Support for WebAssembly (Wasm)

The Flutter and Dart teams are excited to add WebAssembly as a compilation target when building applications for the web.

Background

To compile Dart and Flutter to Wasm, you need a browser that supports WasmGC. The Wasm standard plans to add WasmGC to help garbage-collected languages like Dart execute code in an efficient manner.

Chromium and V8 released stable support for WasmGC in Chromium 119, and the Firefox team announced stable support in Firefox 120. Note that Chrome on iOS uses WebKit, which doesn’t yet support WasmGC. To see the current status of WasmGC and other proposals, check out the WebAssembly roadmap.

Try it out

To try a pre-built Flutter web app using Wasm, check out the Material 3 WasmGC preview demo.

To experiment with Wasm in your own apps, follow the steps below.

Switch to the Flutter master channel and upgrade

Wasm compilation is only available on the latest builds of the master channel.

To learn more about Flutter build release channels and how to switch to the master channel, check out the Flutter wiki.

To then ensure you are running the latest version, run flutter upgrade.

To verify if your Flutter install supports Wasm, run flutter build web --help.

At the bottom of the output, you should find experimental Wasm options like:

Experimental options
    --wasm                                              Compile to WebAssembly rather than JavaScript.
                                                        See https://flutter.dev/wasm for more information.
    --omit-type-checks                                  Omit type checks in Wasm output.
                                                        Reduces code size and improves performance, but might affect runtime correctness. Use with care.
    --wasm-opt                                          Optimize output wasm using the Binaryen (https://github.com/WebAssembly/binaryen) tool.

          [debug]                                       Similar to `full`, but member names are preserved. Debugging is easier, but size is a bit bigger.
          [full] (default)                              wasm-opt is run. Build time is slower, but output is smaller and faster.
          [none]                                        wasm-opt is not run. Fastest build; bigger, slower output.

Pick a (simple) Flutter web application

Choose a Flutter application without platform-specific packages or JavaScript interop code. These known limitations cause issues with Wasm.

Run flutter build web --wasm

To build a web application with Wasm, add a --wasm flag to the existing flutter build web command.

flutter build web --wasm

The command sends its output to the build/web_wasm directory relative to package root.

Serve the output locally with an HTTP server

If you don’t have a local HTTP server installed, you can use the dhttpd package:

flutter pub global activate dhttpd

Then change to the build/web_wasm directory and run the server:

> cd build/web_wasm
> dhttpd
Server started on port 8080

Load it in a browser

As of November 21, 2023, two browser families should be able to run Flutter/Wasm content:

• Chromium-based browsers
  • Version 119 or later.
• Firefox
  • Version 120 or later.

If your configured browser meets the requirements, open localhost:8080 in the browser to view the app.

If the application doesn’t load:

1. Check the developer console for errors.
2. Validate a successful build with the typical JavaScript output.

Known limitations

Wasm support has some limitations. The following list covers some common issues.

Chrome 119 and Firefox 120 or later

As mentioned in Load it in a browser, to run Flutter web apps compiled to Wasm, use Chrome 119 or later or Firefox 120 or later.

Some earlier versions supported WasmGC with specific flags enabled, but WasmGC encodings changed once the feature was stabilized. To ensure compatibility, run the latest version of the Flutter master channel and the latest version of Chrome or Firefox.

Requires preview JS-interop to access browser and JS APIs

To support Wasm, Dart is shifting how it targets browser and JavaScript APIs. This shift prevents Dart code that uses dart:html or package:js from compiling to Wasm. Most platform-specific packages, such as package:url_launcher, use these libraries. As a result, they are currently incompatible with Wasm support in Flutter.

To check if a Wasm build failed due to these APIs, review the error output. These often return soon after a build invocation. An API-related error should resemble the following:

Target dart2wasm failed: Exception: ../../../../.pub-cache/hosted/pub.dev/url_launcher_web-2.0.16/lib/url_launcher_web.dart:6:8: Error: Dart library 'dart:html' is not available on this platform.
import 'dart:html' as html;
       ^
Context: The unavailable library 'dart:html' is imported through these packages:

    web_plugin_registrant.dart => package:url_launcher_web => dart:html
    web_plugin_registrant.dart => package:url_launcher_web => package:flutter_web_plugins => dart:html
    web_plugin_registrant.dart => package:flutter_web_plugins => dart:html

You can expect documentation on the replacements to these APIs later in 2023, including updates to the packages owned by the Dart and Flutter teams.

In the meantime, to experiment with Wasm support in Flutter, avoid these APIs.

Only build support

Neither flutter run nor DevTools support Wasm at the moment.

Learn more

Check out Flutter’s existing web support. Flutter to Wasm work continues. Once finished, we believe your existing Flutter web apps shouldn’t need much work to support Wasm.

If you want to learn more, watch this talk from our team at Wasm I/O 2023: Flutter, Dart, and WasmGC: A new model for web applications.

To check out the latest details on the Flutter and Dart WebAssembly effort, visit at flutter.dev/wasm.