FileSystemWritableFileStream

async function saveFile() {
  // 
  const newHandle = await window.showSaveFilePicker();

  //  FileSystemWritableFileStream 
  const writableStream = await newHandle.createWritable();

  // 
  await writableStream.write(imgBlob);

  // 
  await writableStream.close();
}

//  ()
writableStream.write(data);

// 
writableStream.write({ type: "write", position, data });

// 
writableStream.write({ type: "seek", position });

//  size 
writableStream.write({ type: "truncate", size });