diff --git a/src/fib.ts b/src/fib.ts
index 8dc8ddd..57a0d91 100644
--- a/src/fib.ts
+++ b/src/fib.ts
@@ -1,12 +1,27 @@
 // util function that computes the fibonacci numbers
-module.exports = function fibonacci(n) {
+function fibonacci(n: number): number {
+  if (typeof n !== 'number' || isNaN(n)) {
+    throw new TypeError('Input must be a valid number');
+  }
+
   if (n < 0) {
-    return -1;
-  } else if (n == 0) {
+    throw new RangeError('Input must be a non-negative integer');
+  } else if (n === 0) {
     return 0;
-  } else if (n == 1) {
+  } else if (n === 1) {
     return 1;
   }
 
-  return fibonacci(n - 1) + fibonacci(n - 2);
-};
+  // Removed redundant type annotations
+  let a = 0;
+  let b = 1;
+  let temp: number;
+
+  for (let i = 2; i <= n; i++) {
+    temp = a + b;
+    a = b;
+    b = temp;
+  }
+  return b;
+}
+export default fibonacci;