@@ -1,5 +1,7 @@
 'use strict';
 
+// Elements to be visited
+
 class Product {
   constructor(name, price) {
     this.name = name;
@@ -33,6 +35,8 @@ class Service {
   }
 }
 
+// Visitors to be acceped
+
 class Purchase {
   constructor(items, delivery) {
     this.items = [];
@@ -67,7 +71,7 @@ class Purchase {
 
 class Inspection {
   constructor(items) {
-    this.items = items;
+    this.items = [...items];
   }
 
   check() {
 
@@ -1,13 +1,19 @@
 'use strict';
 
+// Elements to be visited
+
 class Product {
   constructor(name, price) {
     this.name = name;
     this.price = price;
   }
 
-  isAvailable(conditions, callback) {
-    callback(true); // Just a stub
+  accept(visitor) {
+    visitor.visit(this, this.inStock());
+  }
+
+  inStock() {
+    return true; // Just a stub
   }
 }
 
@@ -19,35 +25,39 @@ class Service {
     this.name = name;
   }
 
-  isAvailable(conditions, callback) {
-    const day = conditions.date.getDay();
-    callback(day > Service.SUNDAY && day < Service.SATURDAY);
+  accept(visitor) {
+    const now = new Date();
+    visitor.visit(this, this.isAvailableAt(now));
+  }
+
+  isAvailableAt(date) {
+    const day = date.getDay();
+    return day > Service.SUNDAY && day < Service.SATURDAY;
   }
 }
 
+// Visitors to be acceped
+
 class Purchase {
   constructor(items, delivery) {
     this.items = [];
     this.delivery = null;
-    for (const product of items) {
-      product.isAvailable({}, (available) => {
-        if (available) {
-          console.log(`Product "${product.name}" is in stock`);
-          this.items.push(product);
-        } else {
-          console.log(`Product "${product.name}" is out of stock`);
-        }
-      });
+    for (const item of items) {
+      item.accept(this);
+    }
+    delivery.accept(this);
+  }
+
+  visit(item, available) {
+    const proto = Object.getPrototypeOf(item);
+    const className = proto.constructor.name;
+    if (available) {
+      console.log(`${className} "${item.name}" is available`);
+      if (item instanceof Service) this.delivery = item;
+      else this.items.push(item);
+    } else {
+      console.log(`${className} "${item.name}" is not available`);
     }
-    const now = new Date();
-    delivery.isAvailable({ date: now }, (available) => {
-      if (available) {
-        console.log(`Service "${delivery.name}" is available`);
-        this.delivery = delivery;
-      } else {
-        console.log(`Service "${delivery.name}" is not available`);
-      }
-    });
   }
 }
 
@@ -57,14 +67,18 @@ class Inspection {
   }
 
   check() {
-    for (const product of this.items) {
-      product.isAvailable({}, (available) => {
-        if (available) {
-          console.log(`Product "${product.name}" is in stock`);
-        } else {
-          console.log(`Product "${product.name}" is out of stock`);
-        }
-      });
+    for (const item of this.items) {
+      item.accept(this);
+    }
+  }
+
+  visit(item, available) {
+    const proto = Object.getPrototypeOf(item);
+    const className = proto.constructor.name;
+    if (available) {
+      console.log(`${className} "${item.name}" is in stock`);
+    } else {
+      console.log(`${className} "${item.name}" is out of stock`);
     }
   }
 }