CountQueuingStrategy

const queueingStrategy = new CountQueuingStrategy({ highWaterMark: 1 });

const writableStream = new WritableStream(
  {
    // シンクの実装
    write(chunk) {
      // …
    },
    close() {
      // …
    },
    abort(err) {
      console.log("Sink error:", err);
    },
  },
  queueingStrategy,
);

const size = queueingStrategy.size();