The createBidirectionalStream() method of the WebTransport interface opens a bidirectional stream; it returns a WebTransportBidirectionalStream object containing readable and writable properties, which can be used to reliably read from and write to the server.

"Reliable" means that transmission and order of data are guaranteed. This provides slower delivery (albeit faster than with WebSockets) than datagrams, but is needed in situations where reliability and ordering are important, like chat applications.

None.

A WebTransportBidirectionalStream object.

InvalidStateError DOMException

Thrown if createBidirectionalStream() is invoked while the WebTransport is closed or failed.

An initial function is used to get references to the WebTransportBidirectionalStream.readable and WebTransportBidirectionalStream.writable properties. These are references to ReadableStream and WritableStream instances, which can be used to read from and write to the server.

async function setUpBidirectional() {
  const stream = await transport.createBidirectionalStream();
  // stream is a WebTransportBidirectionalStream
  // stream.readable is a ReadableStream
  const readable = stream.readable;
  // stream.writable is a WritableStream
  const writable = stream.writable;

  ...
}

Reading from the ReadableStream can then be done as follows:

async function readData(readable) {
  const reader = readable.getReader();
  while (true) {
    const { value, done } = await reader.read();
    if (done) {
      break;
    }
    // value is a Uint8Array.
    console.log(value);
  }
}

And writing to the WritableStream can be done like this:

async function writeData(writable) {
  const writer = writable.getWriter();
  const data1 = new Uint8Array([65, 66, 67]);
  const data2 = new Uint8Array([68, 69, 70]);
  writer.write(data1);
  writer.write(data2);
}

• Using WebTransport
• WebSockets API
• Streams API
• WebTransport over HTTP/3