Initial implementation.

.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/

README.md

@@ -2,4 +2,118 @@
 
 A [Vite](https://vitejs.dev/) plugin for [Scala.js](https://www.scala-js.org/).
 
-Work in progress.
+## Usage
+
+We assume that you have an existing Vite and Scala.js sbt project.
+If not, [follow the accompanying tutorial](https://github.com/scala-js/scala-js-website/pull/590).
+
+Install the plugin as a development dependency:
+
+```shell
+$ npm install -D @scala-js/vite-plugin-scalajs
+```
+
+Tell Vite to use the plugin in `vite.config.js`:
+
+```javascript
+import { defineConfig } from "vite";
+import scalaJSPlugin from "@scala-js/vite-plugin-scalajs";
+
+export default defineConfig({
+  plugins: [scalaJSPlugin()],
+});
+```
+
+Finally, import the Scala.js output from a `.js` or `.ts` file with
+
+```javascript
+import 'scalajs:main.js';
+```
+
+which will execute the main method of the Scala.js application.
+
+The sbt project must at least be configured to use ES modules.
+For the best feedback loop with Vite, we recommend to emit small modules for application code.
+If your application lives in the `my.app` package, configure the sbt project with the following settings:
+
+```scala
+scalaJSLinkerConfig ~= {
+  _.withModuleKind(ModuleKind.ESModule)
+    .withModuleSplitStyle(
+      ModuleSplitStyle.SmallModulesFor(List("my.app")))
+},
+```
+
+## Configuration
+
+The plugin supports the following configuration options:
+
+```javascript
+export default defineConfig({
+  plugins: [
+    scalaJSPlugin({
+      // path to the directory containing the sbt build
+      // default: '.'
+      cwd: '.',
+
+      // sbt project ID from within the sbt build to get fast/fullLinkJS from
+      // default: the root project of the sbt build
+      projectID: 'client',
+
+      // URI prefix of imports that this plugin catches (without the trailing ':')
+      // default: 'scalajs' (so the plugin recognizes URIs starting with 'scalajs:')
+      uriPrefix: 'scalajs',
+    }),
+  ],
+});
+```
+
+## Importing `@JSExportTopLevel` Scala.js members
+
+`@JSExportTopLevel("foo")` members in the Scala.js code are exported from the modules that Scala.js generates.
+They can be imported in `.js` and `.ts` files with the usual JavaScript `import` syntax.
+
+For example, given the following Scala.js definition:
+
+```scala
+import scala.scalajs.js
+import scala.scalajs.js.annotation._
+
+@JSExportTopLevel("ScalaJSLib")
+class ScalaJSLib extends js.Object {
+  def square(x: Double): Double = x * x
+}
+```
+
+we can import and use it as
+
+```javascript
+import { ScalaJSLib } from 'scalajs:main.js';
+
+const lib = new ScalaJSLib();
+console.log(lib.square(5)); // 25
+```
+
+### Exports in other modules
+
+By default, `@JSExportTopLevel("Foo")` exports `Foo` from the `main` module, which is why we import from `scalajs:main.js`.
+We can also split the Scala.js exports into several modules.
+For example,
+
+```scala
+import scala.scalajs.js
+import scala.scalajs.js.annotation._
+
+@JSExportTopLevel("ScalaJSLib", "library")
+class ScalaJSLib extends js.Object {
+  def square(x: Double): Double = x * x
+}
+```
+
+can be imported with
+
+```javascript
+import { ScalaJSLib } from 'scalajs:library.js';
+```
+
+The Scala.js documentation contains [more information about module splitting](https://www.scala-js.org/doc/project/module.html).

index.ts

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+import { spawn, SpawnOptions } 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 ${task}`];
+  const options: SpawnOptions = {
+    cwd: cwd,
+    stdio: ['ignore', 'pipe', 'inherit'],
+  };
+  const child = process.platform === 'win32'
+    ? spawn("sbt.bat", args.map(x => `"${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
+  });
+
+  return new Promise((resolve, reject) => {
+    child.on('error', err => {
+      reject(new Error(`sbt invocation for Scala.js compilation could not start. Is it installed?\n${err}`));
+    });
+    child.on('close', code => {
+      if (code !== 0)
+        reject(new Error(`sbt invocation for Scala.js compilation failed with exit code ${code}.`));
+      else
+        resolve(fullOutput.trimEnd().split('\n').at(-1)!);
+    });
+  });
+}
+
+export interface ScalaJSPluginOptions {
+  cwd?: string,
+  projectID?: string,
+  uriPrefix?: string,
+}
+
+export default function scalaJSPlugin(options: ScalaJSPluginOptions = {}): VitePlugin {
+  const { cwd, projectID, uriPrefix } = options;
+
+  const fullURIPrefix = uriPrefix ? (uriPrefix + ':') : 'scalajs:';
+
+  let isDev: boolean | undefined = undefined;
+  let scalaJSOutputDir: string | undefined = undefined;
+
+  return {
+    name: "scalajs:sbt-scalajs-plugin",
+
+    // Vite-specific
+    configResolved(resolvedConfig) {
+      isDev = resolvedConfig.mode === 'development';
+    },
+
+    // standard Rollup
+    async buildStart(options) {
+      if (isDev === undefined)
+        throw new Error("configResolved must be called before buildStart");
+
+      const task = isDev ? "fastLinkJSOutput" : "fullLinkJSOutput";
+      const projectTask = projectID ? `${projectID}/${task}` : task;
+      scalaJSOutputDir = await printSbtTask(projectTask, cwd);
+    },
+
+    // standard Rollup
+    resolveId(source, importer, options) {
+      if (scalaJSOutputDir === undefined)
+        throw new Error("buildStart must be called before resolveId");
+
+      if (!source.startsWith(fullURIPrefix))
+        return null;
+      const path = source.substring(fullURIPrefix.length);
+
+      return `${scalaJSOutputDir}/${path}`;
+    },
+  };
+}