> ## 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.startLocalBioAuth

> Start biometric authentication (fingerprint or face recognition).

Use this API to authenticate the user using biometric methods such as fingerprint or face recognition.

## Parameters

| Property     | Type     | Required | Description                                                                                                                                   |
| ------------ | -------- | -------- | --------------------------------------------------------------------------------------------------------------------------------------------- |
| promptText   | String   | Yes      | The text displayed on the biometric prompt. Use short, clear text to state the authentication purpose to avoid truncation.                    |
| fallbackType | Number   | No       | The alternative authentication method when biometric authentication fails: `0` - No fallback (default). `1` - Fallback to device credentials. |
| success      | Function | No       | Callback on success                                                                                                                           |
| fail         | Function | No       | Callback on failure                                                                                                                           |
| complete     | Function | No       | Callback that always executes                                                                                                                 |

## Success Callback Parameters

| Property     | Type    | Description                                                                                                                      |
| ------------ | ------- | -------------------------------------------------------------------------------------------------------------------------------- |
| success      | Boolean | Whether the biometric authentication succeeded. Returned by the `success` or `complete` callback.                                |
| error        | Number  | The error code. Returned by the `fail` callback, or by the `complete` callback when `success` is `false`. See Error Codes below. |
| errorMessage | String  | The error message corresponding to the error code.                                                                               |

## Code Example

### Basic Authentication

```javascript theme={null}
my.startLocalBioAuth({
  promptText: 'Authenticate to unlock',
  success(res) {
    if (res.success) {
      console.log('Biometric verified');
    }
  },
  fail(res) {
    console.error('Authentication failed:', res.error);
  }
});
```

### Fallback to Device Credentials

```javascript theme={null}
my.startLocalBioAuth({
  promptText: 'Verify your identity',
  fallbackType: 1,
  success(res) {
    if (res.success) {
      console.log('Authenticated');
    }
  },
  fail(res) {
    if (res.error === 11) {
      // The user cancelled the authentication
    }
  }
});
```

### Secure Payment Flow

```javascript theme={null}
Page({
  async confirmPayment() {
    // First verify user identity
    my.startLocalBioAuth({
      promptText: 'Confirm payment',
      fallbackType: 1,
      success: (res) => {
        if (res.success) {
          this.processPayment();
        } else {
          my.showToast({
            content: 'Verification failed',
            type: 'fail'
          });
        }
      },
      fail: (res) => {
        // Biometric not available or cancelled, fall back to PIN
        this.showPinEntry();
      }
    });
  },

  processPayment() {
    // Proceed with payment
  },

  showPinEntry() {
    // Show PIN input as fallback
  }
});
```

### Check Biometric Availability First

```javascript theme={null}
Page({
  async authenticateUser() {
    // Check if biometric is available
    my.checkLocalBioAuthSupported({
      success: (res) => {
        if (res.success) {
          my.startLocalBioAuth({
            promptText: 'Verify your identity',
            success: (verifyRes) => {
              if (verifyRes.success) {
                this.onAuthSuccess();
              }
            }
          });
        } else {
          // Biometric not available
          this.useFallbackAuth();
        }
      }
    });
  },

  onAuthSuccess() {
    my.showToast({ content: 'Authenticated' });
  },

  useFallbackAuth() {
    // Use password or PIN
  }
});
```

## Error Codes

| Code | Description                                                                                     |
| ---- | ----------------------------------------------------------------------------------------------- |
| 11   | The authentication process was canceled by the user.                                            |
| 12   | Either the device lacks biometric support or the super app cannot access the biometric feature. |
| 13   | Biometric credentials are not enabled on the user's device.                                     |
| 99   | An undefined error has occurred. Please contact technical support for assistance.               |

<Warning>
  Biometric authentication may not be available on all devices. Always provide a fallback authentication method.
</Warning>

## Related APIs

<CardGroup cols={2}>
  <Card title="my.checkLocalBioAuthSupported" icon="fingerprint" href="/jsapi/device/biometric/my.checkLocalBioAuthSupported">
    Check availability
  </Card>
</CardGroup>