fix(url): handle empty and root path in withPath, prevent double slashes, add tests (#5091)

packages/better-auth/src/init.test.ts

@@ -219,4 +219,30 @@ describe("init", async () => {
 		});
 		expect(initFn).toHaveBeenCalled();
 	});
+
+	it("handles empty basePath", async () => {
+		const res = await init({
+			database,
+			baseURL: "http://localhost:5147/",
+			basePath: "",
+		});
+		expect(res.baseURL).toBe("http://localhost:5147");
+	});
+
+	it("handles root basePath", async () => {
+		const res = await init({
+			database,
+			baseURL: "http://localhost:5147/",
+			basePath: "/",
+		});
+		expect(res.baseURL).toBe("http://localhost:5147");
+	});
+
+	it("normalizes trailing slashes with default path", async () => {
+		const res = await init({
+			database,
+			baseURL: "http://localhost:5147////",
+		});
+		expect(res.baseURL).toBe("http://localhost:5147/api/auth");
+	});
 });

packages/better-auth/src/utils/url.ts

@@ -4,7 +4,8 @@ import { BetterAuthError } from "../error";
 function checkHasPath(url: string): boolean {
 	try {
 		const parsedUrl = new URL(url);
-		return parsedUrl.pathname !== "/";
+		const pathname = parsedUrl.pathname.replace(/\/+$/, "") || "/";
+		return pathname !== "/";
 	} catch (error) {
 		throw new BetterAuthError(
 			`Invalid base URL: ${url}. Please provide a valid base URL.`,
@@ -17,8 +18,15 @@ function withPath(url: string, path = "/api/auth") {
 	if (hasPath) {
 		return url;
 	}
+
+	const trimmedUrl = url.replace(/\/+$/, "");
+
+	if (!path || path === "/") {
+		return trimmedUrl;
+	}
+
 	path = path.startsWith("/") ? path : `/${path}`;
-	return `${url.replace(/\/+$/, "")}${path}`;
+	return `${trimmedUrl}${path}`;
 }
 
 export function getBaseURL(