diff --git a/src/fib.ts b/src/fib.ts
index 8dc8ddd..7c8802c 100644
--- a/src/fib.ts
+++ b/src/fib.ts
@@ -1,12 +1,9 @@
-// util function that computes the fibonacci numbers
-module.exports = function fibonacci(n) {
-  if (n < 0) {
-    return -1;
-  } else if (n == 0) {
-    return 0;
-  } else if (n == 1) {
-    return 1;
-  }
-
+// src/fib.ts
+const fibonacci = (n: number): number => {
+  if (n < 0) return -1;
+  if (n === 0) return 0;
+  if (n === 1) return 1;
   return fibonacci(n - 1) + fibonacci(n - 2);
 };
+
+module.exports = fibonacci; // OK because `fibonacci` is typed