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fix: App Router discovery for nested parallel-slot sub-routes #458

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fix: App Router discovery for nested parallel-slot sub-routes #458

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a407051
Fix nested parallel slot routes
JaredStowell Mar 11, 2026
a1d4c02
Fix nested parallel slot discovery
JaredStowell Mar 11, 2026
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3 changes: 1 addition & 2 deletions packages/vinext/src/routing/app-router.ts
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Original file line number Diff line number Diff line change
Expand Up @@ -236,10 +236,9 @@ function discoverSlotSubRoutes(
}
}

if (subPathMap.size === 0) continue;

// Find the default.tsx for the children slot at the parent directory
const childrenDefault = findFile(parentPageDir, "default", matcher);
if (subPathMap.size === 0 || !childrenDefault) continue;
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This is the right fix — previously, if childrenDefault was null, synthetic routes were created with pagePath: null, which would 500 at render time. Combining the two guards into one continue is clean.

Minor: the original code had the subPathMap.size === 0 check first (cheap) before the findFile call (does filesystem I/O). The new code always calls findFile even when subPathMap is empty. Consider preserving the short-circuit:

Suggested change
if (subPathMap.size === 0 || !childrenDefault) continue;
if (subPathMap.size === 0) continue;
// Find the default.tsx for the children slot at the parent directory
const childrenDefault = findFile(parentPageDir, "default", matcher);
if (!childrenDefault) continue;

This avoids a pointless findFile call when there are no sub-paths. Not a big deal since fs.existsSync is fast, but it's a free optimization and arguably clearer about the two independent exit conditions.


for (const [subPath, slotPages] of subPathMap) {
// Convert sub-path segments to URL pattern parts
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18 changes: 18 additions & 0 deletions tests/routing.test.ts
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Expand Up @@ -476,6 +476,24 @@ describe("appRouter - route discovery", () => {
});
});

it("does not create nested @slot sub-routes without a children default fallback", async () => {
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Good test. The name accurately describes what it asserts.

Note that this test gives @modal a default.tsx (line 484), so the slot passes the discoverParallelSlots filter. What's being tested is purely the childrenDefault guard in discoverSlotSubRoutes.

It might be worth adding a companion test for the other case: @modal with no root page.tsx/default.tsx and only @modal/profile/page.tsx — which tests that discoverParallelSlots itself excludes such a slot. That would document the current behavior and guard against future regressions if someone relaxes the filter.

await withTempDir("vinext-app-slot-missing-children-default-", async (tmpDir) => {
const appDir = path.join(tmpDir, "app");
await mkdir(path.join(appDir, "inbox", "@modal", "profile"), { recursive: true });
await writeFile(path.join(appDir, "inbox", "page.tsx"), EMPTY_PAGE);
await writeFile(path.join(appDir, "inbox", "@modal", "default.tsx"), EMPTY_PAGE);
await writeFile(path.join(appDir, "inbox", "@modal", "profile", "page.tsx"), EMPTY_PAGE);

invalidateAppRouteCache();
const routes = await appRouter(appDir);
const patterns = routes.map((route) => route.pattern);

expect(patterns).toContain("/inbox");
expect(patterns).not.toContain("/inbox/profile");
expect(matchAppRoute("/inbox/profile", routes)).toBeNull();
});
});

it("rejects non-terminal catch-all intercept targets", async () => {
await withTempDir("vinext-app-intercept-nonterminal-catchall-", async (tmpDir) => {
const appDir = path.join(tmpDir, "app");
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