Browser WASM

A WebImage artifact with backend = "wasm-gc" compiles Java or Kotlin to WasmGC: a WebAssembly module whose objects live in the browser’s garbage-collected heap, plus a JavaScript loader that fetches and starts it. To run an existing .wasm file with Elide, see WebAssembly.

Prerequisites

• Binaryen 119 or later, with wasm-as on PATH at build time.
• Code that meets the Native Image requirements: reachability metadata for reflection, resources and proxies; no native dependencies.

Build

hello-wasm/
  elide.pkl
  src/main/java/Hello.java

public class Hello {
  public static void main(String[] args) {
    System.out.println("Hello from WasmGC");
  }
}

amends "elide:project.pkl"
import "elide:Jvm.pkl" as jvm
import "elide:NativeImage.pkl" as nativeImage

name = "hello-wasm"

artifacts {
  ["app-jar"] = new jvm.Jar {
    sources { "main" }
  }
  ["web"] = new nativeImage.WebImage {
    from { "app-jar" }
    name = "hello"
    entrypoint = "Hello"
    backend = "wasm-gc"
    runtime = "browser"
  }
}

elide build

Output goes to .dev/artifacts/native-image/:

Other properties and compiler options: WebImage.

Load it in a page

Put index.html next to the bundle:

<!doctype html>
<html lang="en">
  <meta charset="utf-8" />
  <title>Hello WasmGC</title>
  <body>
    <script src="./hello.js"></script>
  </body>
</html>

elide dev --no-tui --port 8000 .dev/artifacts/native-image

Open http://localhost:8000/; Hello from WasmGC appears in the browser console.

• The loader requests the module at its own URL plus .wasm, so keep hello.js and hello.js.wasm in the same directory.
• Load it with a classic <script>. With type="module" the loader requests the page’s URL plus .wasm instead.
• Serve over HTTP or HTTPS; file:// URLs do not work.
• System.out writes to the browser console.

To deploy, copy hello.js, hello.js.wasm and the page to any static host.

Browser support

WasmGC bundles need WebAssembly garbage collection and exception handling with exnref:

An older browser rejects the module with a CompileError when the page loads. To serve hosts without WasmGC, see Edge WASM.