The createWritable() method of the FileSystemFileHandle interface creates a FileSystemWritableFileStream that can be used to write to a file. The method returns a Promise which resolves to this created stream.

Any changes made through the stream won't be reflected in the file represented by the file handle until the stream has been closed. This is typically implemented by writing data to a temporary file, and only replacing the file represented by file handle with the temporary file when the writable filestream is closed.

options Optional

An object with the following properties:

keepExistingData Optional

A Boolean. Default false. When set to true if the file exists, the existing file is first copied to the temporary file. Otherwise the temporary file starts out empty.

A Promise which resolves to a FileSystemWritableFileStream object.

NotAllowedError DOMException

Thrown if the PermissionStatus.state for the handle is not 'granted' in readwrite mode.

The following asynchronous function writes the given contents to the file handle, and thus to disk.

async function writeFile(fileHandle, contents) {
  // Create a FileSystemWritableFileStream to write to.
  const writable = await fileHandle.createWritable();

  // Write the contents of the file to the stream.
  await writable.write(contents);

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

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