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…

Keeping web app data local

The problem#

Users don't tend to have a lot of control over their data in web apps. Most often, the data is stored on a server the user does not control—or, if they do control it, we're talking about self-hosting which is much more involved then just navigating to a web app in a browser. Alternatively, the data may be stored locally, but using various browser-specific mechanisms which make it difficult for the user to share, backup, or otherwise reason about the data the web app manipulates.

While desktop apps can replicate these problems, usually they store data in files either explicitly chosen by the user or in well-known locations.

The solution#

Files are a flexible interface to let users do whatever they want with their data, so let's use them for web apps, too.

To save a file to the user's computer, modified from this example:

function saveFile(filename, data, mimeType) {
  const element = document.createElement("a");
  const url = URL.createObjectURL(new Blob([data],
                                  { type: mimeType }));
  element.setAttribute("href", url);
  element.setAttribute("download", filename);
// Save a JSON file:
  JSON.stringify({"Hello": "World!"}, null, 2),

(Consider using beforeunload if the user has unsaved changes to make sure they really do have their data in the file, and not just in the browser.)

To load a file from the user's computer:

function loadFile() {
  const element = document.createElement("input");
  element.type = "file";
  return new Promise((resolve, reject) => {
      () => resolve(element.files[0]));
      () => reject("User canceled."));
// loadFile() must be called from a real user click.
  async (e) => myLoadFunc(await loadFile()));

The details#

Read more…