Introduction to Backend Concepts

After this lesson, you will be able to: Understand how a backend server works and build a simple HTTP API in Node.js using Express that responds to requests.

Up to now, everything you've built runs in the browser. The backend is the other half: code that runs on a server, talks to a database, and serves data to your frontend. This lesson covers what a backend is, why it exists, and walks through a tiny Node.js + Express API you can run in your editor.

Prerequisites:Your First Full Website Project

Why we need a backend

Anything that has to be private, shared between users, or hidden from view has to live on a server, not in the browser. Examples: storing user accounts, talking to a payment API with secret keys, or making sure two users see the same up-to-date data. A backend is just a program that listens for HTTP requests and returns responses. JSON data, HTML, or files.

Browser

frontend

Request / Response⇄

Backend server

Node + Express

SQL / rows⇄

Database

PostgreSQL

The classic three-tier app. The browser never talks to the database directly; the backend sits in between.

Node.js. JavaScript outside the browser

Node.js is a runtime that lets you run JavaScript on a server. It comes with npm, the package manager you'll use to install thousands of open-source libraries. Everything you've learned about JavaScript still applies, but on the server, you also get access to the file system, the network, and (with libraries) databases.

Hello World with Express

Express is the most common Node.js framework for building APIs. It takes about 10 lines to get a server running.

tsx

// server.js
const express = require("express");
const app = express();
app.use(express.json());

app.get("/", (req, res) => {
  res.send("Hello from the backend!");
});

app.get("/api/greet", (req, res) => {
  const name = req.query.name || "friend";
  res.json({ message: `Hello, ${name}!` });
});

app.listen(3000, () => {
  console.log("Server running on http://localhost:3000");
});

Handling POST requests

POST routes accept data in the request body. Combined with express.json(), you can build full APIs.

tsx

const messages = [];

app.get("/api/messages", (req, res) => {
  res.json(messages);
});

app.post("/api/messages", (req, res) => {
  const { text } = req.body;
  if (!text) return res.status(400).json({ error: "text required" });
  messages.push({ text, createdAt: Date.now() });
  res.status(201).json({ ok: true });
});

Try it: spin up your first Express API

Use the in-browser editor, no local setup required.

1
Create a new "Node.js" repl in your editor
2
In the Shell tab, run npm install express
3
Paste the Hello World code into index.js (or server.js)
4
Click Run. your editor will show a URL where your server is reachable
5
Visit /api/greet?name=Alex in your browser, you should see JSON

Quick Check

Why can't we put a Stripe secret key in our frontend JavaScript?

Hint: think about what "secret" means.

←Your First Full Website Project

Back to Web Development

Databases and Data Storage→