Function step

lib/internal/webstreams/readablestream.js:1600–1660

()

  }

  async function step() {
    if (shuttingDown) return true;

    if (dest[kState].backpressure) {
      await writerReadyPromise(writer).promise;
      if (shuttingDown) return true;
    }

    const controller = source[kState].controller;

    // Fast path: batch reads when data is buffered in a default controller.
    // This avoids creating PipeToReadableStreamReadRequest objects and
    // reduces promise allocation overhead.
    if (source[kState].state === 'readable' &&
        isReadableStreamDefaultController(controller) &&
        controller[kState].queue.length > 0) {

      while (controller[kState].queue.length > 0) {
        if (shuttingDown) return true;

        const chunk = dequeueValue(controller);

        if (controller[kState].closeRequested && !controller[kState].queue.length) {
          readableStreamDefaultControllerClearAlgorithms(controller);
          readableStreamClose(source);
        }

        // Write the chunk - we're already in a separate microtask from enqueue
        // because we awaited the writer ready promise above.
        state.currentWrite = writableStreamDefaultWriterWrite(writer, chunk);
        markPromiseAsHandled(state.currentWrite);

        // Check backpressure after each write
        if (dest[kState].backpressure) {
          // Backpressure - stop batch and wait for ready
          break;
        } else if (dest[kState].state !== 'writable' || writableStreamCloseQueuedOrInFlight(dest)) {
          // Closing or erroring - stop batch and wait for shutdown
          break;
        }
      }

      // Trigger pull if needed after batch
      readableStreamDefaultControllerCallPullIfNeeded(controller);

      // Check if stream closed during batch
      if (source[kState].state === 'closed') {
        return true;
      }

      // Yield to microtask queue between batches to allow events/signals to fire
      return false;
    }

    // Slow path: use read request for async reads
    const promise = PromiseWithResolvers();
    // eslint-disable-next-line no-use-before-define
    readableStreamDefaultReaderRead(reader, new PipeToReadableStreamReadRequest(writer, state, promise));