Added a comment

product-inventory/.gitignore

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+node_modules
+package-lock.json

product-inventory/package.json

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+{
+  "name": "product-inventory",
+  "version": "1.0.0",
+  "main": "index.js",
+  "scripts": {
+    "test": "jest"
+  },
+  "keywords": [],
+  "author": "",
+  "license": "ISC",
+  "description": "",
+  "devDependencies": {
+    "jest": "^29.7.0"
+  }
+}

product-inventory/products.js

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+const products = [
+  {Item: 'Laptop', Quantity: 50,  Price: 1000},
+  {Item: 'Smartphone', Quantity: 200, Price: 1200},
+  {Item: 'Headphones', Quantity: 150, Price: 500},
+  {Item: 'Keyboard', Quantity: 75, Price: 200},
+  {Item: 'Mouse', Quantity: 120, Price: 50},
+];
+
+function calculateDiscount(price, discountRate) {
+    if (typeof price !== 'number' || typeof discountRate !== 'number') {
+        return null;
+    }
+    if (discountRate < 0 || discountRate > 1) {
+        return null;
+    }
+
+    return price-price*discountRate;
+}
+
+console.log(calculateDiscount(100, 0.1));
+
+function filterProducts(products, callback) {
+    if (!Array.isArray(products) || typeof callback !== 'function'){
+
+     return [];
+    }
+		return products.filter(callback);
+}
+console.log(filterProducts(products, product => product.price<500));
+
+function sortInventory(inventory, key) {
+if (!Array.isArray(inventory) || typeof key !== 'string'){
+	return [];
+	}
+    return inventory.sort((a,b) => (b[key]-a[key]));
+    };
+console.log(sortInventory(products, "quantity"));
+
+module.exports = {
+    calculateDiscount,
+    filterProducts,
+    sortInventory
+}

product-inventory/tests/products.test.js

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+const { calculateDiscount, filterProducts, sortInventory } = require('../products.js');
+
+const products = [
+    { Item: 'Laptop', Quantity: 50, Price: 1000 },
+    { Item: 'Smartphone', Quantity: 200, Price: 1200 },
+    { Item: 'Headphones', Quantity: 150, Price: 500 },
+    { Item: 'Keyboard', Quantity: 75, Price: 200 },
+    { Item: 'Mouse', Quantity: 120, Price: 50 },
+];
+
+describe("apply dicount function", () => {
+    test("applies a valid discount rate", () => {
+        expect(calculateDiscount(100, 0.1)).toBe(90);
+    });
+
+    test("handles an invalid discount rate gracefully", () => {
+        expect(calculateDiscount(100, -0.1)).toBe(null);
+    });
+
+    test("handles edge case with price of 0", () => {
+        expect(calculateDiscount(0, 0.2)).toBe(0);
+    });
+
+//This test case was added later
+    test("accepts numbers for price not strings",() =>{
+        expect(calculateDiscount(100,"string")).toBe(null);
+    })
+})
+
+const cat = "Nick";
+describe("filter products by price", () => {
+    test("filters products less than 500", () => {
+        expect(filterProducts(products, product => product.Price < 500)).toEqual([
+            { Item: 'Keyboard', Quantity: 75, Price: 200 },
+            { Item: 'Mouse', Quantity: 120, Price: 50 },
+        ])
+    })
+    test("Negative Test Case: Should give empty array for non-array first argument", () => {
+        expect(filterProducts(cat, product => product.Price < 500)).toEqual([]);
+    });
+
+    test("Edge Test Case: Should handle filtering everything out of array", () => {
+        expect(filterProducts(products, product => product.Price < 20)).toEqual([]);
+    });
+});
+
+//sortInventory
+describe("inventoryFunctions.sortInventory", function () {
+    test("Sort by Quantity", () => {
+        expect(sortInventory(products, "quantity")).toEqual([
+            { Item: 'Laptop', Quantity: 50, Price: 1000 },
+            { Item: 'Smartphone', Quantity: 200, Price: 1200 },
+            { Item: 'Headphones', Quantity: 150, Price: 500 },
+            { Item: 'Keyboard', Quantity: 75, Price: 200 },
+            { Item: 'Mouse', Quantity: 120, Price: 50 }
+        ]);
+    });
+
+    test("Number input instead of a string", () => {
+        expect(sortInventory(products, 587609)).toEqual([]);
+    });
+
+    test("Empty array input", () => {
+        expect(sortInventory([], "quantity")).toEqual([]);
+    });
+});