JavaScript: File System Access API: Cannot enqueue a chunk into a closed readable stream

Question

I am using the JavaScript File System Access API streaming a readable stream's output to a FileSystemWritableFileStream. I created a writable stream wrapper to use the writeStream.pipeTo(readStream) method, but I get the same error when performing the same process using the manual way of reading from the ReadableStream and writing to the FileSystemWritableFileStream. I skip the header using seek since it includes the file size, and start writing the data. Here's the code:

function fileWritableStreamToWritableStream(fileStream, headerWriteMethod, headerLengthBytes) {
  let writer = fileStream;
  let bytesWritten = 0;
  const writableStream = new WritableStream({
    // Implement the sink
    async start(controller) {
      if(headerLengthBytes) {
        // returns a promise that resolves when the writer is ready for another operation
        return writer.seek(headerLengthBytes);
      }
    },
    async write(chunk) {
      bytesWritten += chunk.length;
      // returns a promise that resolves when the writer is ready for another operation
      return writer.write({ type: 'write', data: chunk });
    },
    async close() {
      let promise = Promise.resolve();
      if(headerWriteMethod) {
          let headerBuffer = new ArrayBuffer(headerLengthBytes);
          let view = new DataView(headerBuffer);
          let headerBytes = headerWriteMethod(view, bytesWritten);
          promise = writer.seek(0)
            .then(() => writer.write({type: 'write', data: headerBytes.buffer }));
      }
      return promise.then(() => writer.close());
    }
  });
  return writableStream;
}

This was used elsewhere like so:

audioStream.pipeTo(writableStream);

As soon as I start writing, I get the following error in the console:

Errors: Uncaught (in promise) TypeError: Failed to execute 'write' on
'UnderlyingSinkBase': required member type is undefined.
        wav-recording.service.js:262 Uncaught TypeError: Failed to execute 'enqueue' on 'ReadableStreamDefaultController': Cannot enqueue a chunk into a closed readable stream
            at MessagePort.port.onmessage

Answer 1

It turns out the stream was closed because of an error: The File System Access API currently does not support seeking beyond the end of a file. the solution was to use truncate to resize the file:

async start(controller) {
  if(headerLengthBytes) {
    //this call will resize the file to headerLengthBytes so that seek doesn't go past the end of the file.
    await writer.truncate(headerLengthBytes);
    return writer.seek(headerLengthBytes);
  }
}

I will note that the devs are aware of this problem and plan to resolve this issue in the future. However, I think this post will be useful to anyone who gets the same console error for a reason other than the specific one here.