files/en-us/web/api/rtcrtpscripttransformer/writable/index.md
{{APIRef("WebRTC")}}
The writable read-only property of the {{domxref("RTCRtpScriptTransformer")}} interface returns a {{domxref("WritableStream")}} instance that can be used as a sink for encoded media frames enqueued on the corresponding {{domxref("RTCRtpScriptTransformer.readable")}}.
When the corresponding {{domxref("RTCRtpScriptTransform")}} is inserted into the WebRTC sender and receiver pipelines, encoded media frames ({{domxref("RTCEncodedVideoFrame")}} or {{domxref("RTCEncodedAudioFrame")}}) may be enqueued on the {{domxref("RTCRtpScriptTransformer.readable")}}.
A WebRTC encoded transform can read the frames from readable, modify them as needed, and then send them back into the WebRTC pipeline by sending them to this writable.
A common way to perform this operation is to pipe the frames through a {{domxref("TransformStream")}}.
A {{domxref("WritableStream")}}.
The following example shows how {{domxref("RTCRtpScriptTransformer.readable")}} is piped through a {{domxref("TransformStream")}} to RTCRtpScriptTransformer.writable.
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
event.transformer.readable
.pipeThrough(transform)
.pipeTo(event.transformer.writable);
});
The code implements a handler for the {{domxref("DedicatedWorkerGlobalScope.rtctransform_event", "rtctransform")}} event, which is fired at the global worker object on construction of the corresponding {{domxref("RTCRtpScriptTransform")}}, and when new frames are enqueued for processing.
event.transformer is the {{domxref("RTCRtpScriptTransformer")}} that has the writable and readable properties.
A different {{domxref("TransformStream")}} is created to process outgoing and incoming frames, using createSenderTransform() or createReceiverTransform(), respectively (implementations not shown).
The event handler chooses the correct transform stream to use based on options passed through from the RTCRtpScriptTransform constructor and assigns it to transform.
The code calls {{domxref("ReadableStream.pipeThrough()")}} on the readable to pipe encoded frames through the selected TransformStream, and then {{domxref("ReadableStream.pipeTo()")}} to pipe them to the RTCRtpScriptTransformer.writable.
{{Specifications}}
{{Compat}}