WritableStream.locked

var locked = writableStream.locked;

const writableStream = new WritableStream({
  write(chunk) {
    ...
  },
  close() {
    ...
  },
  abort(err) {
    ...
  }
}, queuingStrategy);

...

const writer = writableStream.getWriter();

writableStream.locked
// should return true, as the stream has been locked to a writer