diff --git a/product-inventory/app.js b/product-inventory/app.js
new file mode 100644
index 00000000..d34d9d96
--- /dev/null
+++ b/product-inventory/app.js
@@ -0,0 +1,29 @@
+function calculateDiscount(price, discountRate) {
+    if (typeof price !== 'number' || typeof discountRate !== 'number') return null;
+    if (discountRate < 0 || discountRate > 1) return null;
+    return price - (price * discountRate);
+}
+
+function filterProducts(products, callback) {
+    if (!Array.isArray(products) || typeof callback !== 'function') {
+        return [];
+    }
+    return products.filter(callback);
+}
+
+function sortInventory(inventory, key) {
+    if (!Array.isArray(inventory) || typeof key !== 'string') return [];
+    if (!inventory.every(item => key in item)) return [];
+    
+    return [...inventory].sort((a, b) => {
+        if (a[key] < b[key]) return -1;
+        if (a[key] > b[key]) return 1;
+        return 0;
+    });
+}
+
+module.exports = {
+    calculateDiscount,
+    filterProducts,
+    sortInventory
+};
diff --git a/product-inventory/app.test.js b/product-inventory/app.test.js
new file mode 100644
index 00000000..3a36cd21
--- /dev/null
+++ b/product-inventory/app.test.js
@@ -0,0 +1,82 @@
+const store = require('./app.js');
+
+describe("calculateDiscount", () => {
+    // Positive test
+    test("should calculate discounted price correctly", () => {
+        expect(store.calculateDiscount(100, 0.1)).toBe(90);
+    });
+
+    // Negative tests
+    test("should return null if discountRate > 1", () => {
+        expect(store.calculateDiscount(100, 10)).toBeNull();
+    });
+
+    test("should return null if price or discountRate is not a number", () => {
+        expect(store.calculateDiscount("100", 0.1)).toBeNull();
+        expect(store.calculateDiscount(100, "0.1")).toBeNull();
+    });
+
+    // Edge case
+    test("should return the original price if discountRate is 0", () => {
+        expect(store.calculateDiscount(100, 0)).toBe(100);
+    });
+});
+
+describe("filterProducts", () => {
+    const products = [
+        { name: "Apple", category: "Fruit" },
+        { name: "Carrot", category: "Vegetable" },
+        { name: "Banana", category: "Fruit" }
+    ];
+
+    // Positive test
+    test("should return only products matching the callback", () => {
+        const result = store.filterProducts(products, p => p.category === "Fruit");
+        expect(result).toEqual([
+            { name: "Apple", category: "Fruit" },
+            { name: "Banana", category: "Fruit" }
+        ]);
+    });
+
+    // Negative test
+    test("should return empty array if callback is invalid", () => {
+        expect(store.filterProducts(products, null)).toEqual([]);
+        expect(store.filterProducts(null, p => true)).toEqual([]);
+    });
+
+    // Edge case
+    test("should return empty array if no products match", () => {
+        const result = store.filterProducts(products, p => p.category === "Dairy");
+        expect(result).toEqual([]);
+    });
+});
+
+describe("sortInventory", () => {
+    const inventory = [
+        { name: "Milk", price: 3 },
+        { name: "Bread", price: 2 },
+        { name: "Cheese", price: 5 }
+    ];
+
+    // Positive test
+    test("should sort by price ascending", () => {
+        const result = store.sortInventory(inventory, "price");
+        expect(result).toEqual([
+            { name: "Bread", price: 2 },
+            { name: "Milk", price: 3 },
+            { name: "Cheese", price: 5 }
+        ]);
+    });
+
+    // Negative test
+    test("should return empty array if key does not exist", () => {
+        expect(store.sortInventory(inventory, "quantity")).toEqual([]);
+    });
+
+    // Edge case
+    test("should return empty array if inventory is empty or invalid", () => {
+        expect(store.sortInventory([], "price")).toEqual([]);
+        expect(store.sortInventory(null, "price")).toEqual([]);
+        expect(store.sortInventory(inventory, null)).toEqual([]);
+    });
+});