Day 13: Route Params, Query Params & Middleware in Express.js

🧠 What You’ll Learn

• What are route parameters?

• What are query parameters?

• What is middleware in Express?

• Real-life project: Filtering and accessing student info

• Full code with explanation

• FAQs

---

🔍 1. What are Route Parameters?

Route parameters are dynamic values in the URL.

🧾 Example:

GET /students/5

Here, 5 is a route parameter (student ID).

app.get("/students/:id", (req, res) => {
  const id = req.params.id;
  res.send(`You requested student with ID: ${id}`);
});

---

🔍 2. What are Query Parameters?

Query parameters are used to filter/search data.
They are sent in URL after a question mark (?).

🧾 Example:

GET /students?city=delhi

app.get("/students", (req, res) => {
  const city = req.query.city;
  res.send(`Filter students from city: ${city}`);
});

---

🔍 3. What is Middleware?

Middleware is a function that runs between request and response.

🧾 It can be used for:

• Logging

• Authentication

• Validation

• Adding extra headers

function logger(req, res, next) {
  console.log(`${req.method} ${req.url}`);
  next(); // move to next middleware/route
}

app.use(logger); // applied to all routes

---

🔧 Let’s Build a Real-Life Example (Student API)

We’ll add:

• Route param: Get student by id

• Query param: Filter students by city

• Middleware: Log every request

---

📁 Folder Structure

express-routing/
└── index.js

---

✅ Step 1: Setup Project

mkdir express-routing
cd express-routing
npm init -y
npm install express

---

✅ Step 2: Create index.js

const express = require("express");
const app = express();
const port = 5000;

// Sample student data
const students = [
  { id: 1, name: "Payal", city: "Delhi" },
  { id: 2, name: "Amit", city: "Mumbai" },
  { id: 3, name: "Ravi", city: "Delhi" },
];

// 🧠 Middleware to log all requests
app.use((req, res, next) => {
  console.log(`${req.method} request on ${req.url}`);
  next();
});

// ✅ Route: Get all students OR filter by city
app.get("/students", (req, res) => {
  const city = req.query.city;

  if (city) {
    const filtered = students.filter((s) => s.city.toLowerCase() === city.toLowerCase());
    res.json(filtered);
  } else {
    res.json(students);
  }
});

// ✅ Route: Get single student by ID
app.get("/students/:id", (req, res) => {
  const id = parseInt(req.params.id);
  const student = students.find((s) => s.id === id);

  if (!student) {
    return res.status(404).json({ message: "Student not found" });
  }

  res.json(student);
});

// Server start
app.listen(port, () => {
  console.log(`🚀 Server running on http://localhost:${port}`);
});

---

🧪 Try These in Browser/Postman

✅ 1. Get all students

GET http://localhost:5000/students

✅ 2. Filter students by city

GET http://localhost:5000/students?city=delhi

✅ 3. Get student by ID

GET http://localhost:5000/students/2

---

🧠 Summary of What You Used

---

📊 Real-Life Use Cases

---

❓ FAQs

---

1. Can we use multiple query parameters?

Yes ✅ Example:

/students?city=delhi&age=22

In Express:

const city = req.query.city;
const age = req.query.age;

---

2. What if ID is not found in route?

Use this check:

if (!student) {
  return res.status(404).json({ message: "Student not found" });
}

---

3. Is middleware optional?

Yes, but it's powerful and often used for:

• Logging

• Authentication

• Error handling

---

4. Can I apply middleware to specific routes only?

Yes ✅

app.get("/students", myMiddleware, (req, res) => { ... });

---

5. What happens if I don’t use next() in middleware?

Your app will hang and never reach the route handler.
Always use next() to pass control to next handler.

---

🏁 Practice for Students

1. Add new route: /students/:id/grade → Return dummy grade

2. Use query param to filter students by name

3. Create a middleware that counts how many times /students is called

---

✅ Final Summary

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