electron/spec/fixtures/pages/filesystem.html

<script>
const {ipcRenderer} = require('electron')
function onInitFs (fs) {
  fs.root.getFile('log.txt', {create: true}, function (fileEntry) {
    fileEntry.createWriter(function (fileWriter) {
      var blob = new Blob(['Lorem Ipsum'], {type: 'text/plain'});
      fileWriter.onwriteend = function() {
        ipcRenderer.send('file-system-write-end')
      };
      fileWriter.onerror = errorHandler
      fileWriter.write(blob);
    }, errorHandler);
  }, errorHandler);

}

navigator.webkitPersistentStorage.requestQuota(5 * 1024 * 1024, function (granted) {
  webkitRequestFileSystem(TEMPORARY, granted, onInitFs, errorHandler);
}, errorHandler)

function errorHandler(e) {
  ipcRenderer.send('file-system-error', e)
}
</script>