File System Access API (ponyfill)

Introduction

This is an implementation of the File System Access specification. It is a ponyfill that uses the native browser implementation when available and falls back to a custom one otherwise. It also includes several storage adapters which can be used in the browser, but also in other environments, such as NodeJS or Deno.

The library roughly contains the following:

1. Ponyfills for showDirectoryPicker, showOpenFilePicker and showSaveFilePicker, with fallbacks to regular input elements.
2. Ponyfills for FileSystemFileHandle and FileSystemDirectoryHandle interfaces.
3. Ponyfill for FileSystemWritableFileStream to truncate and write data.
4. An implementation of navigator.storage.getDirectory() (getOriginPrivateDirectory) which can read & write data to and from several sources called adapters, not just the browser sandboxed file system
5. An optional polyfill for DataTransferItem.prototype.getAsFileSystemHandle()

This package builds upon native-file-system-adapter, adding several bug fixes, updates for compliance with the latest spec, browser support improvements (especially Safari), support for bundlers, stricter error handling and more. It is fully rewritten in TypeScript and provides type-safe declarations out-of-the-box.

File system adapters

When getOriginPrivateDirectory is called with no arguments, the browser's native sandboxed file system is used, just like calling navigator.storage.getDirectory().

Optionally, a file system backend adapter can be provided as an argument. This ponyfill ships with a few backends built in:

• node: Uses NodeJS's fs module
• deno: Interact with filesystem using Deno
• sandbox (deprecated): Uses requestFileSystem. Only supported in Chromium-based browsers using the Blink engine.
• indexeddb: Stores files into the browser's IndexedDB object database.
• memory: Stores files in-memory. Thus, it is a temporary file store that clears when the user navigates away.
• cache: Stores files with the browser's Cache API in request/response pairs.

You can even load in your own underlying adapter and get the same set of API's by implementing the FileSystemFileHandleAdapter and FileSystemFolderHandleAdapter interfaces

The API is designed in such a way that it can work with or without the ponyfill if you choose to remove or add this.
It's not trying to interfere with the changing spec by using other properties that may conflict with the feature changes to the spec.

Basic usage

ES import in browser

You can directly import the module using an absolute URL:

<script type="module">
import { getOriginPrivateDirectory } from 'https://cdn.jsdelivr.net/npm/file-system-access/lib/es2018.js'

// Get a directory handle for a sandboxed virtual file system
// same as calling navigator.storage.getDirectory()
const dirHandle1 = await getOriginPrivateDirectory()

// or use an adapter (see adapters table above for a list of available adapters)
const dirHandle2 = await getOriginPrivateDirectory(import('https://cdn.jsdelivr.net/npm/file-system-access/lib/adapters/<adapterName>.js'))
</script>

Install via NPM

Works in Node.JS v14.8+ or in the browser, with a module bundler such as Webpack.

npm i file-system-access

import { getOriginPrivateDirectory } from 'file-system-access'
import indexedDbAdapter from 'file-system-access/lib/adapters/indexeddb.js'
import nodeAdapter from 'file-system-access/lib/adapters/node.js'

const dirHandle = await getOriginPrivateDirectory(indexedDbAdapter)
const nodeDirHandle = await getOriginPrivateDirectory(nodeAdapter, './real-dir')

Examples

File system sandbox

You can get a directory handle to a sandboxed virtual file system using the getOriginPrivateDirectory function. This is a legacy name introduced by an older Native File System specification and is kept for simplicity. It is equivalent to the navigator.storage.getDirectory() method introduced by the later File System Access spec.

import { getOriginPrivateDirectory, support } from 'file-system-access'

// Uses only native implementation - same as calling navigator.storage.getDirectory()
if (support.adapter.native) {
  handle = await getOriginPrivateDirectory()
}
// Blinks old sandboxed api
if (support.adapter.sandbox) {
  handle = await getOriginPrivateDirectory(import('file-system-access/lib/adapters/sandbox.js'))
}
// fast in-memory file system
handle = await getOriginPrivateDirectory(import('file-system-access/lib/adapters/memory.js'))
// Using indexDB
handle = await getOriginPrivateDirectory(import('file-system-access/lib/adapters/indexeddb.js'))
// Store things in the new Cache API as request/responses (bad at mutating data)
if (support.adapter.cache) {
  handle = await getOriginPrivateDirectory(import('file-system-access/lib/adapters/cache.js'))
}

// Node only variant:
handle = await getOriginPrivateDirectory(import('file-system-access/lib/adapters/memory.js'))
handle = await getOriginPrivateDirectory(import('file-system-access/lib/adapters/node.js'), './starting-path')

// Deno only variant:
handle = await getOriginPrivateDirectory(import('file-system-access/src/adapters/memory.js'))
handle = await getOriginPrivateDirectory(import('file-system-access/src/adapters/deno.js'), './starting-path')

File and directory pickers

import { showDirectoryPicker, showOpenFilePicker } from 'file-system-access'

// The polyfilled (file input) version will turn into a memory adapter
// You will have read & write permission on the memory adapter,
// you might want to transfer (copy) the handle to another adapter
const [fileHandle] = await showOpenFilePicker({_preferPolyfill: boolean, ...sameOpts})
const dirHandle = await showDirectoryPicker({_preferPolyfill: boolean, ...sameOpts})

Drag and drop

// Apply polyfill for DataTransferItem.prototype.getAsFileSystemHandle()

import { polyfillDataTransferItem } from 'file-system-access'
await polyfillDataTransferItem();

// or just use a static import

import 'file-system-access/lib/polyfillDataTransferItem.js'

window.ondrop = async evt => {
  evt.preventDefault()
  for (let item of evt.dataTransfer.items) {
    const handle = await item.getAsFileSystemHandle()
    console.log(handle)
  }
}

Copy file handles to sandboxed file system

import { showOpenFilePicker, getOriginPrivateDirectory } from 'file-system-access'

// request user to select a file
const [fileHandle] = await showOpenFilePicker({
  types: [], // default
  multiple: false, // default
  excludeAcceptAllOption: false, // default
  _preferPolyfill: false // default
})

// returns a File Instance
const file = await fileHandle.getFile()

// copy the file over to a another place
const rootHandle = await getOriginPrivateDirectory()
const fileHandle = await rootHandle.getFileHandle(file.name, { create: true })
const writable = await fileHandle.createWritable()
await writable.write(file)
await writable.close()

Save / download a file

import { showSaveFilePicker } from 'file-system-access'

const fileHandle = await showSaveFilePicker({
  _preferPolyfill: false,
  suggestedName: 'Untitled.png',
  types: [
    { accept: { "image/png": [ ".png" ] } },
    { accept: { "image/jpg": [ ".jpg" ] } },
    { accept: { "image/webp": [ ".webp" ] } }
  ],
  excludeAcceptAllOption: false // default
})

// Look at what extension they have chosen
const extensionChosen = fileHandle.name.split('.').pop()

const blob = {
  jpg: generateCanvasBlob({ type: 'blob', format: 'jpg' }),
  png: generateCanvasBlob({ type: 'blob', format: 'png' }),
  webp: generateCanvasBlob({ type: 'blob', format: 'webp' })
}[extensionChosen]

await blob.stream().pipeTo(fileHandle.createWritable())
// or
var writer = fileHandle.getWritable()
await writer.write(blob)
await writer.close()

Supported browsers

When importing as an ES module, browsers that support dynamic imports and ES2018 features are a minimum requirement. When using a bundler, this restriction is no longer applicable.

When the directory picker falls back to input elements, the browser must support webkitdirectory and webkitRelativePath. Because of this, support for picking directories is generally poor on Mobile browsers.

For drag and drop, the getAsFileSystemHandle() polyfill depends on the File and Directory Entries API support, more specifically FileSystemDirectoryEntry, FileSystemFileEntry and webkitGetAsEntry.

Limitations

• Storing a file handle in IndexedDB or sharing it with postMessage isn't currently possible unless you use native.
• showDirectoryPicker and showOpenFilePicker will not throw any AbortErrors (e.g. user cancellations) when using a fallback input element
• showDirectoryPicker will return a flat hierarchy when a fallback input element is used and webkitRelativePath is not supported (e.g. mobile Safari). This can be detected by checking if the name attribute of the root directory handle is an empty string.
• showSaveFilePicker may not actually show any prompt when using a fallback input (e.g. on Chrome the file is auto-saved to the browser's preferred download folder)
• Cache adapter only works in secure (HTTPS) contexts
• IndexedDB adapter may not work in some browsers in Private mode
• Mobile Safari scrambles original file names

A note when downloading with the polyfilled version

Saving/downloading a file is borrowing some of ideas from StreamSaver.js. The difference is:

• Using service worker is optional choice with this adapter.
• It does not rely on some man-in-the-middle or external hosted service worker.
• If you want to stream large data to the disk directly instead of taking up much RAM you need to set up a service worker yourself.
  (note that it requires https - but again worker is optional)

to set up a service worker you have to basically copy the example and register it:

navigator.serviceWorker.register('sw.js')

Without service worker you are going to write all data to the memory and download it once it closes.

Seeking and truncating won't do anything. You should be writing all data in sequential order when using the polyfilled version.

Testing

• In project folder, run npx http-server -p 3000 .
• For browser tests, open http://localhost:3000/example/test.html in your browser.
• For node: npm run test-node
• For deno: npm run test-deno

Resources

I recommend to follow up on this links for you to learn more about the API and how it works

• https://web.dev/file-system-access/
• https://wicg.github.io/file-system-access/
• https://github.com/WICG/file-system-access

Alternatives

• native-file-system-adapter: The original implementation in vanilla JavaScript by @jimmywarting.
• browser-fs-access by @tomayac: A similar, more like a shim (without getSystemDirectory).
• StreamSaver by @jimmywarting: A way to save large data to the disk directly with a writable stream
  (same technique can be achieved if service worker are setup properly)
• FileSaver by @eligrey: One among the first libs to save blobs to the disk

License

file-system-access is licensed under the MIT License. See LICENSE for details.