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## Resource: Define Backend Event Handlers

## Article: Define Backend Event Handlers

## Article Link: https://dev.wix.com/docs/develop-websites/articles/coding-with-velo/backend-code/events/define-backend-event-handlers.md

## Article Content:

# Define Backend Event Handlers

You can define [backend event](https://dev.wix.com/docs/develop-websites/articles/coding-with-velo/backend-code/events/about-backend-events.md) handlers in the `events.js` backend file.

> **Note**: When developing websites, you can only handle events using Velo. Handling site events using the JavaScript SDK [isn't currently supported](https://dev.wix.com/docs/develop-websites/articles/coding-with-velo/develop-with-the-sdk/velo-to-sdk-api-mapping.md#velo-apis-you-should-continue-to-use).

## Step 1 | Add an `events.js` backend file 

The way that you add an `events.js` file to the backend depends on which IDE you're using.

### Editor

1. Hover over **Backend** in the **Public & Backend** section of the code sidebar.
2. Click the **More Actions** icon  ![plus icon](https://wixmp-833713b177cebf373f611808.wixmp.com/images/e45800424595befdc6fe5931a1b7c0b4.png) and then click **Handle backend events**.

![Add events.js](https://wixmp-833713b177cebf373f611808.wixmp.com/images/a1fac75ac6a2e949a10e8098f84d04ea.png)

### Wix IDE or your local IDE

Add an `events.js` file to the `src/backend` folder.

## Step 2 | Define a backend event handler

To define an event handler in the `events.js` file, export a function using a [supported export syntax](https://dev.wix.com/docs/develop-websites/articles/coding-with-velo/java-script-velo/module-export-syntax.md). 
The function name should consist of the Wix module and the name of the event, separated by an underscore. Check the **Events** subcategory in most APIs in the [API documentation](https://dev.wix.com/docs/velo.md) to confirm the exact function naming and to see what information is passed to the event handler when it is called. 

Here is an example of an event handler that [triggers when an invoice is paid](https://dev.wix.com/docs/velo/apis/wix-billing-backend/events/on-invoice-paid.md):

```javascript
export function wixBilling_onInvoicePaid(event) {
  const invoiceId = event.id.id;
  const email = event.customer.email;
  // Add your own logic here.
}
```

## Step 3 | Test and debug your function

There are two ways to [test your backend event handlers](https://dev.wix.com/docs/develop-websites/articles/coding-with-velo/backend-code/events/about-backend-events.md#testing-and-debugging): 

+ In preview mode, use [functional testing](https://dev.wix.com/docs/develop-websites/articles/workspace-tools/testing-monitoring/functional-testing/test-backend-functions-with-functional-testing.md) to call your event handler function manually.
+ Publish your site, then trigger the event that activates your event handler.

Watch a demo of how to set up backend event handlers:

`youtube:https://youtu.be/-LAtdo0mbiY?t=162`