fix: better internal url detection

src/lib/isInternalURL.ts

@@ -1 +1,6 @@
-export const isInternalURL = (url: string): boolean => /^\/[^\/]/.test(url);
+export const isInternalURL = (url: string): boolean => {
+	const isInternal = /^(\/(?!\/)|#)/.test(url);
+	const isSpecialLink = !isInternal && !/^https?:\/\//.test(url);
+
+	return isInternal && !isSpecialLink;
+};

test/lib-isInternalURL.test.ts

@@ -0,0 +1,19 @@
+import test from "ava";
+
+import { isInternalURL } from "../src/lib/isInternalURL";
+
+test("returns true for internal URLs", (t) => {
+	t.true(isInternalURL("/"));
+	t.true(isInternalURL("/internal"));
+	t.true(isInternalURL("#anchor"));
+});
+
+test("returns false for external URLs", (t) => {
+	t.false(isInternalURL("//example.com"));
+	t.false(isInternalURL("//example.com/image.png"));
+	t.false(isInternalURL("//example.com#anchor"));
+	t.false(isInternalURL("https://example.com"));
+	t.false(isInternalURL("mailto:example.com"));
+	t.false(isInternalURL("tel:example.com"));
+	t.false(isInternalURL("ftp:example.com"));
+});