test: add vercel.json config validation tests

app/src/tests/unit/config/vercel.test.ts

@@ -0,0 +1,84 @@
+import * as fs from 'fs';
+import * as path from 'path';
+import { describe, expect, test } from 'vitest';
+
+/**
+ * These tests validate that vercel.json is configured correctly for SPA routing.
+ *
+ * The vercel.json file must have a rewrite rule that sends all requests to the
+ * root index.html, allowing React Router to handle client-side routing.
+ *
+ * Without this configuration, direct URL access or page refreshes on sub-routes
+ * (e.g., /us/research/some-article) will return 404 errors because Vercel
+ * looks for a matching file instead of serving the SPA.
+ */
+describe('vercel.json configuration', () => {
+  const vercelJsonPath = path.resolve(__dirname, '../../../../vercel.json');
+
+  test('given vercel.json exists then file is present', () => {
+    // When
+    const exists = fs.existsSync(vercelJsonPath);
+
+    // Then
+    expect(exists).toBe(true);
+  });
+
+  test('given vercel.json then contains valid JSON', () => {
+    // Given
+    const content = fs.readFileSync(vercelJsonPath, 'utf-8');
+
+    // When
+    const parseJson = () => JSON.parse(content);
+
+    // Then
+    expect(parseJson).not.toThrow();
+  });
+
+  test('given vercel.json then has rewrites array', () => {
+    // Given
+    const content = fs.readFileSync(vercelJsonPath, 'utf-8');
+    const config = JSON.parse(content);
+
+    // Then
+    expect(config).toHaveProperty('rewrites');
+    expect(Array.isArray(config.rewrites)).toBe(true);
+    expect(config.rewrites.length).toBeGreaterThan(0);
+  });
+
+  test('given vercel.json rewrites then has SPA fallback rule', () => {
+    // Given
+    const content = fs.readFileSync(vercelJsonPath, 'utf-8');
+    const config = JSON.parse(content);
+
+    // When - Find the catch-all SPA routing rule
+    const spaRewrite = config.rewrites.find(
+      (rule: { source: string; destination: string }) =>
+        rule.source === '/(.*)' && rule.destination === '/'
+    );
+
+    // Then - Must have the SPA fallback rule
+    expect(spaRewrite).toBeDefined();
+    expect(spaRewrite.source).toBe('/(.*)');
+    expect(spaRewrite.destination).toBe('/');
+  });
+
+  test('given vercel.json then does NOT have routes array (use rewrites for SPA)', () => {
+    // Given
+    const content = fs.readFileSync(vercelJsonPath, 'utf-8');
+    const config = JSON.parse(content);
+
+    // Then - routes array breaks SPA routing when combined with rewrites
+    expect(config.routes).toBeUndefined();
+  });
+
+  test('given root vercel.json then does NOT exist (app/vercel.json is canonical)', () => {
+    // Given - Root vercel.json caused 404 errors by overriding app/vercel.json
+    const rootVercelJsonPath = path.resolve(__dirname, '../../../../../vercel.json');
+
+    // When
+    const exists = fs.existsSync(rootVercelJsonPath);
+
+    // Then
+    expect(exists).toBe(false);
+  });
+});