Putting It Together

The previous pages covered each concept in isolation. This page shows what a complete Express + MongoDB server looks like when everything is assembled.

The server below handles three endpoints for a small bookstore: fetching all books, looking up a single book by ID, and creating a new book.

const express = require("express");
const cors = require("cors");
const { MongoClient, ObjectId } = require("mongodb");

const app = express();
app.use(cors());
app.use(express.json());

const client = new MongoClient("mongodb://localhost:27017");
let books;

app.get("/books", async (req, res) => {
    try {
        const all = await books.find().toArray();
        res.json(all);
    } catch (err) {
        res.status(500).json({ error: "Database error" });
    }
});

app.get("/books/:id", async (req, res) => {
    if (!ObjectId.isValid(req.params.id)) {
        return res.status(400).json({ error: "Invalid book ID" });
    }

    try {
        const book = await books.findOne({ _id: new ObjectId(req.params.id) });

        if (!book) {
            return res.status(404).json({ error: "Book not found" });
        }

        res.json(book);
    } catch (err) {
        res.status(500).json({ error: "Database error" });
    }
});

app.post("/books", async (req, res) => {
    const { title, author } = req.body;

    if (!title || !author) {
        return res.status(400).json({ error: "title and author are required" });
    }

    try {
        const book = { title, author };
        const result = await books.insertOne(book);
        res.status(201).json({ ...book, _id: result.insertedId });
    } catch (err) {
        res.status(500).json({ error: "Database error" });
    }
});

async function start() {
    await client.connect();
    const db = client.db("bookstore");
    books = db.collection("books");
    app.listen(3000, () => {
        console.log("Server running on port 3000");
    });
}

start();

A few things worth noting about the overall structure:

• Routes are registered at the top level, outside start(). This keeps the startup logic separate from the route definitions.
• books is declared at the top level with let and assigned inside start() after the connection resolves. Because app.listen() is called after that assignment, the server does not accept requests until books is ready.
• Validation runs before the try/catch. A missing field is a client error (400), not a server error (500).
• Each early return on an error response prevents Express from trying to send a second response after the function continues.