A Weird Imagination

Loading multiple files without ZIP

The problem#

Last time, I showed how you can let a user have control over their data stored in a web app's OPFS by transferring directories in or out of the browser as ZIP files. But it would be more convenient if the user could just transfer folders instead without needing the extra step of going through an archive manager. There's no shortcut for getting data out of the browser: we can only save one file at a time (unless we use the Chrome-only File System Access API). But there is a cross-browser way to load multiple files, or even nested directories, into the browser.

The solution#

The HTML Drag and Drop API supports transferring multiple files and directories, although the details are a bit messy:

const target = document.getElementById("dropTarget");
// Required to make drop work.
target.addEventListener("dragover", (e) => e.preventDefault());
target.addEventListener("drop", async (e) => {
  await Promise.allSettled([...e.dataTransfer.items]
    .map(async (item) => {
      if (item.getAsFileSystemHandle) {
        await processFileSystemHandle(dir,
          await item.getAsFileSystemHandle());
      } else {
        await processFileSystemEntry(dir,

As you can see, for cross-browser support, we need to handle both getAsFileSystemHandle() and webkitGetAsEntry(). And, unfortunately, they return different types, so those two process*() functions really are pretty different:

async function processFileSystemHandle(dir, handle) {
  if (handle.kind === "directory") {
    const subdir = await dir.getDirectoryHandle(handle.name,
                             {create: true});
    for await (const entry of handle.values()) {
      await processFileSystemHandle(subdir, entry);
  } else /* handle.kind === "file" */ {
    await writeFile(await dir.getFileHandle(handle.name,
                              {create: true}),
                    await handle.getFile());
async function processFileSystemEntry(dir, entry) {
  async function readDirectory(directory) {
    let dirReader = directory.createReader();
    let getEntries = async () => {
      const results = await (new Promise((resolve, reject) =>
        dirReader.readEntries(resolve, reject)));
      if (results.length) {
        return [...results, ...await getEntries()];
      return [];

    return await getEntries();

  if (entry.isDirectory) {
    const subdir = await dir.getDirectoryHandle(entry.name,
                             {create: true});
    for (const el of await readDirectory(entry)) {
      await processFileSystemEntry(subdir, el);
  } else /* entry.isFile */ {
    const file = new Promise((resolve, reject) =>
      entry.file(resolve, reject));
    await writeFile(await dir.getFileHandle(entry.name,
                              {create: true}),
                    await file);

(These assume the writeFile() helper from last week's post to handle writing inside a Web Worker as necessary.)

The details#

Read more…

ZIP web app local data

The problem#

In my previous post, I gave some tips for making a web app save and load its data as a file to give the user control over their data. But for many applications, it's useful to think of the user's data as multiple files, possibly organized into directories. OPFS lets a web app store local data with a filesystem-like API, but, due to security concerns, there's no direct access to the user's real filesystem, so there's no straightforward way for the user to view or manipulate that data.

The solution#

The common way to deal with this kind of issue is to stuff all of the files into one file, reducing it to a solved problem. We'll use the ZIP archive file format as it's pretty universally supported, so the user can likely use such files. In these examples, I use the zip.js library, so you'll have to import zip-fs.min.js (or equivalent) to use them.

In these functions, dir is an OPFS directory: either the root directory from navigator.storage.getDirectory() or a subdirectory's FileSystemDirectoryHandle. They input/output the ZIP files as Blobs; use the helpers from my previous post to actually connect to the user's filesystem.

Downloading a directory as a ZIP is simple:

async function zipDirectory(dir) {
  const zipFs = new zip.fs.FS();
  await zipFs.root.addFileSystemHandle(dir);
  return await zipFs.exportBlob();

Reading a ZIP file into OPFS is more complicated and must be done inside a Web Worker (due to using createSyncAccessHandle()):

async function unzipToDirectory(zipfile, dir) {
  const z = new zip.fs.FS();
  await z.importBlob(zipfile);

  async function extract(z, dir) {
    if (z.directory) {
      const childDir = z.name
        ? await dir.getDirectoryHandle(z.name,
                    { create: true })
        : dir;
      for (const child of z.children) {
        await extract(child, childDir);
    } else {
      await writeFile(
        await dir.getFileHandle(z.name, { create: true }),
        await (await z.getBlob()).arrayBuffer());

  await extract(z.root, dir);
async function writeFile(file, contents) {
  const handle = await file.createSyncAccessHandle();
  if (contents.arrayBuffer) contents = await contents.arrayBuffer();

The details#

Read more…