This code belongs to this article: https://hesam-kamalan.medium.com/flutter-web-download-file-from-firebase-6a5c35ef7613

file_downloader.dart

// Dart imports:
import 'dart:convert';
import 'dart:typed_data';

// Flutter imports:
import 'package:flutter/foundation.dart' show kIsWeb;

// Package imports:
import 'package:get_it/get_it.dart';
import 'package:universal_html/html.dart';
import 'package:url_launcher/url_launcher.dart';

// Project imports:
import './modules/managers/firebase_manager.dart';

/// Open file in a browser
Future<void> launchURL(String? url) async {
  if (url == null) return;
  await canLaunch(url) ? await launch(url) : throw 'Could not launch $url';
}

/// Download file from firebase
Future<void> downloadFile(String fileName, String pathToFile, String url) async {
  if (pathToFile.isEmpty) return;
  
  if (!kIsWeb) return;

  Uint8List? data = await FirebaseStorage.instance.ref(pathToFile).getData();
  // Fallback option
  if (data == null) return await launchURL(url);
  String encodedData = base64Encode(data);

  try {
    AnchorElement(href: 'data:application/octet-stream;charset=utf-8;base64,$encodedData')
      ..setAttribute('download', fileName)
      ..click();
  } catch (error) {
    // Fallback option
    await launchURL(url);
  }
}