ReadableStreamDefaultReader.closed

The closed read-only property of the ReadableStreamDefaultReader interface returns a Promise that fulfills when the stream closes or the reader's lock is released, or rejects if the stream throws an error. This property enables you to write code that responds to an end to the streaming process.

A Promise.

In this snippet, a previously-created reader is queried to see if the stream has been closed. When it is closed, the promise fulfills and the message is logged to the console.

reader.closed.then(() => {
  console.log('reader closed');
})