Introduction to Express.js

Last Updated : Oct 2026

Express.js is a web application framework for Node.js. It provides a minimal set of features for building web applications and APIs.It unopinionated about how your app is structured,but most developers Follows a MVC (Model-View-Controller) architecture and some other patterns.

Model - Data layer,where you store your data or structure your database.
View - Presentation layer,where you render your data to the user.
Controller - Logic layer,where you handle user input and logic.
Router - Routing layer,where you map URLs to controllers.
Middleware - Middleware layer,where you add additional functionality to your app.
Utils - Utility layer,Contains helper functions.
Lib - Library layer,Contains third-party libraries or actions.
Error - Error handling layer,where you handle errors in your app.
Config - Configuration layer,where you configure your app.

Note : Each layer is responsible for a specific task and can be separated from the rest of the app.We make folders for each layer,generally named conventions is lowercase in plural form like models,views,etc.

Prerequisites Knowledge for Express.js

Node.js
JavaScript

Installation of Express.js

Step 1: Install node.js from nodejs.org.

Step 2: Create a directory for your application and navigate into it.

mkdir myapp
cd myapp

Step 3: Install Express.js using npm.

npm init -y
npm install express

Step 4: Create a basic Express.js app.

create a index.js file and add the following code:

import express from "express";

const app = express();

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

app.listen(3000, () => {
  console.log("Server started on port 3000 Click http://localhost:3000");
});

Let's break down the code:

const express = require("express"); imports the express module from the express package.
const app = express(); creates an instance of the express application.The instance have some common methods and properties.
- app.get() is used to define a route for the HTTP GET method.also same as all other HTTP methods
- app.use() is used to add middleware to the application.
- app.listen() is used to start the server and listen for incoming requests.
- app.set() is used to set configuration options for the application.
- app.all() is used to define a route for all HTTP methods.
- app.route() is used to create chainable routes.
app.get("/", (req, res) => {...}) defines a route for the root URL (/) with a handler function that sends a response with the text "Hello, Express.js!".
- req is the request object,which contains information about the incoming request.it includes things like headers,query parameters,etc.
- res is the response object,which is used to send the response back to the client.it includes following methods like:
  - res.send() sends a response with the specified content.
  - res.render() renders a view template with the specified data.
  - res.json() sends a JSON response with the specified data.
  - res.status() sets the status code for the response.
  - res.redirect(status, url) redirects the client to a different URL,with status code.
  - res.cookie() sets a cookie in the response.
  - res.end() ends the response.
app.listen(3000, () => {...}) starts the server on port 3000 and logs a message to the console.

Step 5: Change package.json file.

{
  "name": "myapp",
  "version": "1.0.0",
  "type": "module",
  "main": "index.js",
  "scripts": {
    "test": "echo \"Error: no test specified\" && exit 1",
    "start": "node index.js"
  },
  "keywords": [],
  "author": "",
  "license": "ISC",
  "description": "",
  "dependencies": {
    "express": "^5.2.1"
  }
}

Note: In whole tutorial We always use ESM for importing modules in.You are free to use CJS if you want.

Step 6: Run the app.

node index.js

Step 6: Open your browser and navigate to http://localhost:3000 to see the app running.

Note : You can also use npm start to start the app.

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