🧩 Node.js Lab – Error Handling in Express

📘 Overview

In this lab, you will build a simple Product Management API using Node.js and Express that demonstrates proper error handling practices in backend development.

The objective is to help you understand how to:

• Handle different types of runtime and operational errors in Express.
• Structure error-handling middleware.
• Validate and manage request data properly.
• Work with asynchronous file operations using the Node.js fs/promises module.

---

🧠 Problem Statement

You are building a Product Management REST API that allows users to:

• View all products
• View a specific product by ID
• Add a new product

The product data will be stored locally in a products.json file inside the data folder.

Your main focus is to implement proper error handling across the application.

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🏗️ Project Structure

Your project should have the following folder layout:

Error-Handling/
│
├── controllers/
│   └── productController.js
├── data/ 
│   └── products.json          
├── middleware/            
│   └── errorHandler.js               
├── routes/
│   └── productRoutes.js                
├── src/
│   └── server.js                  
├── test/               
│   └── product.test.js
└── README.md                   

---

🧩 Tasks to Complete

1️⃣ Global Error Handling Middleware

Create a centralized error-handling middleware inside
/middleware/errorHandler.js that catches all errors in one place.

Example:

app.use((err, req, res, next) => {
  // Logic to handle errors globally
});

Error should be sent in JSON format like :

{  "error": "Internal Server Error" }

---

2️⃣ Route for Undefined Endpoints (404 Handler)

In your app.js, define a route handler for any undefined routes.

Example:

app.use((req, res) => {
  // Logic to handle undefined routes
});

Error should be sent in JSON format like :

{  "error": "Route not found" }

---

3️⃣ Controller: Get All Products

Inside controllers/productController.js, implement:

Example:

export const getAllProducts = async (req, res, next) => {
  try {
    // Read from products.json using fs/promises
    // Send the list of products as JSON
  } catch (err) {
    next(err); // Pass error to middleware
  }
};

Should return a list of all products in JSON format.

{  "products": products }

---

4️⃣ Controller: Get Product by ID

Implement:

Example:

export const getProductById = async (req, res, next) => {
    // Extract id from params
    // Find product by id
    // If not found, return 404 with { error: "Product not found" }
};

• If product not found → respond with :

{  "error": "Product not found" }

and 404 Not Found status code.

• On success → return:

{ "id": 1, "name": "Product Name", "price": 100 }

---

5️⃣ Controller: Add Product

Implement:

Example:

export const addProduct = async (req, res, next) => {
  try {
    // Validate name and price fields
    // Read existing products from file
    // Create new product object with auto-incremented ID
    // Write back to products.json
    // Return 201 Created with the new product
  } catch (err) {
    next(err);
  }
};

• If validation fails → respond with :

{ "error": "Invalid product data" }

and 400 Bad Request status code.

• On success → return:

{ "id": 1, "name": "Product Name", "price": 100 }

⚡ Error Types to Handle {#error-types-to-handle}

Your application should gracefully handle the following:

Type	Example	Response
Validation Error	Missing name or invalid price	{ "error": "Invalid product data" } (400)
Not Found Error	Product ID doesn't exist	{ "error": "Product not found" } (404)
File System Error	products.json is missing/corrupted	{ "error": "Internal Server Error" } (500)
Undefined Route	/random endpoint	{ "error": "Route not found" } (404)

---

Installing and Running Your Application

Run the following command to install all required dependencies:

npm install

Run the following command to start the server:

npm run dev

🧪 Testing Your Implementation {#testing-your-implementation}

A complete product.test.js file is provided inside /test.

Run tests with:

npm test

You should see test cases covering:

• ✅ Successful CRUD operations (Create, Read, Update, Delete)

• ❌ Error handling (Invalid data, Not Found, Corrupted File)

Keep running tests until all pass successfully!!