feat(vercel): Support `trailingSlash` (#3176)

2 files changed, 48 insertions, 5 deletions

diff --git a/.changeset/thirty-boxes-shave.md b/.changeset/thirty-boxes-shave.md
new file mode 100644
index 000000000..bb270722c
--- /dev/null
+++ b/.changeset/thirty-boxes-shave.md
@@ -0,0 +1,5 @@
+---
+'@astrojs/vercel': patch
+---
+
+Support trailingSlash
diff --git a/packages/integrations/vercel/src/index.ts b/packages/integrations/vercel/src/index.ts
index 439eeeb04..35d20d42f 100644
--- a/packages/integrations/vercel/src/index.ts
+++ b/packages/integrations/vercel/src/index.ts
@@ -87,11 +87,49 @@ export default function vercel(): AstroIntegration {
 					version: 3,
 					basePath: '/',
 					pages404: false,
-					rewrites: staticRoutes.map((route) => ({
-						source: route.pathname,
-						regex: route.pattern.toString(),
-						destination: `/${ENTRYFILE}`,
-					})),
+					redirects:
+						// Extracted from Next.js v12.1.5
+						_config.trailingSlash === 'always'
+							? [
+									{
+										source: '/:file((?!\\.well-known(?:/.*)?)(?:[^/]+/)*[^/]+\\.\\w+)/',
+										destination: '/:file',
+										internal: true,
+										statusCode: 308,
+										regex: '^(?:/((?!\\.well-known(?:/.*)?)(?:[^/]+/)*[^/]+\\.\\w+))/$',
+									},
+									{
+										source: '/:notfile((?!\\.well-known(?:/.*)?)(?:[^/]+/)*[^/\\.]+)',
+										destination: '/:notfile/',
+										internal: true,
+										statusCode: 308,
+										regex: '^(?:/((?!\\.well-known(?:/.*)?)(?:[^/]+/)*[^/\\.]+))$',
+									},
+							  ]
+							: _config.trailingSlash === 'never'
+							? [
+									{
+										source: '/:path+/',
+										destination: '/:path+',
+										internal: true,
+										statusCode: 308,
+										regex: '^(?:/((?:[^/]+?)(?:/(?:[^/]+?))*))/$',
+									},
+							  ]
+							: undefined,
+					rewrites: staticRoutes.map((route) => {
+						let source = route.pathname as string;
+
+						if (_config.trailingSlash === 'always' && !source.endsWith('/')) {
+							source += '/';
+						}
+
+						return {
+							source,
+							regex: route.pattern.toString(),
+							destination: `/${ENTRYFILE}`,
+						};
+					}),
 					dynamicRoutes: dynamicRoutes.map((route) => ({
 						page: `/${ENTRYFILE}`,
 						regex: route.pattern.toString(),