Express.js Essentials

Express.js is the most widely used Node.js framework for building APIs and web apps. Let’s go step by step through the essentials.

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📦 1. Setting Up Express

npm install express

const express = require("express");
const app = express();
 
app.use(express.json()); // Middleware to parse JSON bodies
 
const PORT = process.env.PORT || 3000;
app.listen(PORT, () => console.log(`Server running on port ${PORT}`));

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🌐 2. Routing

Express routes define how your app responds to HTTP requests.

// GET route
app.get("/", (req, res) => {
  res.send("Home Page");
});
 
// POST route
app.post("/users", (req, res) => {
  const user = req.body;
  res.status(201).json({ message: "User created", user });
});
 
// PUT route
app.put("/users/:id", (req, res) => {
  const { id } = req.params;
  res.send(`Updating user with ID: ${id}`);
});
 
// DELETE route
app.delete("/users/:id", (req, res) => {
  const { id } = req.params;
  res.send(`Deleting user with ID: ${id}`);
});

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⚙️ 3. Middleware

Middleware functions run before route handlers and can modify req, res, or handle errors.

Example: Logging Middleware

app.use((req, res, next) => {
  console.log(`${req.method} ${req.url}`);
  next(); // Must call next() to continue
});

Example: Custom Middleware for Auth

const authMiddleware = (req, res, next) => {
  const token = req.headers["authorization"];
  if (!token) return res.status(401).json({ error: "Unauthorized" });
  next();
};
 
app.use("/users", authMiddleware);

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📄 4. req & res Objects

• req (request): info about client request

  • req.body → JSON or form data

  • req.params → URL parameters

  • req.query → query string parameters

  • req.headers → HTTP headers

• res (response): sending data back

  • res.send() → send text/html

  • res.json() → send JSON

  • res.status() → set HTTP status

  • res.redirect() → redirect client

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🔗 5. Query Params vs URL Params

URL Params

Defined in the route:

app.get("/users/:id", (req, res) => {
  res.send(`User ID: ${req.params.id}`);
});

Request: /users/123 → req.params.id = '123'

Query Params

Passed in the URL after ?:

app.get("/search", (req, res) => {
  res.send(`Search query: ${req.query.q}`);
});

Request: /search?q=nodejs → req.query.q = 'nodejs'

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❌ 6. Error Handling

Express has special middleware for errors: (err, req, res, next)

// Route that throws an error
app.get("/error", (req, res) => {
  throw new Error("Something went wrong!");
});
 
// Error-handling middleware
app.use((err, req, res, next) => {
  console.error(err.stack);
  res.status(500).json({ message: err.message });
});

• Errors can also be passed with next(err):

app.get("/fail", (req, res, next) => {
  next(new Error("Failed request"));
});

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✅ Summary

• Routing: map HTTP methods + paths to handlers.

• Middleware: reusable functions that run before or after routes.

• req & res: request info and response methods.

• Params: URL params (req.params) vs query params (req.query).

• Error handling: use error-handling middleware (err, req, res, next) for clean centralized error responses.

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