Transformation and destination developer reference

Note: It's worth reading the building transformation and destination overview for a quick introduction to how to build your own.

plugin.json file

A plugin.json file is structured as follows:

{
  "name": "<plugin_display_name>",
  "url": "<repo_url>",
  "description": "<description>",
  "main": "<entry_point>",
  "config": [
    {
      "markdown": "A Markdown block.\n[Use links](http://example.com) and other goodies!"
    },
    {
      "key": "param1",
      "name": "<param1_name>",
      "type": "<param1_type>",
      "default": "<param1_default_value>",
      "hint": "<param1_hint_value>",
      "required": true,
      "secret": true
    },
    {
      "key": "param2",
      "name": "<param2_name>",
      "type": "<param2_type>",
      "default": "<param2_default_value>",
      "required": false
    }
  ]
}

Here's an example plugin.json file from our 'Hello world transformation':

{
  "name": "Hello World",
  "url": "https://github.com/PostHog/helloworldplugin",
  "description": "Greet the World and Foo a Bar, JS edition!",
  "main": "index.js",
  "config": [
    {
      "markdown": "This is a sample transformation!"
    },
    {
      "key": "bar",
      "name": "What's in the bar?",
      "type": "string",
      "default": "baz",
      "hint": "This will be sent in a **property**",
      "required": false
    }
  ]
}

Most options in this file are self-explanatory, but there are a few worth exploring further:

main

main determines the entry point for your transformation or destination, where your setupPlugin and processEvent functions are. More on these later.

config

config consists of an array of objects that each pertain to a specific configuration field or markdown explanation for your plugin.

Each object in a config can have the following properties:

Note: You can have a config field that only contains markdown. This won't be used to configure your transformation or destination but can be placed anywhere in the config array and is useful for customizing the content of the configuration step in the PostHog UI.

PluginMeta

Check out types for a full spec of types.

Every plugin server function is called by the server with an object of type PluginMeta that can include global, attachments, and config, which you can use in your logic.

Here's what they do:

config

Gives you access to the transformation or destination's config values as described in plugin.json and configured via the PostHog interface.

Example:

export function processEvent(event, { config }) {
    event.properties['greeting'] = config.greeting
    return event
}

global

The global object is used for sharing functionality between setupPlugin and the rest of the special functions, like processEventor composeWebhook, since global scope does not work in the context of PostHog transformations or destinations. global is not shared across worker threads

Example:

export function setupPlugin({ global, config }) {
    global.eventsToTrack = (config.eventsToTrack || '').split(',') 
}

export function processEvent(event, { global, config }) {
    if(global.eventsToTrack.includes(event.event)) {
        // Do something
    }
}

attachments

attachments gives access to files uploaded by the user for config parameters of type attachment. An attachment has the following type definition:

interface PluginAttachment {
    content_type: string
    file_name: string
    contents: any
}

As such, accessing the contents of an uploaded file can be done with attachments.attachmentName.contents.

Example:

export function setupPlugin({ attachments, global }: Meta) {
    if (attachments.maxmindMmdb) {
        global.ipLookup = new Reader(attachments.maxmindMmdb.contents)
    }
}

setupPlugin function

setupPlugin is a function you can use to dynamically set transformation or destination configuration based on the user's inputs at the configuration step.

You could, for example, check if an API Key inputted by the user is valid and throw an error if it isn't, prompting PostHog to ask for a new key.

It takes only an object of type PluginMeta as a parameter and does not return anything.

Example:

export function setupPlugin({ global, config }) {
    global.eventsToTrack = (config.eventsToTrack || '').split(',')
}

On PostHog Cloud and email-enabled instances of PostHog, project members are notified by email of the plugin being disabled automatically. This is to ensure that action is taken if the plugin is important for data integrity.

processEvent function

processEvent is the juice of your transformation or destination.

In essence, it takes an event as a parameter and returns an event as a result. In the process, this event can be:

• Modified
• Not returned (preventing ingestion)

It takes an event and an object of type PluginMeta as parameters and returns an event.

Here's an example (from the 'Hello World transformation'):

export function processEvent(event, { config }) {
    if (!event.properties) event.properties = {}
    event.properties['greeting'] = config.greeting
    return event
}

As you can see, the function receives the event before it is ingested by PostHog, adds properties to it (or modifies them), and returns the enriched event, which will then be ingested by PostHog (after all transformations and destinations run).

Note: You cannot use storage nor cache nor external calls in transformations or destinations in PostHog Cloud. Furthermore you can only define one of non-async processEvent or composeWebhook per transformation or destination.

composeWebhook function

Minimum PostHog hash: https://github.com/PostHog/posthog/commit/0137b9d40d8c0b4a7183fd6bb3c718a35d116b95

composeWebhook is a non-async function that is executed at the end of the pipeline. It allows users to submit data to their own HTTP endpoint when an event happens. The function can return

• null if for a specific event we don't want to trigger an HTTP request.

• Webhook object, for which we'll trigger an HTTP request to the url with the payload, method and headers returned. If you are interested in exporting large amounts of event data from PostHog, look into batch exports.

Here's a quick example:

function composeWebhook(event) {
    if (event.event == '$autocapture') {
        return null
    }
    return {
        url: "http://pineapples-make-pizzas-delicious.com",
        body: JSON.stringify(event),
        headers: {
            'Content-Type': 'application/json',
        },
        method: 'POST',
    }
}

Note: You cannot use storage nor cache in transformations or destinations in PostHog Cloud. Furthermore you can only define one of processEvent or composeWebhook per transformation or destination.

Testing

In order to ensure transformations and destinations are stable and work as expected for all their users, we highly recommend writing tests for every one you build.

Adding testing capabilities

You will need to add Jest and our testing scaffold to your project in your package.json file:

"jest": {
    "testEnvironment": "node"
},
"scripts": {
    "test": "jest ."
},
"devDependencies": {
    "@posthog/plugin-scaffold": "*",
    "jest": "^27.0.4"
}

Next, create your test files e.g. index.test.js or index.test.ts for testing your index.js or index.ts file.

Writing tests

Write tests in Jest, you can learn more about the syntax and best practices in the Jest documentation. We recommend writing tests to cover the primary functions (e.g. does it create events in the expected format) and also for edge cases (e.g. does it crash if no data is sent).

Logs

Transformations and destinations can make use of the console for logging and debugging. console.log, console.warn, console.error, console.debug, console.info are all supported.

These logs can be seen on the 'Logs' page of each, which can be accessed on the 'Data Pipelines' page of the PostHog UI.

Do not log a line for every event in PostHog Cloud as that would create a lot of spam and waste storage.

Types

PostHog supports TypeScript natively, without you having to compile the TypeScript yourself (although you can also do that).

To build a TypeScript transformation or destination, you'll probably need some types, so read on.

Installation

To use the types, you can install them as follows:

# if using yarn
yarn add --dev @posthog/plugin-scaffold

# if using npm
npm install --save-dev @posthog/plugin-scaffold

Then, in your transformation or destination, you can use them like so:

import { PluginEvent, PluginMeta } from '@posthog/plugin-scaffold'

export function processEvent(event: PluginEvent, meta: PluginMeta) {
    if (event.properties) {
        event.properties['hello'] = 'world'
    }
    return event
}