Node.js<\/a> is an open-source, cross-platform JavaScript runtime environment built on Chrome’s V8 JavaScript engine<\/a>. It enables developers to run JavaScript code outside the browser, making it a popular choice for backend development, APIs, and microservices. Node.js uses an event-driven, non-blocking I\/O model, which makes it highly efficient and suitable for building scalable network applications.<\/p>\n In backend systems, data seeding refers to pre-populating a database with initial data. This is typically achieved by writing scripts in Node.js that insert default or sample records into the database. Data seeding is especially valuable for:<\/p>\n Data seeding in Node.js applications ensures that developers, testers, and CI\/CD pipelines start with a consistent dataset every time, reducing environment-specific bugs and easing onboarding for new team members.<\/p>\n To set up Node.js on Windows, download the installer from here<\/a>. Run the installer, which includes the NPM package manager, and follow the setup wizard. After installation, you can verify Node.js and npm by running:<\/p>\n PostgreSQL<\/a> is a powerful, open-source relational database system. To set it up locally:<\/p>\n Here, we will set up the project, write scripts to insert sample data into the Before seeding data, set up a Node.js project:<\/p>\n This script demonstrates how to create a PostgreSQL database, define schemas, and insert sample data using Node.js:<\/p>\n This Node.js script connects to a PostgreSQL database using the Add a script to This Execute the seed script in development mode:<\/p>\n This command sets the environment to Upon successful execution, the console will show:<\/p>\n This output shows that the script inserted users, products, orders, and the many-to-many relationships between orders and products without errors, confirming that the database seeding was successful and complete.<\/p>\n Unit testing the seed script helps validate logic without interacting with a real database. By mocking database queries, we can simulate PostgreSQL responses and verify that the seed functions are executing correctly.<\/p>\n This Jest-based test suite mocks the Maintain clear documentation to facilitate onboarding, collaboration, and consistency:<\/p>\n\n
1.1 Setting up Node.js<\/h3>\n
node -v\nnpm -v\n<\/pre>\n
![\"Fig.](\"https:\/\/www.javacodegeeks.com\/wp-content\/uploads\/2025\/08\/node-npm-installation-img1.jpg\")
<\/a><\/a>2. Setting Up PostgreSQL<\/h2>\n
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psql<\/code> or a GUI tool like pgAdmin.<\/li>\npg<\/code> npm package in your Node.js project to connect to PostgreSQL.<\/li>\n<\/ul>\n2.1 Installing PostgreSQL Client for Node.js<\/h3>\n
npm install pg\n<\/pre>\n
<\/a>3. Code Example<\/h2>\n
users<\/code> and orders<\/code> tables, and demonstrate how to manage environment-specific seeding.<\/p>\n3.1 Setup<\/h3>\n
mkdir pg-seed-example\ncd pg-seed-example\nnpm init -y\nnpm install pg\n<\/pre>\n
3.2 Seed Script<\/h3>\n
\nconst { Pool } = require('pg');\n\n\/\/ PostgreSQL connection pool\nconst pool = new Pool({\n user: 'your_db_user',\n host: 'localhost',\n database: 'your_db_name',\n password: 'your_db_password',\n port: 5432,\n});\n\n\/\/ Create tables if not exist\nasync function createSchema() {\n await pool.query(`\n CREATE TABLE IF NOT EXISTS users (\n id VARCHAR(50) PRIMARY KEY,\n name VARCHAR(100),\n email VARCHAR(100) UNIQUE,\n street VARCHAR(200),\n city VARCHAR(100),\n zip VARCHAR(20)\n );\n `);\n\n await pool.query(`\n CREATE TABLE IF NOT EXISTS products (\n id VARCHAR(50) PRIMARY KEY,\n name VARCHAR(100),\n price NUMERIC(10,2)\n );\n `);\n\n await pool.query(`\n CREATE TABLE IF NOT EXISTS orders (\n id VARCHAR(50) PRIMARY KEY,\n user_id VARCHAR(50) REFERENCES users(id)\n );\n `);\n\n await pool.query(`\n CREATE TABLE IF NOT EXISTS order_items (\n order_id VARCHAR(50) REFERENCES orders(id),\n product_id VARCHAR(50) REFERENCES products(id),\n quantity INT,\n PRIMARY KEY (order_id, product_id)\n );\n `);\n\n console.log('Database schema created or verified.');\n}\n\n\/\/ Seed user data\nasync function seedUsers() {\n const users = [\n { id: 'user1', name: 'Alice', email: 'alice@example.com', street: '123 Elm St', city: 'Metropolis', zip: '12345' },\n { id: 'user2', name: 'Bob', email: 'bob@example.com', street: '456 Oak St', city: 'Gotham', zip: '67890' },\n ];\n\n for (const user of users) {\n await pool.query(\n `INSERT INTO users(id, name, email, street, city, zip) VALUES($1,$2,$3,$4,$5,$6) ON CONFLICT (id) DO NOTHING`,\n [user.id, user.name, user.email, user.street, user.city, user.zip]\n );\n console.log(\\`Seeded user: \\${user.name}\\`);\n }\n}\n\n\/\/ Seed product data\nasync function seedProducts() {\n const products = [\n { id: 'prod1', name: 'Laptop', price: 1200 },\n { id: 'prod2', name: 'Mouse', price: 20 },\n { id: 'prod3', name: 'Keyboard', price: 50 },\n ];\n\n for (const product of products) {\n await pool.query(\n `INSERT INTO products(id, name, price) VALUES($1,$2,$3) ON CONFLICT (id) DO NOTHING`,\n [product.id, product.name, product.price]\n );\n console.log(\\`Seeded product: \\${product.name}\\`);\n }\n}\n\n\/\/ Seed order data\nasync function seedOrders() {\n const orders = [\n { id: 'order1', userId: 'user1' },\n { id: 'order2', userId: 'user1' },\n { id: 'order3', userId: 'user2' },\n ];\n\n for (const order of orders) {\n await pool.query(\n `INSERT INTO orders(id, user_id) VALUES($1,$2) ON CONFLICT (id) DO NOTHING`,\n [order.id, order.userId]\n );\n console.log(\\`Seeded order: \\${order.id} for user \\${order.userId}\\`);\n }\n}\n\n\/\/ Seed order items (many-to-many relationship)\nasync function seedOrderItems() {\n const orderItems = [\n { orderId: 'order1', productId: 'prod1', quantity: 1 },\n { orderId: 'order1', productId: 'prod2', quantity: 2 },\n { orderId: 'order2', productId: 'prod2', quantity: 1 },\n { orderId: 'order3', productId: 'prod3', quantity: 1 },\n ];\n\n for (const item of orderItems) {\n await pool.query(\n `INSERT INTO order_items(order_id, product_id, quantity) VALUES($1,$2,$3) ON CONFLICT (order_id, product_id) DO NOTHING`,\n [item.orderId, item.productId, item.quantity]\n );\n console.log(\\`Seeded order item: order \\${item.orderId} -> product \\${item.productId}\\`);\n }\n}\n\n\/\/ Main seeding function\nasync function seed() {\n if (process.env.NODE_ENV === 'development') {\n console.log('Running development seed...');\n await createSchema();\n await seedUsers();\n await seedProducts();\n await seedOrders();\n await seedOrderItems();\n } else {\n console.log('No seeding performed for environment:', process.env.NODE_ENV);\n }\n await pool.end();\n}\n\nseed()\n .then(() => {\n console.log('Seeding complete.');\n process.exit(0);\n })\n .catch((error) => {\n console.error('Seeding failed:', error);\n process.exit(1);\n });\n<\/pre>\n3.2.1 Code Explanation<\/h4>\n
pg<\/code> module, creates required tables if they do not exist, and inserts sample data for development environments. The createSchema<\/code> function defines tables for users<\/code>, products<\/code>, orders<\/code>, and order_items<\/code> using CREATE TABLE IF NOT EXISTS<\/code>, ensuring proper primary and foreign key relationships to maintain data integrity, including a many-to-many link between orders and products. Separate seeding functions (seedUsers<\/code>, seedProducts<\/code>, seedOrders<\/code>, seedOrderItems<\/code>) insert predefined data using parameterized INSERT<\/code> queries with ON CONFLICT<\/code> clauses to avoid duplicate entries. The seed<\/code> function orchestrates schema creation and data insertion, executing only when NODE_ENV<\/code> is set to “development”, and finally closes the database connection pool, logging success or failure messages for tracking purposes.3.3 package.json Scripts<\/h3>\n
package.json<\/code> for running seeds:<\/p>\n{\n \"name\": \"pg-seed-example\",\n \"version\": \"1.0.0\",\n \"main\": \"seed.js\",\n \"scripts\": {\n \"seed:dev\": \"NODE_ENV=development node seed.js\"\n },\n \"dependencies\": {\n \"pg\": \"^8.0.0\"\n }\n}\n<\/pre>\n3.3.1 Code Explanation<\/h4>\n
package.json<\/code> file defines a Node.js project named “pg-seed-example” with version 1.0.0 and a main entry point of seed.js<\/code>. It includes a scripts<\/code> section with a custom script seed:dev<\/code> that sets the environment variable NODE_ENV<\/code> to “development” and runs the seed script. The dependencies<\/code> section lists pg<\/code> version 8.0.0 as the required package to enable PostgreSQL database connectivity from Node.js.<\/p>\n3.4 Running the Seed Script<\/h3>\n
npm run seed:dev\n<\/pre>\n
development<\/code> and triggers the seed.js<\/code> script, ensuring that seeding only happens in the intended environment.<\/p>\n3.5 Output<\/h3>\n
\nRunning development seed...\nDatabase schema created or verified.\nSeeded user: Alice\nSeeded user: Bob\nSeeded product: Laptop\nSeeded product: Mouse\nSeeded product: Keyboard\nSeeded order: order1 for user user1\nSeeded order: order2 for user user1\nSeeded order: order3 for user user2\nSeeded order item: order order1 -> product prod1\nSeeded order item: order order1 -> product prod2\nSeeded order item: order order2 -> product prod2\nSeeded order item: order order3 -> product prod3\nSeeding complete.\n<\/pre>\n
<\/a>4. Unit Testing the Seed Script<\/h2>\n
4.1 Benefits of Unit Testing Seed Scripts<\/h3>\n
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4.2 Example Unit Test using Jest<\/h3>\n
\n\/\/ seed.test.js\nconst { seedUsers } = require('.\/seed'); \/\/ import function to test\nconst { Pool } = require('pg');\n\n\/\/ Mock the pg module\njest.mock('pg', () => {\n const mPool = { query: jest.fn(), end: jest.fn() };\n return { Pool: jest.fn(() => mPool) };\n});\n\ndescribe('Seed Script Tests', () => {\n let pool;\n\n beforeAll(() => {\n pool = new Pool();\n });\n\n test('should insert users correctly', async () => {\n pool.query.mockResolvedValueOnce({}); \/\/ mock query result\n await seedUsers();\n expect(pool.query).toHaveBeenCalled();\n expect(pool.query).toHaveBeenCalledWith(\n expect.stringContaining('INSERT INTO users'),\n expect.any(Array)\n );\n });\n});\n<\/pre>\npg<\/code> module so that no real database connection is made. It intercepts calls to pool.query<\/code> and verifies that the correct SQL statement (an INSERT INTO<\/code> command) is executed with the expected parameters. This ensures the seed function logic is correct, prevents accidental data modification, and provides fast, isolated feedback during development.<\/p>\n<\/a>5. Documenting Seeding Practices<\/h2>\n