Express.js Installation and First Application

Express.js is a lightweight Node.js framework for building web applications and REST APIs.

• Simplifies routing and HTTP request handling.
• Supports middleware for extending application functionality.
• Enables fast and scalable server-side development.

System Requirements:

• Node.js
• NPM

Getting Started with Express.js

Follow these steps to create and run your first Express.js application.

Step 1: Create a Project Directory

Create a new folder for your Express application and navigate into it.

Open Command Prompt or Terminal and run:

mkdir my-express-app
cd my-express-app

Step 2: Initialize the Project

Initialize a new Node.js project.

npm init -y

This command creates a package.json file that stores your project's metadata and dependencies.

Step 3: Install Express

Install Express as a project dependency.

npm install express

Your package.json will include Express under dependencies.

Step 4: Create the Server File

Create a file named app.js (or server.js) in the project directory.

Project Structure

Add the following code:

const express = require("express");

const app = express();
const PORT = 3000;

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

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

• Import the Express module using require().
• Create an Express application with express().
• Define the server port.
• Create a GET route for the root (/) URL.
• Start the server using app.listen().

Step 5: Run the Application

Start the server.

node app.js

Open your browser and visit:

http://localhost:3000

Step 6: Add Routes

Routes define how your application responds to different HTTP requests.

Syntax

app.METHOD(path, callback);

Example: GET Route

app.get("/", (req, res) => {
  res.status(200).send("Welcome to the Home Page");
});

Example: Another GET Route

app.get("/hello", (req, res) => {
  res.set("Content-Type", "text/html");
  res.status(200).send("<h1>Hello Express!</h1>");
});

Visit:

http://localhost:3000/hello

The browser renders the HTML response.

Example: POST Route

app.post("/submit", (req, res) => {
  res.send("Form Submitted");
});

Common HTTP methods include:

• GET
• POST
• PUT
• PATCH
• DELETE

Step 7: Receive Data from the Client

Use the built-in JSON middleware to parse incoming JSON requests.

const express = require("express");
const app = express();
app.use(express.json());
app.post("/", (req, res) => {
  const { name } = req.body;
  res.send(`Welcome ${name}`);
});
const PORT = 3000;
app.listen(PORT, () => {
  console.log(`Server is running at http://localhost:${PORT}`);
});

You can test this endpoint using tools like Postman, Thunder Client, fetch(), Axios, or cURL.

Example request body:

{
  "name": "Ryan"
}

Response:

Step 8: Send Different Types of Responses

Express provides several response methods.

Send Plain Text

res.send("Hello World");

Send JSON

res.json({
  success: true,
  message: "Data received"
});

Set Status Code

res.status(201).send("User Created");

Set Response Headers

res.set("Content-Type", "text/html");

Step 9: Serve Static Files

To serve CSS, JavaScript, images, or HTML files, use the express.static() middleware.

Project Structure:

In app.js:

const express = require("express");
const path = require("path");
const app = express();
app.use(
  "/static",
  express.static(path.join(__dirname, "Static Files"))
);
app.listen(3000, () => {
  console.log("Server is running at http://localhost:3000");
});

Access files using:

http://localhost:3000/static/index.html

Step 10: Send a Single File

Use sendFile() when you want to send one specific file.

const express = require("express");
const path = require("path");
const app = express();
const PORT = 3000;
app.get("/file", (req, res) => {
  res.sendFile(path.join(__dirname, "Static Files", "image.jpg"));
});
app.listen(PORT, () => {
  console.log(`Server is running at http://localhost:${PORT}`);
});

Visit:

http://localhost:3000/file