Transformations is one of RudderStack's key features. It gives you the ability to code custom JavaScript functions to implement specific use-cases on your event data, like:

  • Filtering/sampling events.
  • Enriching events by implementing static logic or leveraging an external API.
  • Cleaning/aggregating data.
  • Data masking or removing sensitive PII information in the events to ensure data privacy.
  • Implementing external actions on the events using an API.

Transformations can be used across your Event Stream, Reverse ETL and Cloud Extract pipelines.

You can refer to our Sample Transformations GitHub repository for some useful templates that you can use to create your own transformations.

Transformations only work for the Cloud mode destinations.

To know more about the Cloud mode in RudderStack, check out the RudderStack Connection Modes guide.

Adding a transformation

Adding a Transformation
  • Click on the Create New option as shown:
Create New Transformation
  • Next, assign a name for this new transformation. Enter the transformation function's code in the Transformation window, as shown:
Assign a transformation name
  • Add your transformation code within the transformEvent function in the Transformation window. You can also add other functions and call them from within transformEvent.
transformEvent function

Our Transformations GitHub repository contains some useful templates that you can use to create your own transformations.

You can copy-paste the entire code of any of the functions present in this repository into the Transformation window.

Remember to delete the pre-populated transformEvent function in such cases, before pasting your code.

  • RudderStack also gives you the ability to test your transformation function with the Run Test option as shown:
Run Test
  • In case you want to perform any aggregation/roll-up operation on a micro batch of events, use the transformBatch function, as shown:
export function transformBatch(events, metadata) {
return events;

Disclaimer for using transformBatch:

  • Make sure you pass on the messageId from the input event onto the output event. If the messageId is passed on, the order of delivery of events is preserved. Without it, this order may not be maintained.
  • It is highly recommended to use transformEvent in every possible case as it ensures event ordering in all cases.
  • In case a transformBatch is required and event ordering is important, make sure to pass on the messageId.


One of the key features of RudderStack Transformations is the ability to reuse code written for a transformation in other transformations as well. For this, RudderStack lets you create libraries or functions that can be reused in different transformations.

To create a library, follow these steps:

  • Click on the Transformations link in the left nav bar and go to the Libraries section. Click on the Create New option to add new libraries, as shown:
Create new library
  • Add the library's Name, an optional Description, and include the custom functions that you need to reuse across all other transformations.
Add library name and description
  • You can add multiple functions to a single library, as shown:
Add more functions under a single library

RudderStack does not support the deletion of libraries as of now. This is to ensure that you don't break any existing transformations that use the library that you might be trying to delete.

Using libraries in transformations

To use the libraries in your existing transformation, simply take the name of your library and convert it into camel case without spaces - this becomes your library handle.

For example, if your library name is is rudder email, then the library handle would be isRudderEmail.

Using libraries in transformations

Let's say you want to import a function called rudderEmail, which returns true for the emails from a specific domain (e.g. RudderStack) and false otherwise, from the is rudder email library. Also, you want to use this function to filter the events that don't have the email address of the specified domain.

The following code snippet demonstrates how we can implement this use-case:

import { rudderEmail } from "isRudderEmail";
export function transformEvent(event) {
const email =
event.context && event.context.traits &&;
if (email) {
if (!rudderEmail(email)) return;
return event;

On running a test, an example event not having the specified email domain is filtered out, as shown:

Event filtered out without specifying email domain

Importing multiple functions from a single library

When importing a single function or multiple functions from a library, it is important to keep the import statement to one line. Breaking the import function into multiple lines will lead to an error.

The following snippets highlight how to properly import functions from a library:

// Correct Way
// ---------------
import { getLoss } from "getFinanceData";
// OR
import { getLoss, getRevenue, getProfit } from "getFinanceData";
import {
} from "getFinanceData";
// For default getPrice import
import getPrice, { getRevenue, getProfit } from "getFinanceData";
// alias imports
import getPrice as gp, { getRevenue as gr, getProfit } from "getFinanceData";
// usuage: gp(event), gr(event), getProfit(ev)
import * as GFD from "getFinanceData";
// usuage: GFD.getRevenue(ev), GFD.getProfit(ev)
// for default import: GFD.default(ev)
// Incorrect Way
// -----------------
import * from "getFinanceData";
// OR
import getPrice as gp from "getFinanceData";

Accessing metadata

RudderStack injects a function metadata(event) into your transformations as an argument. This allows you to access the event metadata variables that help you customize your transformations.

metadata() takes the event as input and returns the metadata of the event.

The following properties, if available, are present in the metadata response:

Property NameDescription
sourceIdThis refers to the ID of the source configured on your RudderStack dashboard. Note that it different from the source Write Key. Refer to the image below for more details.
destinationIdID of the destination configured on your RudderStack dashboard.
messageIdCorresponds to the unique ID for each event.
sessionIdIf sessions are enabled, this corresponds to the value of the session ID.
Source ID Source ID Source ID

Since you may not need the metadata in every transformation, you can optionally access it by including it in your function signature as an argument wherever required.

An example of this is as shown below:

export function transformEvent(event, metadata) {
const meta = metadata(event);
event.sourceId = meta.sourceId;
return event;

External API requests

You can make any number of external API requests in your transformation functions and use the response to enrich your events data. RudderStack injects an asynchronous fetch(url) function into your transformations. It makes an API call to the given URL and returns the response in a JSON format.

An example of how to use the fetch function in transformations is shown below:

export async function transformEvent(event, metadata) {
const res = await fetch("post_url", {
method: "POST", // POST, PUT, DELETE, GET, etc.
headers: {
"Content-Type": "application/json;charset=UTF-8",
Authorization: "Bearer <your_authorization_token>"
body: JSON.stringify(event)
event.response = JSON.stringify(res);
return event;

To see how this feature is used in more detail, check out the Clearbit enrichment example which uses the fetch function.

For improved performance, we recommend using batch API requests instead of a separate API request for each event whenever possible.


FetchV2 is a wrapper around the fetch call to provide more access to fetch the response properties.

The following properties are present in a fetchV2 response:

Property NameDescription
statusStatus code of fetch response, e.g. 200.
urlURL of the Fetch API.
headersResponse headers
bodyResponse body in JSON or TEXT. By default, it is JSON.

Use the try-catch block to capture the cause for failures during the fetch call, e.g. when a timeout is applied.

An example of how to use the fetchV2 function in RudderStack transformations is shown below:

export async function transformEvent(event) {
try {
const res = await fetchV2("url", { timeout: 1000});
if (res.status == 200) {
event.response = JSON.stringify(res.body);
} catch (err) {
return event;

Debugging with logs

You can access event-related logs while running a test by including the log() function in your transformation code. An example of this is as shown:

export function transformEvent(event, metadata) {
const meta = metadata(event);
event.sourceId = meta.sourceId;
log("Event Name is", event.event, ";", "Message Id is", event.messageId);
log("Source Id is", meta.sourceId);
return event;

On running a test on the above code, you can see the logs in the Logs section of the dashboard, as shown:

Adding a Transformation

Arguments to the log function can be a String, Number or Object.


You should take into account the memory and time limits when adding a new transformation. Each invocation of the transformation should not exceed the following limits:

Memory Limit8 MB
Time Limit4 seconds

The user transformation fails if these limits are exceeded.

Contact us

For queries on any of the sections covered in this guide, you can contact us or start a conversation in our Slack community.