WritableStream: locked property

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