base configuration option

src/config.ts

@@ -19,22 +19,27 @@ export interface TableOfContents {
 
 export interface Config {
   title?: string;
+  base?: string;
   pages?: (Page | Section)[]; // TODO rename to sidebar?
   toc?: TableOfContents;
 }
 
 export async function readConfig(root: string): Promise<Config | undefined> {
   for (const ext of [".js", ".ts"]) {
+    let config;
     try {
       const configPath = join(process.cwd(), root, ".observablehq", "config" + ext);
       const configStat = await stat(configPath);
       // By using the modification time of the config, we ensure that we pick up
       // any changes to the config on reload. TODO It would be better to either
       // restart the preview server when the config changes, or for the preview
       // server to watch the config file and hot-reload it automatically.
-      return (await import(`${configPath}?${configStat.mtimeMs}`)).default;
+      config = (await import(`${configPath}?${configStat.mtimeMs}`)).default;
     } catch {
       continue;
     }
+    if (typeof config?.base === "string" && !config.base.match(/^[/]\w+[/]$/))
+      throw new Error(`invalid base: ${config.base}`);
+    return config;
   }
 }

src/preview.ts

@@ -51,6 +51,17 @@ class Server {
     try {
       const url = new URL(req.url!, "http://localhost");
       let {pathname} = url;
+      const config = await readConfig(this.root);
+      const {base = "/"} = config ?? {};
+      if (!base || !pathname.startsWith(base)) {
+        if (pathname === "/") {
+          res.writeHead(302, {Location: base});
+          res.end();
+          return;
+        }
+        throw new HttpError("Not found", 404);
+      }
+      pathname = pathname.slice(base.length - 1);
       if (pathname === "/_observablehq/runtime.js") {
         send(req, "/@observablehq/runtime/dist/runtime.js", {root: "./node_modules"}).pipe(res);
       } else if (pathname.startsWith("/_observablehq/")) {