diff --git a/src/fib.ts b/src/fib.ts
index 8dc8ddd..6247674 100644
--- a/src/fib.ts
+++ b/src/fib.ts
@@ -1,12 +1,16 @@
-// util function that computes the fibonacci numbers
-module.exports = function fibonacci(n) {
+/**
+ * @param n The index of the Fibonacci number to compute.
+ * @returns The nth Fibonacci number, or -1 for negative input.
+ */
+export function fibonacci(n: number): number {
   if (n < 0) {
     return -1;
-  } else if (n == 0) {
+  }
+  if (n === 0) {
     return 0;
-  } else if (n == 1) {
+  }
+  if (n === 1) {
     return 1;
   }
-
   return fibonacci(n - 1) + fibonacci(n - 2);
-};
+}
\ No newline at end of file