> For the complete documentation index, see [llms.txt](https://docs.caf.io/llms.txt). Markdown versions of documentation pages are available by appending `.md` to page URLs; this page is available as [Markdown](https://docs.caf.io/caf-sdk/web-javascript/getting-started/document-detector/sdk-event-listeners.md).

# Event listeners

The SDK provides various event listeners to handle different events during the document capture process. These listeners allow you to execute custom logic when specific events occur.

## addEventListener <a href="#sr1l558mx177" id="sr1l558mx177"></a>

The `addEventListener` method is used to listen to events of the specified `type`.

```javascript
documentDetector.addEventListener(type, listener, options);
```

**Parameters**

* `type` ([EventType](#66n5usmgj32o)): The type of event to listen for (check [EventType](#66n5usmgj32o) for details).
* `listener` (function): The function to be executed when the event is triggered.
* `options` (object, optional): Additional options for the event listener

## removeEventListener <a href="#id-2umdj3vdpe3t" id="id-2umdj3vdpe3t"></a>

The `removeEventListener` method is used to remove an event listener of the specified `type`.

```javascript
documentDetector.removeEventListener(type, callback, options);
```

**Parameters**

* `type` ([EventType](#66n5usmgj32o)): The type of event to listen for (check [EventType](#66n5usmgj32o) for details).
* `callback` (function): The function to be removed.
* `options` (object, optional): Additional options for the event listener

## EventType <a href="#id-66n5usmgj32o" id="id-66n5usmgj32o"></a>

<table data-header-hidden><thead><tr><th></th><th width="468"></th></tr></thead><tbody><tr><td><strong>Type</strong></td><td><strong>Description</strong></td></tr><tr><td><strong><code>"front_capture_started"</code></strong></td><td>Triggered when the capture of the front of the document has started.</td></tr><tr><td><strong><code>"back_capture_started"</code></strong></td><td>Triggered when the capture of the back of the document has started.</td></tr><tr><td><strong><code>"capture_invalid"</code></strong></td><td>Triggered when an error occurs in some validation performed by the SDK.</td></tr><tr><td><strong><code>"capture_failed"</code></strong></td><td>Triggered when an error occurs that is not handled by the SDK.</td></tr><tr><td><strong><code>"capture_ready"</code></strong></td><td><p>Triggered when the SDK capture screen (camera preview or upload UI) is rendered and ready for user interaction.</p><p>Event detail: <code>mode</code>, <code>expectedDocumentType</code>, <code>expectedDocumentSide</code>.</p></td></tr><tr><td><strong><code>"upload_button_clicked"</code></strong></td><td>Triggered when the user clicks the upload button in upload mode.</td></tr><tr><td><strong><code>"capture_result"</code></strong></td><td><p>Triggered after a document is successfully captured and validated.</p><p>Event detail: <code>mode</code>, <code>isValid</code>, <code>expectedDocumentType</code>, <code>expectedDocumentSide</code>.</p></td></tr></tbody></table>

## Example <a href="#h196wa5m346d" id="h196wa5m346d"></a>

```javascript
// Listen for the event when the capture of the front of the document starts
documentDetector.addEventListener('front_capture_started', () => {
    console.debug('Document front capture event fired');
});

// Listen for the event when the capture of the back of the document starts
documentDetector.addEventListener('back_capture_started', () => {
    console.debug('Document back capture event fired');
});

// Listen for the event when the SDK UI is ready for user interaction
documentDetector.addEventListener('capture_ready', (event) => {
    console.debug('SDK is ready for capture', {
        mode: event.detail?.mode,
        expectedDocumentType: event.detail?.expectedDocumentType,
        expectedDocumentSide: event.detail?.expectedDocumentSide,
    });
});

// Listen for the event when the user clicks the upload button
documentDetector.addEventListener('upload_button_clicked', () => {
    console.debug('Upload button clicked');
});

// Listen for the event when a capture result is available
documentDetector.addEventListener('capture_result', (event) => {
    console.debug('Capture result', {
        mode: event.detail?.mode,
        isValid: event.detail?.isValid,
        expectedDocumentType: event.detail?.expectedDocumentType,
        expectedDocumentSide: event.detail?.expectedDocumentSide,
    });
});
```


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