ran npm run tdd & generated TDD

test/sortedIndex/sortedIndex.js

@@ -0,0 +1,5 @@
+module.exports = (arr, n) => {
+const isDescending = arr[0] > arr[arr.length - 1];
+const index = arr.findIndex(el => (isDescending ? n >= el : n <= el));
+return index === -1 ? arr.length : index;
+};