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

# Functions

Functions are used to mutate webhook payloads (event data) before they are dispatched based on a passed user-defined Javascript function.

Convoy uses [goja](https://github.com/dop251/goja) to provide a JavaScript runtime environment in Go, providing full ECMAScript 5.1 support (including regex and strict mode). Most of the ECMAScript 6 spec is implemented, but this is a work in progress. To enhance the runtime, console support from [goja\_nodejs](https://github.com/dop251/goja_nodejs) was also added.

### Importing Modules

`require` support also exists but is currently disabled.

### Caveats

Certain constraints exist while using functions:

* Multiple functions can be written in the file, but only the `transform` function is invoked.
* Only the first argument is used in the `transform` function and that is the payload data.
* The `transform` function must return a value.

## Use cases

### Subscriptions

Functions can be configured when creating or updating a subscription.
This is currently only possible in [incoming projects](../product-manual/organizations-and-projects#incoming-project).
The function is used to mutate ingested event payloads from webhook providers before sending them to the destination endpoints.

### Sources

Functions can be configured when creating a message broker source in an [outgoing project](../product-manual/organizations-and-projects#outgoing-project).
The function is used to mutate ingested event payloads from the message broker before sending them to the destination endpoints.

<Frame>
  <img src="https://mintcdn.com/convoy/a36zzjqCEpay4boE/images/subscription-create-function.png?fit=max&auto=format&n=a36zzjqCEpay4boE&q=85&s=0979762a64c22f3a2142f711aa6d141d" alt="Create Webhook Function" width="2530" height="1672" data-path="images/subscription-create-function.png" />
</Frame>

## Example

```json sample payload theme={null}
{
	"blockchain": "optimism",
	"chainId": "10",
	"connectedAddresses": [
		{
			"address": "0xca3a901d1bd21be11b88e4e9d534fb48d35dde48",
			"blockchain": "ethereum",
			"chainId": "1",
			"timestamp": "2023-05-18T16:07: 22Z"
		}
	],
	"profileImageContentValue": {
		"image": {
			"original": "10/caster/9613/profile_image/original_image.jpg"
		}
	},
	"userAssociatedAddresses": ["0x4e4814f10e341882438264c85ba8f02ef8f8a592", "0xca3a901d1bd21be11b88e4e9d534fb48d35dde41", "0x3628bbc094720495489f6ac726ef7233c3d9587e"],
	"userId": "9613"
}
```

```js sample function theme={null}
function transform(payload) {
	return {
		event_type: 'chain.connected',
		data: {
			addresses: payload.connectedAddresses[0].address,
			created_at: new Date(payload.connectedAddresses[0].timestamp).getTime(),
			user_id: payload.userId
		}
	};
}
```

```json sample output theme={null}
{
	"data": {
		"addresses": "0xca3a901d1bd21be11b88e4e9d534fb48d35dde48",
		"created_at": 1684426042000,
		"user_id": "9613"
	},
	"event_type": "chain.connected"
}
```