Swift for Wasm: Q3 2025 Updates

Platform Swift for WebAssembly
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Since official support for Wasm in Swift 6.2 was announced at WWDC this year, Swift contributors have done a huge amount of work. To provide better visibility and to track progress, here’s a list of notable changes:

Surely I missed a few changes in this list unintentionally, so please feel free to post what you found notable in this thread. Many thanks to all of the contributors pushing the ecosystem forward!

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(ง^_^)ง LET'S GOOO!!!

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This is very cool. I really need to try out some WASM stuff soon if just to play with it!

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Are there any plans for Carton?

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We are soft-deprecating Carton in favor of swift package js SwiftPM Package Plugin provided by JavaScriptKit, which just generates an ES Module + .wasm file. You can still use Carton for now, but I no longer have any further plans to invest in Carton for now.

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I see. What are the plans for the debugging features?

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We actually added enhanced Swift support in the Wasm debugging extension for Chrome DevTools early this year: Debugging - Swift and WebAssembly

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I see. So that feature wasn’t necessarily required to be used with Carton. I apologize. I feel like I have been updating and changing how I build my project that I have lost track of where all the parts come into play. I have stayed with 5.10 so I can keep using carton. But I gotta move to 6.1+, so now I’m trying to move to whatever the newest way to do things is. I just finished building a new Ubuntu VM to test. The project compiled fine using the instructions form the page your provided.

Is there a mechanism to run 'wasm-opt’ after it is built?

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Yes, swift package js -c release automatically applies wasm-opt after the build if the tool is installed.

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I see. So the tool is installed. But the binary is at 120mb (should be like 20mb after wasm-opt). Is the stripped file being moved somewhere else? I guess, what I mean is there a folder like “Bundle” that is being created somewhere?

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  1. Hmm, if you build with release configuration, debug symbols should be stripped unless you explicitly pass --debug-info-format option. Can you share your build command?
  2. The "Bundle" directory is generated under .build/plugins/PackageToJS/outputs/Package by default. If you pass --output option with --allow-writing-to-package-directory plugin option, you can keep the existing directory layout:
$ swift package --swift-sdk "${SWIFT_SDK_ID}" -c release plugin --allow-writing-to-package-directory js --output ./Bundle
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I was just building with

swift package --swift-sdk 6.1-RELEASE-wasm32-unknown-wasi js --use-cdn

I used your suggested command (which is a life saver thank you!)

$ swift package --swift-sdk 6.1-RELEASE-wasm32-unknown-wasi -c release plugin --allow-writing-to-package-directory js --output ./Bundle

The .wasm file is now 50mb. Still not the almost 20mb it was when building with 5.10 and Carton.

Is there an optimization option that I could pass?

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The major binary size difference in the recent versions is ICU data. Recent Foundation brought full-featured ICU data into our binary, so you might be able to reduce the size into what we had before by replacing the data with slimmed version: GitHub - GoodNotes/swift-icudata-slim: A slimmed-down version of ICU for Swift

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I did not realize at all that the increase in ICU data would cause an approaching 30MB (I fathom that some of the size increase is from other stuff too?) . I suppose it is not awful. The site will just take about twice as long to load, which given the niche-ness of it that is ok.

Lastly is there an argument to get the index.html file to copy into the ./Bundle folder? Other than that (which I can definitely do myself), this looks like it is going to be mostly a good replacement.

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I think you actually don't need to copy index.html to ./Bundle. Instead, you can:

$ mkdir dist
$ cat <<EOS > dist/index.html
<!DOCTYPE html>
<html>
<body>
  <script type="module">
    import { init } from "./Bundle/index.js";
    init();
  </script>
</body>
</html>
EOS
$ swift package js --output ./dist/Bundle
$ echo "./dist/Bundle" >> .gitignore
$ # Serve ./dist by your HTTP server
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So I am a little confused, when I do --wasm-optimizations none with Carton on 5.10, the resulting binary is only about 32MB (with optimization it is about 20MB), but when I run it using the new method under 6.1 the resulting binary is larger than when there is no optimization at all on the old version.

I am wondering if something else is happening here that I am either skipping or missing. Like maybe certain unused functions are not being stripped?

Surly it isn't all ICU tables? Is it?

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If you can provide me a reproducible example, I can take a closer look :folded_hands: