Assignment 08: Introduction to Express.js — Books API

Goal

Build your first server. It will answer requests for a list of books — adding one, changing one, and deleting one — all before any frontend exists.

Why This Matters

So far, your code has run in the browser. The browser is the client — it asks for things. Today you build the server — the computer that answers. A server is a program that waits for requests and sends back responses. Express is a tool that makes building a server in Node much faster.

You will test your server with Postman, a tool for sending requests without a browser. There is no frontend yet — that comes later. Today is just about the mechanics of CRUD: Create, Read, Update, Delete.

Resources

• Express docs: https://expressjs.com/en/starter/hello-world.html
• Postman download: https://www.postman.com/downloads/

Setup

This is a Node project, so there is no Vite step.

mkdir books-api
cd books-api
npm init -y
npm install express

Create one file: app.js. Everything for this assignment lives in this single file.

Paste this at the top of app.js. This is your starting data — an array, which is just a list:

let books = [
  { id: 1, title: "The Pragmatic Programmer", author: "David Thomas", genre: "Tech", available: true },
  { id: 2, title: "Educated", author: "Tara Westover", genre: "Memoir", available: true },
  { id: 3, title: "Dune", author: "Frank Herbert", genre: "Sci-Fi", available: false },
  { id: 4, title: "Sapiens", author: "Yuval Noah Harari", genre: "History", available: true },
  { id: 5, title: "The Alchemist", author: "Paulo Coelho", genre: "Fiction", available: true },
];

let nextId = 6; // use this for any new book you create

---

Part 1: Start the Server

Why: Before writing any routes, confirm the server itself can turn on.

Steps:

• Require express: const express = require("express");
• Create the app: const app = express();
• Add this line: app.use(express.json()); — it lets your server read JSON that's sent to it. Without it, req.body will always be empty.
• At the bottom, start the server: app.listen(8080, () => console.log("Server running on port 8080"));
• In your terminal, run node app.js.

Check it: Your terminal shows Server running on port 8080.

---

Part 2: Your First Route

Why: A route is a rule: "when a request comes to this address, run this code." Write the smallest possible route first, before touching your book data.

Steps:

• Above app.listen, add: app.get("/", (req, res) => res.send("Books API is running"));
• Stop your server (Ctrl+C) and run node app.js again. (Express does not restart itself — you must do this every time you change app.js.)
• Open http://localhost:8080 in your browser.

Check it: The page shows "Books API is running".

---

Part 3: GET All Books

Why: This is the simplest way to read data — send back the whole list.

Steps:

• Add: app.get("/api/books", (req, res) => res.json(books));
• Restart your server.
• Open Postman. Send a GET request to http://localhost:8080/api/books.

Check it: Postman shows all 5 books as JSON.

---

Part 4: GET One Book by Id

Why: Now you'll read a single item using a route parameter — a piece of the URL itself, like the 2 in /api/books/2.

Steps:

• Add a new route: app.get("/api/books/:id", (req, res) => { ... })
• Inside, read the id from req.params.id. Important: this value is always a string, so wrap it in Number() before comparing it to the ids in your array.
• Find the matching book in the books array.
• If no book matches, send back status 404 and stop (use return).
• If a book matches, send it back as JSON.
• Restart your server.

Check it: GET /api/books/2 returns "Educated". GET /api/books/99 returns a 404 status.

---

Part 5: POST a New Book

Why: Now you'll let the client create something new, instead of only reading data.

Steps:

• Add: app.post("/api/books", (req, res) => { ... })
• Read title, author, and genre from req.body.
• Build a new book object. Use nextId for its id, then increase nextId by 1.
• Add the new book to the books array.
• Send back status 201 and the new book.
• Restart your server.

Check it: In Postman, set the body to raw → JSON, and send { "title": "Clean Code", "author": "Robert Martin", "genre": "Tech" } to POST /api/books. Then GET /api/books and confirm 6 books are now in the list.

---

Part 6: PATCH an Existing Book

Why: PATCH changes only the fields you send — not the whole object.

Steps:

• Add: app.patch("/api/books/:id", (req, res) => { ... })
• Find the book by id (Number() again). If it's not found, send 404 and stop.
• Copy the fields from req.body onto the existing book, without erasing the fields you didn't send. (Object.assign(book, req.body) does this for you.)
• Send back status 200 and the updated book.
• Restart your server.

Check it: Send { "available": false } to PATCH /api/books/1. Only available should change.

---

Part 7: DELETE a Book

Why: The last CRUD action — removing something completely.

Steps:

• Add: app.delete("/api/books/:id", (req, res) => { ... })
• Find the book's position in the array by id. If it's not found, send 404 and stop.
• Remove it from the books array.
• Send back status 204 with no body: res.sendStatus(204).
• Restart your server.

Check it: Delete a book, then GET /api/books and confirm it's gone.

---

Common Gotchas

• req.params.id is always a string. "2" === 2 is false — always wrap it in Number() first.
• req.body is undefined if you forget app.use(express.json()).
• Every route must send back exactly one response. Forgetting return before res.sendStatus(404) will cause a "Cannot set headers after they are sent" crash.
• PATCH should only update the fields sent in the request, not replace the whole object.
• 204 No Content should have no body. Use res.sendStatus(204), not res.status(204).json(...).
• Forgot to restart your server after a change? Stop it (Ctrl+C) and run node app.js again.
• In Postman, set the request body format to raw → JSON for POST and PATCH routes.

How to Submit Your Work

Steps:

• Open your terminal. Make sure you are inside your books-api folder.
• Run git init.
• Run git add .
• Run git commit -m "complete books api assignment"
• Go to github.com. Click the + icon, then New repository.
• Name it books-api. Leave every checkbox unchecked. Click Create repository.
• Copy the three commands GitHub shows you under "...or push an existing repository from the command line." Paste them into your terminal and press enter.
• Refresh the GitHub page in your browser to confirm your files are there.

Submit: Copy your repo's URL and submit that link.

Industry Standards

• Check for required fields before touching the data store — validate at the boundary.
• Send back the status code that matches what happened: 200 read/updated, 201 created, 204 deleted, 404 not found, 400 bad input.
• Real projects split routes into separate files by resource instead of one big file — you'll practice that pattern in the stretch challenges.

Stretch Challenges

Only attempt these after Parts 1–7 work and are tested in Postman. Try them roughly in this order.

• Filter by genre: Add a query string to your GET-all route. If req.query.genre has a value, send back only books matching that genre. Example: GET /api/books?genre=Sci-Fi.
• Validation: On POST /api/books, return 400 if title or author is missing from req.body.
• Error handling: Wrap your route logic in try/catch. Log the error, and respond with 500 if something unexpected happens.
• Organize your code: Right now everything is in one file. Split it up:
  • Create an api/ folder with index.js and books.js.
  • Move your book routes into api/books.js, using express.Router() instead of app.
  • In api/index.js, mount the books router.
  • In app.js, mount your api/index.js router under /api, and remove the routes you moved out.
• Add dev middleware: Install cors and morgan. Add app.use(cors()) so a future frontend can talk to your server, and app.use(morgan("dev")) to log every request in your terminal.
• Reviews (a second, nested resource): Create api/reviews.js. Reviews belong to a book through a bookId field.

  let reviews = [
    { id: 1, bookId: 1, reviewer: "Alice", rating: 5, comment: "Must-read for any developer." },
    { id: 2, bookId: 1, reviewer: "Bob", rating: 4, comment: "Dense but rewarding." },
    { id: 3, bookId: 3, reviewer: "Charlie", rating: 5, comment: "One of the best sci-fi novels ever written." },
  ];
  
  let nextReviewId = 4;

  • GET /api/books/:bookId/reviews — return all reviews where review.bookId === Number(req.params.bookId).
  • POST /api/books/:bookId/reviews — read reviewer, rating, comment from req.body. Create a new review with the next id and the correct bookId. Respond 201.
  • DELETE /api/reviews/:id — delete a review by its own id. Respond 204.
  • On the POST review route, return 400 if rating is missing or is not a number between 1 and 5.
  • Mount this router in api/index.js under both /books and /reviews, since it needs to answer to both URL shapes.
• Add GET /api/books/:id/reviews that also includes the book's own data, not just the reviews.
• Add pagination to GET /api/books using ?page=1&limit=3 query params.
• Add PATCH /api/reviews/:id to update a review.
• Add GET /api/books/available that returns only books where available is true. (Hint: this route must be defined before /api/books/:id, or Express will treat "available" as an id.)
• Add middleware that counts every request that hits your server and logs the running total.

Finished Checklist

Before submitting, verify:

• node app.js starts without errors.
• GET /api/books returns all 5 starter books.
• GET /api/books/:id returns one book, or 404 if not found.
• POST /api/books creates a new book; GET /api/books confirms it's there.
• PATCH /api/books/:id changes only the fields you sent.
• DELETE /api/books/:id removes the book; GET /api/books confirms it's gone.
• Your work has been committed and pushed to GitHub.