The FileSystemWritableFileStream interface of the File System Access API is a WritableStream object with additional convenience methods, which operates on a single file on disk. The interface is accessed through the FileSystemFileHandle.createWritable() method.

Inherits properties from its parent, WritableStream.

Inherits methods from its parent, WritableStream.

FileSystemWritableFileStream.write

Writes content into the file the method is called on, at the current file cursor offset.

FileSystemWritableFileStream.seek

Updates the current file cursor offset to the position (in bytes) specified.

FileSystemWritableFileStream.truncate

Resizes the file associated with the stream to be the specified size in bytes.

This asynchronous function opens the 'Save File' picker, which returns a FileSystemFileHandle once a file is selected. From which a writable stream is then created using the FileSystemFileHandle.createWritable() method.

A user defined Blob is then written to the stream which is subsequently closed.

async function saveFile() {
  // create a new handle
  const newHandle = await window.showSaveFilePicker();

  // create a FileSystemWritableFileStream to write to
  const writableStream = await newHandle.createWritable();

  // write our file
  await writableStream.write(imgBlob);

  // close the file and write the contents to disk.
  await writableStream.close();
}

The following show different examples of options that can be passed into the write() method.

// just pass in the data (no options)
writableStream.write(data);

// writes the data to the stream from the determined position
writableStream.write({ type: "write", position, data });

// updates the current file cursor offset to the position specified
writableStream.write({ type: "seek", position });

// resizes the file to be size bytes long
writableStream.write({ type: "truncate", size });

• File System Access API
• The File System Access API: simplifying access to local files