5.2.3. Data Persistence

Apps aren’t very useful if they forget everything when you refresh the page. Data persistence is about storing information in a way that outlives a single session — like saving user accounts, blog posts, or todo list items.

That’s where databases come in.

A database lets your application remember things. You can store data, look it up later, update it, or delete it. Most beginner web apps use a database to handle Create, Read, Update, and Delete (CRUD) operations.

If you’ve built a form on a website and saved the results — congratulations, you’ve used data persistence.

Common Types of Databases

When you are starting out, you'll be told there are two main types of databases:

• Relational databases (e.g., SQLite, PostgreSQL, MySQL)
  • Store data in tables, like a spreadsheet
  • Use SQL (Structured Query Language) to read/write data
  • Have relationships between rows (e.g., a users table and a posts table)
• NoSQL databases (e.g., MongoDB, Firebase)
  • Store data as documents or key-value pairs (they're basically a huge hash map)
  • More flexible, but less strict about structure

As a beginner, you’ll probably use SQLite or MongoDB first.

Simple Example (SQL)

Here’s what saving and retrieving a todo might look like in SQL:

-- Create a table
CREATE TABLE todos (
  id INTEGER PRIMARY KEY,
  task TEXT,
  completed BOOLEAN
);

-- Add a new task
INSERT INTO todos (task, completed) VALUES ('Learn SQL', false);

-- Get all tasks
SELECT * FROM todos;

Connecting a Web App to a Database

Here’s the simplest possible JavaScript code snippet to connect to an SQLite database using the sqlite3 package in Node.js:

const sqlite3 = require("sqlite3");

// Connect to (or create) a database file
const db = new sqlite3.Database("my-database.db");

// Create a table
db.run(`CREATE TABLE IF NOT EXISTS todos (
  id INTEGER PRIMARY KEY,
  task TEXT,
  completed BOOLEAN
)`);

// Insert a row
db.run(`INSERT INTO todos (task, completed) VALUES (?, ?)`, [
  "Learn SQLite",
  false,
]);

// Read rows
db.all(`SELECT * FROM todos`, (err, rows) => {
  console.log(rows);
});

Knowledge Checklist

• I understand what data persistence means and why it's important.
• I’ve used a database (like SQLite) to store and retrieve information.
• I can perform READ, CREATE, UPDATE, and DELETE (CRUD) with SQL
• I understand the difference between relational and NoSQL databases.
• I can describe the concept of primary keys and relationships between tables.
• I’ve connected a backend app to a database and queried it dynamically.