Payload Transformations

Payload transformations let you modify the HTTP method, URL, headers, and body of a webhook before Mittr delivers it. Each endpoint can have its own transformation — written in JavaScript.

How it works

An event is routed to an endpoint with a transformation configured
Mittr executes the JavaScript transformation with the event payload as input
The transformed result replaces the original delivery parameters
Mittr delivers the modified request

Transformations run in a sandboxed environment with a 1-second timeout and 128MB memory limit.

Configure a transformation

Set the transformation field when creating or updating an endpoint:

curl -X POST https://app.mittr.io/api/v1/endpoints \
  -H "X-API-Key: mtr_your_key" \
  -H "Content-Type: application/json" \
  -d '{
    "url": "https://api.example.com/orders",
    "eventTypes": ["order.*"],
    "transformation": "return { payload: { orderId: payload.id, total: payload.amount / 100, currency: \"usd\" } }"
  }'

Transformation API

Your transformation code receives these variables and must return an object:

Input variables

Variable	Type	Description
`payload`	object	The event payload
`eventType`	string	The event type (e.g., `order.created`)
`headers`	object	The current delivery headers

Return object

Field	Type	Description
`payload`	object	Modified payload (replaces original)
`method`	string	HTTP method override (default: `POST`)
`url`	string	URL override (replaces endpoint URL for this delivery)
`headers`	object	Additional headers to merge
`cancel`	boolean	Set to `true` to skip this delivery entirely

All fields are optional. Omitted fields keep their original values.

Examples

Reshape a payload

// Extract only the fields the destination needs
return {
  payload: {
    id: payload.orderId,
    customer: payload.customerEmail,
    items: payload.lineItems.map(i => ({ sku: i.sku, qty: i.quantity })),
    total: payload.totalCents / 100
  }
}

Add custom headers

// Add authorization and routing headers
return {
  headers: {
    "Authorization": "Bearer " + payload.apiToken,
    "X-Tenant-ID": payload.tenantId
  }
}

Change HTTP method

// Use PUT instead of POST for update events
if (eventType === "order.updated") {
  return {
    method: "PUT",
    url: "https://api.example.com/orders/" + payload.orderId
  }
}
return {}

Conditionally cancel delivery

// Only deliver high-value orders
if (payload.amount < 10000) {
  return { cancel: true }
}
return {
  payload: {
    ...payload,
    priority: "high"
  }
}

Route to different URLs by event type

const routes = {
  "order.created": "https://api.example.com/orders",
  "order.updated": "https://api.example.com/orders/" + payload.id,
  "customer.created": "https://api.example.com/customers"
}
return {
  url: routes[eventType] || "https://api.example.com/webhooks",
  method: eventType.includes("updated") ? "PUT" : "POST"
}

Limits

Limit	Value
Execution timeout	1 second
Memory limit	128 MB

Transformations that exceed these limits are terminated and the delivery proceeds with the original payload.