The Native File System API: Simplifying access to local files

What is the Native File System API?

The new Native File System API enables developers to build powerful web apps that interact with files on the user's local device, like IDEs, photo and video editors, text editors, and more. After a user grants a web app access, this API allows web apps to read or save changes directly to files and folders on the user's device.

Current status

Using the Native File System API

To show off the true power and usefulness of the Native File System APIs, I wrote a single file text editor. It lets you open a text file, edit it, save the changes back to disk, or start a new file and save the changes to disk. It's nothing fancy, but provides enough to help you understand the concepts.

Enabling via chrome://flags

If you want to experiment with the Native File System API locally, enable the #native-file-system-api flag in chrome://flags.

Read a file from the local file system

The first use case I wanted to tackle was to ask the user to choose a file, then open and read that file from disk.

Ask the user to pick a file to read

The entry point to the Native File System API is window.chooseFileSystemEntries(). When called, it shows a file picker dialog, and prompts the user to select a file. After selecting a file, the API returns a handle to the file. An optional options parameter lets you influence the behavior of the file picker, for example, allowing the user to select multiple files, or directories, or different file types. Without any options specified), the file picker allows the user to select a single file, perfect for our text editor.

Like many other powerful APIs, calling chooseFileSystemEntries() must be done in a secure context, and must be called from within a user gesture.

let fileHandle;
butOpenFile.addEventListener('click', async (e) => {
  fileHandle = await window.chooseFileSystemEntries();
  // Do something with the file handle
});

Once the user selects a file, chooseFileSystemEntries() returns a handle, in this case a FileSystemFileHandle that contains the properties and methods needed to interact with the file.

It’s helpful to keep a reference to the file handle around so that it can be used later. It’ll be needed to save changes back to the file, or any other file operations. In the next few milestones, installed Progressive Web Apps will also be able to save the handle to IndexedDB and persist access to the file across page reloads.

Read a file from the file system

Now that you have a handle to a file, you can get the properties of the file, or access the file itself. For now, let’s simply read the contents of the file. Calling handle.getFile() returns a File object, which contains binary data as a blob. To get the data from the blob, call one of the reader methods (slice(), stream(), text(), arrayBuffer()).

const file = await fileHandle.getFile();
const contents = await file.text();

Putting it all together

Putting it all together, when the user clicks the Open button, the browser shows a file picker. Once they’ve selected a file, the app reads the contents and puts them into a <textarea>.

let fileHandle;
butOpenFile.addEventListener('click', async (e) => {
  fileHandle = await window.chooseFileSystemEntries();
  const file = await fileHandle.getFile();
  const contents = await file.text();
  textArea.value = contents;
});

Write the file to the local file system

In the text editor, there are two ways to save a file: Save, and Save As. Save simply writes the changes back to the original file using the file handle we got earlier. But Save As creates a new file, and thus requires a new file handle.

Create a new file

The chooseFileSystemEntries() API with {type: 'saveFile'} will show the file picker in “save” mode, allowing the user to pick a new file they want to use for saving. For the text editor, I also wanted it to automatically add a .txt extension, so I provided some additional parameters.

function getNewFileHandle() {
  const opts = {
    type: 'saveFile',
    accepts: [{
      description: 'Text file',
      extensions: ['txt'],
      mimeTypes: ['text/plain'],
    }],
  };
  const handle = window.chooseFileSystemEntries(opts);
  return handle;
}

Save changes to the original file

To write data to disk, I needed to create a FileSystemWriter by calling createWriter() on the file handle, then call write() to do the write. If permission to write hasn’t been granted already, the browser will prompt the user for permission to write to the file first (during createWriter()). The write() method takes a string, which is what we want for a text editor, but it can also take a BufferSource, or a Blob.

async function writeFile(fileHandle, contents) {
  // Create a writer (request permission if necessary).
  const writer = await fileHandle.createWriter();
  // Make sure we start with an empty file
  await writer.truncate(0);
  // Write the full length of the contents
  await writer.write(0, contents);
  // Close the file and write the contents to disk
  await writer.close();
}

Note: There's no guarantee that the contents are written to disk until the close() method is called.

The keepExistingData option when calling createWriter() isn’t supported yet, so once I get the writer, I immediately call truncate(0) to ensure I started with an empty file. Otherwise, if the length of the new content is shorter than the existing content, the existing content after the new content would remain. keepExistingData will be added in a future milestone.

When createWriter() is called, Chrome checks if the user has granted write permission. If not, it requests permission. If the user grants permission, the app can write the contents to the file. But, if the user does not grant permission, createWriter(), will throw a DOMException, and the app will not be able to write to the file. In the text editor, these DOMExceptions are handled in the saveFile() method.

Note: All the code behind the text editor is on GitHub, and the core file system interactions are in fs-helpers.js.

What else is possible?

Beyond reading and writing files, the Native File System API provides several other new capabilities.

Open a directory and enumerate its contents

To enumerate all files in a directory, call chooseFileSystemEntries() with the type option set to openDirectory. The user selects a directory in a picker, after which a FileSystemDirectoryHandle is returned, which lets you enumerate and access the directory’s files.

const butDir = document.getElementById('butDirectory');
butDir.addEventListener('click', async (e) => {
  const opts = {type: 'openDirectory'};
  const handle = await window.chooseFileSystemEntries(opts);
  const entries = await handle.getEntries();
  for await (const entry of entries) {
    const kind = entry.isFile ? 'File' : 'Directory';
    console.log(kind, entry.name);
  }
});

What’s currently supported?

We’re still working on some of the implementation for the Native File System API, and not everything in the spec (or explainer) has been completed.

As of Chrome 78, the following functionality is not available, or doesn't match the spec:

• Handles are not serializable, meaning they cannot be passed via postMessage(), or stored in IndexedDB.
• Non-atomic writes (i.e. calls to FileSystemFileHandle.createWriter() with inPlace: true).
• Writing to a file using a WritableStream.
• The FileSystemDirectoryHandle.resolve() method.

Security and permissions

We’ve designed and implemented the Native File System API using the core principles we defined in Controlling Access to Powerful Web Platform Features, including user control and transparency, and user ergonomics.

Opening a file or saving a new file

When opening a file, the user provides permission to read a file or directory via the file picker. The open file picker can only be shown via a user gesture when served from a secure context. If the user changes their mind, they can cancel the selection in the file picker and the site does not get access to anything. This is the same behavior as the <input type="file"> element.

Similarly, when a web app wants to save a new file, the browser will show the save file picker, allowing the user to specify the name and location of the new file. Since they are saving a new file to the device (versus overwriting an existing file), the file picker grants the app permission to write to the file.

Restricted folders

To help protect users and their data, the browser may limit the user’s ability to save to certain folders, for example, core operating system folders like Windows, the macOS Library folders, etc. When this happens, the browser will show a modal prompt and ask the user to choose a different folder.

Modifying an existing file or directory

A web app cannot modify a file on disk without getting explicit permission from the user.

Permission prompt

If a person wants to save changes to a file that they previously granted read access, the browser will show a modal permission prompt, requesting permission for the site to write changes to disk. The permission request can only be triggered by a user gesture, for example, clicking a “Save” button.

Alternatively, a web app that edits multiple files, like an IDE, can also ask for permission to save changes at the time of opening.

If the user chooses Cancel, and does not grant write access, the web app cannot save changes to the local file. It should provide an alternative method to allow the user to save their data, for example providing a way to “download” the file, saving data to the cloud, etc.

Transparency

Once a user has granted permission to a web app to save a local file, Chrome will show an icon in the omnibox. Clicking on the omnibox icon opens a popover showing the list of files the user has given access to. The user can easily revoke that access if they choose.

Permission persistence

The web app can continue to save changes to the file without prompting as long as the tab is open. Once a tab is closed, the site loses all access. The next time the user uses the web app, they will be re-prompted for access to the files. In the next few milestones, installed Progressive Web Apps (only) will also be able to save the handle to IndexedDB and persist access to handles across page reloads. In this case, an icon will be shown in the omnibox as long as the app has write access to local files.

Feedback

We want to hear about your experiences with the Native File System API.

Tell us about the API design

Is there something about the API that doesn’t work like you expected? Or are there missing methods or properties that you need to implement your idea? Have a question or comment on the security model?

• File a spec issue on the WICG Native File System GitHub repo, or add your thoughts to an existing issue.

Problem with the implementation?

Did you find a bug with Chrome's implementation? Or is the implementation different from the spec?

• File a bug at https://new.crbug.com. Be sure to include as much detail as you can, simple instructions for reproducing, and set Components to Blink>Storage>FileSystem. Glitch works great for sharing quick and easy repros.

Planning to use the API?

Planning to use the Native File System API on your site? Your public support helps us to prioritize features, and shows other browser vendors how critical it is to support them.

• Share how you plan to use it on the WICG Discourse thread
• Send a Tweet to @ChromiumDev with #nativefs and let us know where and how you’re using it.

Helpful Links

• Public explainer
• Native File System specification & File specification
• Tracking bug
• ChromeStatus.com entry
• Native File System API - Chromium Security Model
• Blink Component: Blink>Storage>FileSystem