The FileSystemWritableFileStream interface of the File System 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.

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

A text string 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("This is my file content");

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

The following examples show different 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 API
• The File System Access API: simplifying access to local files