FileSystemWritableFileStream

Secure context: This feature is available only in secure contexts (HTTPS), in some or all supporting browsers.

Experimental: This is an experimental technology
Check the Browser compatibility table carefully before using this in production.

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.

Instance properties

Inherits properties from its parent, WritableStream.

Instance methods

Inherits methods from its parent, WritableStream.

FileSystemWritableFileStream.write Experimental: Writes content into the file the method is called on, at the current file cursor offset.
FileSystemWritableFileStream.seek Experimental: Updates the current file cursor offset to the position (in bytes) specified.
FileSystemWritableFileStream.truncate Experimental: Resizes the file associated with the stream to be the specified size in bytes.

Examples

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 });

Specifications

Specification
File System Standard # api-filesystemwritablefilestream

Browser compatibility

BCD tables only load in the browser