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This module allows you to easily use the File System Access API on supporting browsers, with a transparent fallback to the <input type="file"> and <a download> legacy methods. This library is a ponyfill.

Read more on the background of this module in my post Progressive Enhancement In the Age of Fugu APIs.

Live Demo

See the library in action:

Usage Example

The module feature-detects support for the File System Access API and only loads the actually relevant code.

// The imported methods will use the File System
// Access API or a fallback implementation.
import {
} from '';

(async () => {
  // Open a file.
  const blob = await fileOpen({
    mimeTypes: ['image/*'],

  // Open multiple files.
  const blobs = await fileOpen({
    mimeTypes: ['image/*'],
    multiple: true,

  // Open all files in a directory,
  // recursively including subdirectories.
  const blobsInDirectory = await directoryOpen({
    recursive: true,

  // Save a file.
  await fileSave(blob, {
    fileName: 'Untitled.png',
    extensions: ['.png'],

API Documentation

Opening files:

// Options are optional.
const options = {
  // List of allowed MIME types, defaults to `*/*`.
  mimeTypes: ['image/*'],
  // List of allowed file extensions (with leading '.'), defaults to `''`.
  extensions: ['.png', '.jpg', '.jpeg', '.webp'],
  // Set to `true` for allowing multiple files, defaults to `false`.
  multiple: true,
  // Textual description for file dialog , defaults to `''`.
  description: 'Image files',

const blobs = await fileOpen(options);

Opening directories:

// Options are optional.
const options = {
  // Set to `true` to recursively open files in all subdirectories,
  // defaults to `false`.
  recursive: true,

const blobs = await directoryOpen(options);

The module also polyfills a webkitRelativePath property on returned files in a consistent way, regardless of the underlying implementation.

Saving files:

// Options are optional.
const options = {
  // Suggested file name to use, defaults to `''`.
  fileName: 'Untitled.txt',
  // Suggested file extensions (with leading '.'), defaults to `''`.
  extensions: ['.txt'],

// Optional file handle to save back to an existing file.
// This will only work with the File System Access API.
// Get a `FileHandle` from the `handle` property of the `Blob`
// you receive from `fileOpen()` (this is non-standard).
const handle = previouslyOpenedBlob.handle;

await fileSave(someBlob, options, handle);

Browser-NativeFS in Action

You can see the module in action in the Excalidraw drawing app.



A similar, but more extensive library called native-file-system-adapter is provided by @jimmywarting.


Thanks to @developit for improving the dynamic module loading and @dwelle for the helpful feedback, issue reports, and the Windows build fix. Directory operations were made consistent regarding webkitRelativePath and parallelized and sped up significantly by @RReverser. The TypeScript type annotations were provided by @nanaian.

License and Note

Apache 2.0.

This is not an official Google product.

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