Test migration to jest by hand

Apparently using regular expressions is way easier.

test/sortedLastIndexBy/sortedLastIndexBy.js

@@ -0,0 +1,10 @@
+const sortedLastIndexBy = (arr, n, fn) => {
+const isDescending = fn(arr[0]) > fn(arr[arr.length - 1]);
+const val = fn(n);
+const index = arr
+.map(fn)
+.reverse()
+.findIndex(el => (isDescending ? val <= el : val >= el));
+return index === -1 ? 0 : arr.length - index;
+};
+module.exports = sortedLastIndexBy;