ReadableStreamDefaultReader.closed

The closed read-only property of the ReadableStreamDefaultReader interface returns a promise that fulfills if the stream becomes closed or the reader's lock is released, or rejects if the stream errors.

var closed = readableStreamDefaultReader.closed;

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');
})