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Transformations are a powerful new Svix feature that allow the modification of certain webhook properties in-flight. When you enable Transformations, your customers can write Javascript code on their endpoints that can change a webhook's HTTP method, target URL, and body payload.

Transformations are a beta feature. While in beta, they are free and unlimited. However, this will likely change when the feature exits the beta stage.

Enabling Transformations

Transformations can be enabled at the environment level. When you enable Transformations for an environment, your customers will be able to use Transformations on their endpoints.

To enable Transformations, navigate to the Settings page in the dashboard. Under the Environment Settings section, click General Settings, and then toggle the Enable Transformations switch. Your customers will need to reenter the Consumer App Portal for the change to take effect.

Environment Settings page with the Enable Transformation toggle

Using Transformations

Once enabled for an environment, you customers can begin using Transformations by logging into the Consumer Portal, clicking on an endpoint, clicking into the Advanced tab, and scrolling down to the Transformations card:

The Transformations card on an endpoint

An endpoint's Transformation can be enabled or disabled at any time by toggling the switch on this card.

Your customers can write Javascript code to edit an endpoint's Transformation, and test their code against an event type's payload or a custom payload to see the resulting webhook.

Editing an endpoint's Transformation

How to write a Transformation

Svix expects a Transformation to declare a function named handler. Svix will pass a WebhookObject to this function as its only argument, and expects the function to always return a WebhookObject.

WebhookObject is a JSON object containing 3 properties:

  • method, a string representing the HTTP method the webhook will be sent with. It is always "POST" by default, and its only valid values are "POST" or "PUT"
  • url, a string representing the endpoint's URL. It can be changed to any valid URL.
  • payload, which contains the webhook's payload as a JSON object. It can be changed as needed.
  • cancel, a Boolean which controls whether or not to cancel the dispatch of a webhook. This value defaults to false. Note that canceled messages appear as successful dispatches.

The Transformation will only work if the handler function returns the modified WebhookObject.

An example Transformation

Suppose that sometimes, your customer wants to redirect webhooks to a custom URL instead of the endpoint's defined URL. They only want to do this redirect if a custom URL is present in the webhook payload. They can write a transformation like this:

function handler(webhook) {
if (webhook.payload.customUrl) {
webhook.url = webhook.payload.customUrl;
return webhook;

Great, the webhook is redirected to the custom URL if the customUrl property exists on the payload. Otherwise, it is sent to the endpoint's defined URL.