WebTransport: closed property

Baseline 2026

Newly available

Since March 2026, this feature works across the latest devices and browser versions. This feature might not work in older devices or browsers.

Secure context: This feature is available only in secure contexts (HTTPS), in some or all supporting browsers.

Note: This feature is available in Web Workers.

The closed read-only property of the WebTransport interface returns a promise that resolves when the transport is closed.

This promise is created when the WebTransport object is created and is resolved when the transport is closed in any way, such as by calling the close() method, when the connection failed, or when the connection is closed by the server. Accessing or awaiting this promise does not trigger any action; it only allows actions to be taken when the transport is closed, similar to listening to an event.

Value

A Promise that resolves to an object containing the following properties:

closeCode: A number representing the error code for the error.
reason: A string representing the reason for closing the WebTransport.

Or, in the case of an unexpected closure, such as network failure, the promise rejects with an error.

Examples

const url = "https://example.com:4999/wt";

async function initTransport(url) {
  // Initialize transport connection
  const transport = new WebTransport(url);

  // The connection can be used once ready fulfills
  await transport.ready;

  // …
}

// …

async function closeTransport(transport) {
  // Respond to connection closing
  try {
    await transport.closed;
    console.log(`The HTTP/3 connection to ${url} closed gracefully.`);
  } catch (error) {
    console.error(`The HTTP/3 connection to ${url} closed due to ${error}.`);
  }
}