problems(js): add “2626. Array Reduce Transformation”

Signed-off-by: Matej Focko <mfocko@redhat.com>

problems/array-reduce-transformation.js

@@ -0,0 +1,15 @@
+/**
+ * @param {number[]} nums
+ * @param {Function} fn
+ * @param {number} init
+ * @return {number}
+ */
+var reduce = function(nums, fn, init) {
+    let val = init;
+
+    for (let x of nums) {
+        val = fn(val, x);
+    }
+
+    return val;
+};