I get this error in electron’s main.js while using nodejs fs.

 fs.readFile('./index.html', 'utf8', ......
[Error: EACCES: permission denied, open './index.html'] {
  errno: -13,
  code: 'EACCES',
  syscall: 'open',
  path: './index.html'
Connections used:
Interface         Plug                   Slot               Notes
browser-support   test:browser-support   :browser-support   -
hardware-observe  test:hardware-observe  :hardware-observe  manual
home              test:home              :home              -
network           test:network           :network           -
network-bind      test:network-bind      :network-bind      -
opengl            test:opengl            :opengl            -
raw-usb           test:raw-usb           :raw-usb           manual
wayland           test:wayland           :wayland           manual
Snapd version:
snap    2.49.1+git814.gc4a4ea6
snapd   2.49.1+git814.gc4a4ea6
series  16
ubuntu  20.04
kernel  5.8.0-45-generic
Any idea why?
    
electron/electron-quick-start/blob/master/main.js
// Modules to control application life and create native browser window
const {app, BrowserWindow} = require('electron')
const path = require('path')
function createWindow () {
  // Create the browser window.
  const mainWindow = new BrowserWindow({
    width: 800,
    height: 600,
    webPreferences: {
      preload: path.join(__dirname, 'preload.js')
  // and load the index.html of the app.
  mainWindow.loadFile('index.html')
  // Open the DevTools.
  // mainWindow.webContents.openDevTools()
  This file has been truncated. show original
    <meta charset="UTF-8">
    <!-- https://developer.mozilla.org/en-US/docs/Web/HTTP/CSP -->
    <meta http-equiv="Content-Security-Policy" content="default-src 'self'; script-src 'self'">
    <meta http-equiv="X-Content-Security-Policy" content="default-src 'self'; script-src 'self'">
    <title>Hello World!</title>
  </head>
    <h1>Hello World!</h1>
    We are using Node.js <span id="node-version"></span>,
    Chromium <span id="chrome-version"></span>,
    and Electron <span id="electron-version"></span>.
    <!-- You can also require other files to run in this process -->
    <script src="./renderer.js"></script>
  </body>
</html>
And before loadFile i added:
fs.readFile('./index.html', 'utf8', function (err, data) {...}
I snapcrafted it on Ubuntu 20.04, and installed it with sudo.
$ snapcraft
$ Snapped test_0.1.0_amd64.snap
$ sudo snap install --dangerous ./test_0.1.0_amd64.snap 
Connections are as previous post.

When i try to run, it gives EACCES
$ sudo snap run test
Mir-kiosk is runned in another terminal…
In snap.yaml:
apps:
  test:
    daemon: simple
    restart-condition: always
    command: desktop-launch xwayland-kiosk-launch "$SNAP/main/electron-quick-start" "--no-sandbox"
    plugs:
    - browser-support
    - network
    - network-bind
    - opengl
    - wayland
    - home
    - raw-usb
    - hardware-observe
parts:
  test:
    plugin: nil
    source: .
    after: [desktop-gtk3]
    override-build: |
        case $SNAPCRAFT_ARCH_TRIPLET in
          "i386-linux-gnu") ARCH="ia32";;
          "x86_64-linux-gnu") ARCH="x64";;
          "arm-linux-gnueabihf") ARCH="armv7l";;
          "aarch64-linux-gnu") ARCH="arm64";;
          *) echo "ERROR: electron does not support the '$SNAPCRAFT_ARCH_TRIPLET' architecture" && exit 1;;
        rm -Rf ./node_modules &&
        npm install -g npm@latest &&
        npm install --unsafe-perm=true --build-from-source &&
        ./node_modules/.bin/electron-rebuild &&
        npm install --unsafe-perm=true electron-packager &&
        ./node_modules/.bin/electron-packager . --overwrite --platform=linux --arch=$ARCH --output=release-build --prune=true
  ...........
  console.log(exec("pwd").toString())
  console.log(exec("ls -la").toString())
  mainWindow.loadFile('index.html')
And now i get:
>>>PWD response
/media/t0skfak3r/drive3/test
>>>ls -la response
ls: cannot open directory '.': Permission denied
(node:548955) UnhandledPromiseRejectionWarning: Error: Command failed: ls -la
ls: cannot open directory '.': Permission denied
    at checkExecSyncError (child_process.js:625:11)
    at execSync (child_process.js:661:15)
    at electron/js2c/asar_bundle.js:5:12164
    at createWindow (/snap/test/x11/main/resources/app/main.js:25:15)
    at /snap/test/x11/main/resources/app/main.js:68:3
              
you have $SNAP_DATA whcih translates to /var/snap/<snapname>/current,  $SNAP_COMMON pointing to /var/snap/<snapname>/common as well as $SNAP_USER_DATA and $SNAP_USER_COMMON pointing to ~/snap/<snapname>/current and ~/snap/<snapname>/common  … all these are available through environment variables at runtime of each snap …
              
Yes, forgot to write here.
Solved it reading: https://askubuntu.com/questions/762354/where-can-ubuntu-snaps-write-data.
And in electron i get the path like this:
var path = process.env.SNAP_USER_DATA ? process.env.SNAP_USER_DATA + '/' : './';
Many thanks!