refactor: change logic & show flowchart

Author: nutkiattisak

2619. array-prototype-last/README.md

@@ -0,0 +1,6 @@
+```mermaid
+flowchart TD
+    A(Start) --> B{Is array empty?}
+    B -- No --> C(Return last element)
+    B -- Yes --> D(Return -1)
+```

2619. array-prototype-last/index.js

@@ -1,9 +1,3 @@
 Array.prototype.last = function () {
-  if (this[this.length - 1] === null) {
-    return null;
-  }
-  if (this[this.length - 1] === undefined) {
-    return -1;
-  }
-  return this[this.length - 1];
-};
+  return this.length === 0 ? -1 : this[this.length - 1]
+}