Fix navigation when base path is set and validate that option's val…

…ue (#1666)

Co-authored-by: Sidharth Vinod <sidharthv96@gmail.com>

.changeset/soft-bikes-bake.md

@@ -0,0 +1,5 @@
+---
+'@sveltejs/kit': patch
+---
+
+Fix navigation when `base` path is set and validate that option's value

documentation/docs/14-configuration.md

@@ -116,7 +116,7 @@ Whether to [hydrate](#ssr-and-javascript-hydrate) the server-rendered HTML with
 An object containing zero or more of the following `string` values:
 
 - `assets` — an absolute path, or a path relative to `base`, where your app's files are served from. This is useful if your files are served from a storage bucket of some kind
-- `base` — a root-relative (i.e. starts with `/`) path that specifies where your app is served from. This allows the app to live on a non-root path
+- `base` — a root-relative path that must start, but not end with `/` (e.g. `/base-path`). This specifies where your app is served from and allows the app to live on a non-root path
 
 ### prerender
 

packages/kit/src/core/load_config/index.js

@@ -135,8 +135,10 @@ export function validate_config(config) {
 	// resolve paths
 	const { paths } = validated.kit;
 
-	if (paths.base !== '' && !paths.base.startsWith('/')) {
-		throw new Error('config.kit.paths.base must be a root-relative path');
+	if (paths.base !== '' && (paths.base.endsWith('/') || !paths.base.startsWith('/'))) {
+		throw new Error(
+			"kit.paths.base option must be a root-relative path that starts but doesn't end with '/'. See https://kit.svelte.dev/docs#configuration-paths"
+		);
 	}
 
 	paths.assets = resolve(paths.base, paths.assets);

packages/kit/src/core/load_config/index.spec.js

@@ -171,7 +171,19 @@ test('fails if paths.base is not root-relative', () => {
 				}
 			}
 		});
-	}, /^config\.kit\.paths\.base must be a root-relative path$/);
+	}, /^kit\.paths\.base option must be a root-relative path that starts but doesn't end with '\/'. See https:\/\/kit\.svelte\.dev\/docs#configuration-paths$/);
+});
+
+test("fails if paths.base ends with '/'", () => {
+	assert.throws(() => {
+		validate_config({
+			kit: {
+				paths: {
+					base: '/github-pages/'
+				}
+			}
+		});
+	}, /^kit\.paths\.base option must be a root-relative path that starts but doesn't end with '\/'. See https:\/\/kit\.svelte\.dev\/docs#configuration-paths$/);
 });
 
 test('fails if prerender.pages are invalid', () => {

packages/kit/src/runtime/client/router.js

@@ -245,7 +245,7 @@ export class Router {
 
 			if (incorrect) {
 				info.path = has_trailing_slash ? info.path.slice(0, -1) : info.path + '/';
-				history.replaceState({}, '', `${info.path}${location.search}`);
+				history.replaceState({}, '', `${this.base}${info.path}${location.search}`);
 			}
 		}