Adiciona função as botões de + e -.

app.js

@@ -0,0 +1,25 @@
+document.addEventListener("DOMContentLoaded", function() {
+    const quantityInput = document.querySelector(".shopping-form-quantity-input");
+    const incrementButton = document.querySelector(".shopping-form-increment-button");
+    const decrementButton = document.querySelector(".shopping-form-decrement-button");
+
+    incrementButton.addEventListener("click", incrementQuantity);
+    decrementButton.addEventListener("click", decrementQuantity);
+
+    function incrementQuantity() {
+        const currentValue = Number(quantityInput.value);
+        const newValue = currentValue + 1;
+
+        quantityInput.value = newValue;
+    }
+
+    function decrementQuantity() {
+        const currentValue = Number(quantityInput.value);
+        const newValue = currentValue - 1;
+
+        if (newValue > 0) {
+            quantityInput.value = newValue;
+        }
+        
+    }
+});

index.html

@@ -38,5 +38,7 @@
                 </form>
             </section>
         </main>
+
+        <script src="app.js"></script>
     </body>
 </html>