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📦 Watcher, bundler, and test runner for your SwiftWasm apps


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carton 📦

Watcher, bundler, and test runner for your SwiftWasm apps

The main goal of carton is to provide a smooth zero-config experience when developing for WebAssembly. It currently supports these features with separate commands:

  • Watching the app for source code changes and reloading it in your browser with swift run carton dev.
  • Running your XCTest suite in the full JavaScript/DOM environment with swift run carton test.
  • Optimizing and packaging the app for distribution with swift run carton bundle.


The main motivation for carton came after having enough struggles with webpack.js, trying to make its config file work, looking for appropriate plugins. At some point the maintainers became convinced that the required use of webpack in SwiftWasm projects could limit the wider adoption of SwiftWasm itself. Hopefully, with carton you can avoid using webpack altogether. carton also simplifies a few other things in your SwiftWasm development workflow such as toolchain and SDK installations.

Getting started



To install carton as a SwiftPM plugin, first add the following line to your Package.swift file:

dependencies: [
    .package(url: "", from: "1.0.0"),


  • If you already use carton before 0.x.x versions via Homebrew, you can remove it with brew uninstall carton and install the new version as a SwiftPM dependency.
  • Also please remove the old .build directory before using the new carton


The swift run carton dev command builds your project with the SwiftWasm toolchain and starts an HTTP server that hosts your WebAssembly executable and a corresponding JavaScript entrypoint that loads it. The app, reachable at, will automatically open in your default web browser. The port that the development server uses can also be controlled with the --port option (or -p for short). You can edit the app source code in your favorite editor and save it, carton will immediately rebuild the app and reload all browser tabs that have the app open. You can also pass a --verbose flag to keep the build process output available, otherwise stale output is cleaned up from your terminal screen by default. If you have a custom index.html page you'd like to use when serving, pass a path to it with a --custom-index-page option.

The swift run carton test command runs your test suite in wasmer, node or using your default browser. You can switch between these with the --environment option, passing either: command, node or browser. Code that depends on JavaScriptKit should pass either --environment node or --environment browser options, depending on whether it needs Web APIs to work. Otherwise the test run will not succeed, since JavaScript environment is not available with --environment command. If you want to run your test suite on CI or without GUI but on browser, you can pass --headless flag. It enables WebDriver-based headless browser testing. Note that you need to install a WebDriver executable in PATH before running tests. You can use the command with a prebuilt test bundle binary instead of building it in carton by passing --prebuilt-test-bundle-path <your binary path>.

The swift run carton bundle command builds your project using the release configuration (although you can pass the --debug flag to it to change that), and copies all required assets to the Bundle directory. You can then use a static file hosting (e.g. GitHub Pages) or any other server with support for static files to deploy your application. All resulting bundle files except index.html are named by their content hashes to enable cache busting. As with swift run carton dev, a custom index.html page can be provided through the --custom-index-page option. You can also pass --debug-info flag to preserve names and DWARF sections in the resulting .wasm file, as these are stripped in the release configuration by default. By default, swift run carton bundle will run wasm-opt on the resulting .wasm binary in order to reduce its file size. That behaviour can be disabled (in order to speed up the build) by appending the --wasm-optimizations none option.

All commands that delegate to swift build and swiftc (namely, dev, test, and bundle) can be passed -Xswiftc arguments, which is equivalent to -Xswiftc in swift build. All -Xswiftc arguments are propagated to swiftc itself unmodified.

All of these commands and subcommands can be passed a --help flag that prints usage info and information about all available options.

How does it work?

carton bundles a WASI polyfill, which is currently required to run any SwiftWasm code, and the JavaScriptKit runtime for convenience. carton also embeds an HTTP server for previewing your SwiftWasm app directly in a browser. The development version of the polyfill establishes a helper WebSocket connection to the server, so that it can reload development browser tabs when rebuilt binary is available. This brings the development experience closer to Xcode live previews, which you may have previously used when developing SwiftUI apps.

carton does not require any config files for these basic development scenarios, while some configuration may be supported in the future, for example for complex asset pipelines if needed. The only requirement is that your Package.swift contains at least a single executable product, which then will be compiled for WebAssembly and served when you start swift run carton dev in the directory where Package.swift is located.

carton is built as a SwiftPM Plugin with SwiftNIO, and supports both macOS and Linux. (Many thanks to everyone supporting and maintaining those projects!)


There is no comprehensive documentation about the internal architecture of carton, but this page might be helpful to understand the current architecture and rationale.

Running swift run carton dev with the release configuration

By default swift run carton dev will compile in the debug configuration. Add the --release flag to compile in the release configuration.

Version compatibility

carton previously embedded runtime parts of the JavaScriptKit library. This runtime allows Swift and JavaScript code to interoperate in Node.js and browser environments. Because of how JavaScriptKit runtime was embedded, older versions of JavaScriptKit were incompatible with different versions of carton and vice versa. This incompatibility between different versions was resolved starting with JavaScriptKit 0.15 and carton 0.15. All version combinations of carton and JavaScriptKit higher than those are compatible with each other.

You still have to keep in mind that older versions of SwiftWasm may be incompatible with newer carton. You can follow the compatibility matrix if you need to use older verions:

carton version SwiftWasm version JavaScriptKit version Tokamak version
1.0+ 5.9.2
0.20+ 5.9
0.15+ 5.6 0.15+ 0.10.1+
0.14 5.6 0.14 0.10
0.13 5.5 0.13 0.9.1
0.12.2 5.5 0.12 0.9.1
0.12.0 5.5 0.11 0.9.0
0.11.0 5.4 0.10.1 0.8.0



If this tool saved you any amount of time or money, please consider sponsoring the SwiftWasm organization. Or you can sponsor some of our maintainers directly on their personal sponsorship pages: @carson-katri, @kateinoigakukun, and @MaxDesiatov. While some of the sponsorship tiers give you priority support or even consulting time, any amount is appreciated and helps in maintaining the project.

Become a gold or platinum sponsor and contact maintainers to add your logo on our README on GitHub with a link to your site.

Coding Style

This project uses SwiftFormat and SwiftLint to enforce formatting and coding style. We encourage you to run SwiftFormat within a local clone of the repository in whatever way works best for you either manually or automatically via an Xcode extension, build phase or git pre-commit hook etc.

To guarantee that these tools run before you commit your changes on macOS, you're encouraged to run this once to set up the pre-commit hook:

brew bundle # installs SwiftLint, SwiftFormat and pre-commit
pre-commit install # installs pre-commit hook to run checks before you commit

Refer to the pre-commit documentation page for more details and installation instructions for other platforms.

SwiftFormat and SwiftLint also run on CI for every PR and thus a CI build can fail with incosistent formatting or style. We require CI builds to pass for all PRs before merging.

Code of Conduct

This project adheres to the Contributor Covenant Code of Conduct. By participating, you are expected to uphold this code. Please report unacceptable behavior to