Initial implementation.

Author: sjrd

.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", `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]}`;
+    },
+  };
+}