | Promise | The rejected Promise instance                    |

## Code Example

### Basic Usage

```javascript theme={null}
App({
  onShow() {
    my.onUnhandledRejection((res) => {
      console.log('Unhandled rejection reason:', res.reason);
      console.log('Rejected promise:', res.promise);
    });
  }
});
```

### Error Logging

```javascript theme={null}
App({
  onLaunch() {
    my.onUnhandledRejection((res) => {
      // Log unhandled rejections
      this.logRejection({
        reason: res.reason,
        timestamp: Date.now()
      });
    });
  },

  logRejection(data) {
    my.request({
      url: 'https://api.example.com/errors/rejections',
      method: 'POST',
      data: data
    });
  }
});
```

### Development Debugging

```javascript theme={null}
App({
  onLaunch() {
    my.getRunScene({
      success: ({ envVersion }) => {
        if (envVersion === 'develop') {
          my.onUnhandledRejection((res) => {
            // Show detailed info in development
            my.alert({
              title: 'Unhandled Promise Rejection',
              content: String(res.reason)
            });
          });
        }
      }
    });
  }
});
```

## Common Causes

Unhandled Promise rejections typically occur when:

<AccordionGroup>
  <Accordion title="Missing .catch() Handler">
    ```javascript theme={null}
    // This will trigger onUnhandledRejection
    my.request({
      url: 'https://api.example.com/data'
    }).then((res) => {
      console.log(res);
    });
    // Missing .catch()

    // Correct approach
    my.request({
      url: 'https://api.example.com/data'
    }).then((res) => {
      console.log(res);
    }).catch((err) => {
      console.error('Request failed:', err);
    });
    ```
  </Accordion>

  <Accordion title="Missing try/catch with async/await">
    ```javascript theme={null}
    // This will trigger onUnhandledRejection
    async function fetchData() {
      const res = await my.request({ url: '...' });
      return res.data;
    }

    // Correct approach
    async function fetchData() {
      try {
        const res = await my.request({ url: '...' });
        return res.data;
      } catch (err) {
        console.error('Failed:', err);
        return null;
      }
    }
    ```
  </Accordion>

  <Accordion title="Errors in Promise Chains">
    ```javascript theme={null}
    // Error in .then() without .catch()
    fetchData()
      .then((data) => {
        throw new Error('Processing failed');
      });

    // Correct approach
    fetchData()
      .then((data) => {
        throw new Error('Processing failed');
      })
      .catch((err) => {
        console.error('Chain error:', err);
      });
    ```
  </Accordion>
</AccordionGroup>

<Tip>
  Always add `.catch()` handlers to your Promises or use `try/catch` with `async/await` to prevent unhandled rejections.
</Tip>

## Related APIs

<CardGroup cols={2}>
  <Card title="my.offUnhandledRejection" icon="xmark" href="/jsapi/event/my.offUnhandledRejection">
    Stop listening for unhandled rejections
  </Card>

  <Card title="my.onError" icon="exclamation-circle" href="/jsapi/event/my.onError">
    Listen for JS errors
  </Card>
</CardGroup>