Add the build script and type definitions.

source/build.ts

@@ -0,0 +1,82 @@
+// Import native Node libraries.
+import path from "node:path";
+import process from "node:process";
+import fsp from "node:fs/promises";
+// Import Esbuild and associated plugins.
+import esbuild from "esbuild";
+import copyPlugin from "esbuild-copy-static-files";
+import {sassPlugin} from "esbuild-sass-plugin";
+// Import PostCSS and associated plugins.
+import cssnano from "cssnano";
+import postcss from "postcss";
+
+/**
+ * Create an absolute path from a given relative one, using the directory
+ * this file is located in as the base.
+ *
+ * @param relative The relative path to make absolute.
+ * @returns The absolute path.
+ */
+function toAbsolutePath(relative: string): string {
+  return new URL(relative, import.meta.url).pathname;
+}
+
+// Create variables based on the environment.
+const dev = process.env.NODE_ENV === "development";
+const watch = process.env.WATCH === "true";
+
+// Create absolute paths to various directories.
+const outDir = toAbsolutePath("../build");
+const sourceDir = toAbsolutePath("../source");
+
+// Ensure that the output directory exists.
+await fsp.mkdir(outDir, {recursive: true});
+
+const cssProcessor = postcss([cssnano()]);
+
+const options: esbuild.BuildOptions = {
+  bundle: true,
+  // Define variables to be replaced in the code. Note that these are replaced
+  // "as is" and so we have to stringify them as JSON, otherwise a string won't
+  // have its quotes for example.
+  define: {
+    $dev: JSON.stringify(dev),
+  },
+  entryPoints: [path.join(sourceDir, "setup.tsx")],
+  format: "esm",
+  logLevel: "info",
+  minify: !dev,
+  outdir: outDir,
+  plugins: [
+    // Copy all files from `source/assets/` to the output directory.
+    copyPlugin({src: path.join(sourceDir, "assets/"), dest: outDir}),
+
+    // Compile SCSS to CSS.
+    sassPlugin({
+      type: "style",
+      async transform(source) {
+        // In development, don't do any extra processing.
+        if (dev) {
+          return source;
+        }
+
+        // But in production, run the CSS through PostCSS.
+        const {css} = await cssProcessor.process(source, {from: undefined});
+        return css;
+      },
+    }),
+  ],
+  // Link sourcemaps in development but omit them in production.
+  sourcemap: dev ? "linked" : false,
+  splitting: true,
+  // Target ES2022, and the first Chromium and Firefox releases from 2022.
+  target: ["es2022", "chrome97", "firefox102"],
+  treeShaking: true,
+};
+
+if (watch) {
+  const context = await esbuild.context(options);
+  await context.watch();
+} else {
+  await esbuild.build(options);
+}

source/packages.d.ts

@@ -0,0 +1,15 @@
+// Type definitions for third-party packages.
+
+declare module "esbuild-copy-static-files" {
+  import {type cpSync} from "node:fs";
+  import {type Plugin} from "esbuild";
+
+  type CopySyncParameters = Parameters<typeof cpSync>;
+
+  type Options = {
+    src?: CopySyncParameters[0];
+    dest?: CopySyncParameters[1];
+  } & CopySyncParameters[2];
+
+  export default function (options: Options): Plugin;
+}

source/types.d.ts

@@ -0,0 +1,6 @@
+// Export something so TypeScript doesn't see this file as an ambient module.
+export {};
+
+declare global {
+  const $dev: boolean;
+}