RTCRtpScriptTransformer: options property

Limited availability

This feature is not Baseline because it does not work in some of the most widely-used browsers.

The options read-only property of the RTCRtpScriptTransformer interface returns the object that was (optionally) passed as the second argument during construction of the corresponding RTCRtpScriptTransform.


An object.


The simplest use of options is for the main thread to indicate whether the corresponding RTCRtpScriptTransform is to be added to the WebRTC sender or receiver pipeline. This is important if the same worker is used for processing both incoming and outgoing encoded frames, as it allows code to determine what transform should be applied to the frames.

Options can also be used to send/transfer the second port of a message channel to the worker-side transform. This channel can then be used to send dynamic information to a transform stream, such as when encryption keys are changed or added. Note that you might also send messages to the transform using Worker.postMessage(), but you would then have to appropriately redirect the messages if the worker is used in different contexts (while a message port option provides a direct channel for a specific transform).


How to indicate the current WebRTC pipeline

RTCRtpScriptTransform is constructed with a particular Worker and options, and then inserted into either the WebRTC outgoing or incoming pipeline by assigning it to RTCRtpSender.transform or RTCRtpReceiver.transform, respectively. If the same worker is used in the transforms for the incoming and outgoing pipeline, then you need to supply options in the constructor to indicate whether encoded frames to be transformed are incoming or outgoing.

The example below shows how this might be done for a RTCRtpScriptTransform added to the sender pipeline after adding a track to the peer connection (RTCPeerConnection), and then adding another transform to the receiver pipeline when a track is received.

// videoSender is an RTCRtpSender.
const videoSender = peerConnection.addTrack(track, mediaStream);
videoSender.transform = new RTCRtpScriptTransform(worker, {
  name: "senderTransform",
peerConnection.ontrack = (event) => {
  // event.receiver is an RTCRtpReceiver
  event.receiver.transform = new RTCRtpScriptTransform(worker, {
    someOption: "receiverTransform",

In each case above we supply an object with a different value for the option object's name property, which indicates the pipeline that the transform was added to. Note that the names and values of properties in options is arbitrary: what matters is that the main thread and worker thread both know what properties and values are used.

The following code how the passed options are used in the worker. First we implement a handler for the rtctransform event, which is fired at the global worker object on construction of the corresponding RTCRtpScriptTransform, and when new frames are enqueued for processing. event.transformer is a RTCRtpScriptTransformer that has a readable, writable, and options property.

addEventListener("rtctransform", (event) => {
  let transform;
  // Select a transform based on passed options
  if (event.transformer.options.name == "senderTransform")
    transform = createSenderTransform(); // A TransformStream
  else if (event.transformer.options.name == "receiverTransform")
    transform = createReceiverTransform(); // A TransformStream
  else return;

  // Pipe frames from the readable to writeable through TransformStream

The code creates a different TransformStream to process outgoing and incoming frames, using createSenderTransform() or createReceiverTransform(), based on the passed options (it then pipes frames from the readable, through the selected TransformStream, to the writable).

Passing a message port to a transform

This example shows how to create a message channel and transfer one of its ports to the WebRTC encoded transform running in the worker. This main thread can then send and transfer objects and messages to the transformer running in the worker after construction, and vice versa.

The code below first creates a MessageChannel and then constructs a RTCRtpScriptTransform passing the port2 value as an property in the options argument. The port is also included in the array passed as the third constructor argument, so that it is transferred into the worker context.

const channel = new MessageChannel();

const transform = new RTCRtpScriptTransform(
  { purpose: "encrypter", port: channel.port2 },

The worker can then get the port from the rtctransform event fired at the global worker object.

let messagePort;
addEventListener("rtctransform", (event) => {
  messagePort = event.transformer.options.port;
  // ... other transformer code

Code in each end of the channel can send and transfer objects to the other end using MessagePort.postMessage(), and listen for incoming messages using its message event.

For example, assuming we had an encryption key in a Uint8Array typed array named encryptionKey, we could transfer it from the main thread to the worker as shown:

const encryptionKeyBuffer = encryptionKey.buffer;
channel.port1.postMessage(encryptionKeyBuffer, [encryptionKeyBuffer]);

The worker would listen for the message event to get the key:

  messagePort.addEventListener("message", (event) => {
    const encryptionKeyBuffer = event.data;
    // ... Use the encryptionKeyBuffer for encryption or any other purpose

See message channel for more information and examples.


WebRTC Encoded Transform
# dom-rtcrtpscripttransformer-options

Browser compatibility

BCD tables only load in the browser

See also