problems(js): add “2619. Array Prototype Last”

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

problems/js/array-prototype-last.js

@@ -0,0 +1,13 @@
+Array.prototype.last = function() {
+    if (!this.length) {
+        // empty array, ideally should throw an exception
+        return -1;
+    }
+
+    return this[this.length - 1];
+};
+
+/**
+ * const arr = [1, 2, 3];
+ * arr.last(); // 3
+ */