Bun.FileSystemRouter: expose the assetPrefix getter on the prototype

src/runtime/api/filesystem_router.classes.ts

@@ -28,6 +28,10 @@ export default [
         getter: "getStyle",
         cache: true,
       },
+      assetPrefix: {
+        getter: "getAssetPrefix",
+        cache: true,
+      },
     },
     klass: {},
   }),

src/runtime/api/filesystem_router.rs

@@ -698,11 +698,14 @@ impl FileSystemRouter {
 
     #[bun_jsc::host_fn(getter)]
     pub fn get_asset_prefix(this: &Self, global_this: &JSGlobalObject) -> JsResult<JSValue> {
-        if let Some(ref asset_prefix) = this.asset_prefix {
-            return Ok(zs_to_js(asset_prefix.leak(), global_this));
-        }
+        // An omitted `assetPrefix` and `assetPrefix: ""` both store no prefix, so report
+        // the empty string the router actually applies rather than null.
+        let prefix = match this.asset_prefix {
+            Some(ref asset_prefix) => asset_prefix.leak(),
+            None => b"",
+        };
 
-        Ok(JSValue::NULL)
+        Ok(zs_to_js(prefix, global_this))
     }
 
     // Codegen's `host_fn_finalize` calls this via `|b| FileSystemRouter::finalize(b)`

test/js/bun/util/filesystem_router.test.ts

@@ -314,6 +314,40 @@ it("assetPrefix, src, and origin", async () => {
   }
 });
 
+it("assetPrefix is readable from the router", () => {
+  const { dir } = make(["index.tsx"]);
+
+  const router = new Bun.FileSystemRouter({
+    dir,
+    style: "nextjs",
+    assetPrefix: "/_next/static/",
+    origin: "https://nextjs.org",
+  });
+
+  const descriptor = Object.getOwnPropertyDescriptor(Object.getPrototypeOf(router), "assetPrefix");
+  expect(typeof descriptor?.get).toBe("function");
+  expect(descriptor?.set).toBeUndefined();
+  expect(Object.hasOwn(router, "assetPrefix")).toBe(false);
+
+  expect(router.assetPrefix).toBe("/_next/static/");
+
+  // the prefix the router reports is the one it applies to `src`
+  expect(router.match("/")!.src).toBe(`${router.origin}${router.assetPrefix}index.tsx`);
+
+  router.reload();
+  expect(router.assetPrefix).toBe("/_next/static/");
+});
+
+it("assetPrefix is an empty string when unset", () => {
+  const { dir } = make(["index.tsx"]);
+
+  for (const assetPrefix of [undefined, ""]) {
+    const router = new Bun.FileSystemRouter({ dir, style: "nextjs", assetPrefix });
+    expect(router.assetPrefix).toBe("");
+    expect(router.match("/")!.src).toBe("index.tsx");
+  }
+});
+
 it(".query works", () => {
   // set up the test
   const { dir } = make(["posts.tsx"]);