[adapter-vercel] Make all root routes be called index instead of an empty string

.changeset/shiny-needles-divide.md

@@ -0,0 +1,5 @@
+---
+'@sveltejs/adapter-vercel': patch
+---
+
+Fix `edge:true, split: true` build error for root routes when deploying to Vercel

packages/adapter-vercel/index.js

@@ -340,7 +340,7 @@ async function v3(builder, external, edge, split) {
 						.slice(1, -2) // remove leading / and trailing $/
 						.replace(/\\\//g, '/')}(?:/__data.json)?$`; // TODO adding /__data.json is a temporary workaround — those endpoints should be treated as distinct routes
 
-					await generate_function(route.id, src, entry.generateManifest);
+					await generate_function(createValidFunctionName(route.id), src, entry.generateManifest);
 				}
 			};
 		});
@@ -385,3 +385,20 @@ function write(file, data) {
 
 	writeFileSync(file, data);
 }
+
+/**
+ * Make all root routes be called `index`:
+ *
+ * @example
+ * ```js
+ * createValidFunctionName("") // "index"
+ * createValidFunctionName("nested") // "nested"
+ * createValidFunctionName("nested/") // "nested/index"
+ * ```
+ *
+ * @param {string} routeId
+ * @returns {string} route id that is a valid function name
+ */
+function createValidFunctionName(routeId) {
+	return routeId.replace(/(\/|^)$/, '$1index');
+}