Initial implementation.

Author: sjrd

.github/workflows/ci.yml

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+name: Node.js test and Build
+
+on:
+  push:
+    branches: [ main ]
+  pull_request:
+    branches: [ main ]
+
+jobs:
+  build:
+    strategy:
+      matrix:
+        os: [ubuntu-latest, windows-latest]
+        node-version: ['16']
+        java: ['8']
+
+    runs-on: ${{ matrix.os }}
+
+    steps:
+    - uses: actions/checkout@v3
+    - name: Use Node.js ${{ matrix.node-version }}
+      uses: actions/setup-node@v3
+      with:
+        node-version: ${{ matrix.node-version }}
+    - name: Set up JDK ${{ matrix.java }}
+      uses: actions/setup-java@v3
+      with:
+        distribution: 'adopt'
+        java-version: ${{ matrix.java }}
+        cache: 'sbt'
+
+    - name: Cache dependencies
+      uses: actions/cache@v3
+      with:
+        path: |
+          **/node_modules
+        key: ${{ runner.os }}-${{ hashFiles('**/package-lock.json') }}
+
+    - name: Install dependencies
+      run: npm install
+    - name: Run sbt once in the test project to make sure sbt is downloaded
+      run: sbt projects
+      working-directory: ./test/testproject
+    - name: Perform unit test
+      run: npm test
+    - name: Build
+      run: npm run build

.gitignore

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+/node_modules/
+/dist/
+target/

index.ts

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+import { spawn, SpawnOptionsWithoutStdio } from "child_process";
+import type { Plugin as VitePlugin } from "vite";
+
+// Utility to invoke a given sbt task and fetch its output
+function printSbtTask(task: string, cwd?: string): Promise<string> {
+  const args = ["--batch", "-no-colors", "-Dsbt.supershell=false", `print ${task}`];
+  const options: SpawnOptionsWithoutStdio = {
+    cwd: cwd,
+  };
+  const child = process.platform === 'win32'
+    ? spawn("sbt.bat", args.map(x => `"${x}"`), {shell: true, ...options})
+    : spawn("sbt", args, options);
+
+  let fullOutput: string = '';
+
+  child.stdout.setEncoding('utf-8');
+  child.stdout.on('data', data => {
+    fullOutput += data;
+    process.stdout.write(data); // tee on my own stdout
+  });
+
+  child.stderr.setEncoding('utf-8');
+  child.stderr.on('data', data => {
+    process.stderr.write(data); // tee on my own stderr
+  });
+
+  return new Promise((resolve, reject) => {
+    child.on('close', code => {
+      if (code !== 0)
+        reject(new Error(`sbt invocation for Scala.js compilation failed with exit code ${code}`));
+      else
+        resolve(fullOutput.trimEnd().split('\n').at(-1)!);
+    })
+  })
+}
+
+export interface ScalaJSPluginOptions {
+  projectID?: string,
+  cwd?: string,
+}
+
+export default function scalaJSPlugin(options: ScalaJSPluginOptions = {}): VitePlugin {
+  const { projectID, cwd } = options;
+
+  let isDev = false; // when resolving the Vite config, this may turn into true
+  let scalaJSOutputDir: string;
+
+  return {
+    name: "scalajs:sbt-scalajs-plugin",
+
+    // Vite-specific
+    configResolved(resolvedConfig) {
+      isDev = resolvedConfig.mode === 'development';
+    },
+
+    // standard Rollup
+    async buildStart(options) {
+      const task = isDev ? "fastLinkJSOutput" : "fullLinkJSOutput";
+      const projectTask = projectID ? `${projectID}/${task}` : task;
+      scalaJSOutputDir = await printSbtTask(projectTask, cwd);
+    },
+
+    // standard Rollup
+    resolveId(source, importer, options) {
+      const parts = /^scalajs:(.*)$/g.exec(source);
+      if (!parts)
+        return null;
+
+      return `${scalaJSOutputDir}/${parts[1]}`;
+    },
+  };
+}