Introduction to Express.js

By Syncfusion®, Inc. · Last updated November 12, 2026

What Express.js is and why it was created, its history and features, Express versus the Node.js http module, Express 4 versus Express 5, how a request flows through an app, what you can build, and how to set up, configure, run and test your first Express 5 server.

What you will learn

• Explain what Express.js is and why developers use it
• Compare Express with the Node.js http module and Express 4 with Express 5
• Set up an Express 5 project with npm, ES modules and npm scripts
• Read the port from environment variables with --env-file or dotenv
• Write, run, test and restart a first Express server

Express.js Introduction

Express.js = a small framework that helps you build web servers with Node.js.

• A web server is a program that waits for requests and sends back answers.
• Node.js lets JavaScript run on a computer, outside the browser.
• Express sits on top of Node.js and does the boring server work for you.
• You write short rules like "when someone asks for /menu, send the menu".
• Most people just call it Express.

Simple lesson: Express helps you write a web server in JavaScript with very little code.

A Real-World Example

Your school canteen gets a new app. Students open it on their phones.

• A student taps Menu. The phone sends a request to the canteen server.
• The server is written with Express.
• Express finds the rule for /menu and runs it.
• The rule sends back the list: Idli, Dosa, Vada.
• The phone shows the list. All of this takes a few milliseconds.

Animation: a phone sends GET /menu to an Express server, and the server sends the menu back

Canteen app Client (phone)

Express on Node.js app.get("/menu") Server

GET /menu

Idli, Dosa, Vada

1. Request 2. Response

The phone asks. Express finds the matching rule and answers.

In this lesson, you will learn:

1. What Express is, why it was made and what it can build.
2. How Express compares with plain Node.js and with Express 4.
3. How a request travels through an Express app.
4. How to set up a project and keep secrets out of your code.
5. How to write, run, test and fix your first server.

Before You Start

Check these before you go on. You will learn faster if you already know them:

• JavaScript basics: variables, functions, arrays and objects.
• Arrow functions: (req, res) => ... is used everywhere.
• Modules: import and export between files.
• Promises, async and await: for work that takes time, such as a database.
• Node.js basics: running a file with node file.js.
• npm basics: installing packages with npm install.
• HTTP basics: requests and responses. The next lesson teaches these.
• Node.js 18 or later: Express 5 needs it. The current LTS version is best.

Simple lesson: Know modern JavaScript and a little Node.js, and you are ready.

Your First Express Server

Here is a complete Express server. It has just three steps: create the app, add a route, and start listening on a port.

Press ▶ Run. Then look at the panel on the right.

• Terminal shows the start command ($ node server.js) and the message from console.log.
• Response shows 200 OK and the welcome text.
• Press the GET /hello button. You get 404 Not Found, because there is no rule for /hello.

Simple lesson: app.get adds a rule, and app.listen starts the server.

Why Express Was Created

Node.js can already make a web server. So why do we need Express?

• Plain Node.js gives you the request as raw text and numbers.
• You must check the URL yourself with many if statements.
• You must set every header, such as the content type, by hand.
• You must read the request body piece by piece.
• Every project ends up writing the same helper code again.
• Express puts these helpers in one small, shared package.

Simple lesson: Express removes the repeated work, so you can focus on what your app does.

History of Express.js

• 2010: TJ Holowaychuk released Express. It was inspired by Sinatra, a small Ruby framework.
• 2014: The company StrongLoop took care of the project.
• 2015 to 2016: IBM bought StrongLoop, and Express moved to the Node.js Foundation.
• Today: Express belongs to the OpenJS Foundation and is looked after by volunteers.
• 2024: Express 5.0 came out, the first big update in about ten years.
• 2025: Express 5.1 became the default version that npm install express gives you.

Simple lesson: Express is old, stable and still updated. This course uses Express 5.

Features of Express.js

• Routing: match a method and a path, such as GET /menu, to your code.
• Middleware: small functions that run one after another on every request.
• Request helpers: read the path, query and headers easily with req.
• Response helpers: send text, JSON, files and status codes with res.
• Body parsing: turn sent JSON or form data into a JavaScript object.
• Static files: serve images, CSS and HTML from a folder.
• Templates: build HTML pages from data with a view engine.
• Huge ecosystem: thousands of ready-made packages work with Express.

Simple lesson: Express gives you the basic tools. You add more only when you need them.

Minimal and Unopinionated

Think about two ways to eat at a canteen.

1. A fixed thali comes with every dish already chosen.
2. A plain dosa counter gives you a dosa, and you pick your own chutneys.
3. The thali is quick to start, but hard to change.
4. The dosa counter takes a few choices, but you get exactly what you like.

Express is the dosa counter.

Minimal

Minimal = Express ships with only the core pieces.

• No database tool is built in.
• No login system is built in.
• No fixed folder layout is built in.
• The package stays small and easy to learn.

Unopinionated

Unopinionated = Express does not force one "right way".

• You choose the database, such as PostgreSQL or MongoDB.
• You choose the folder layout.
• You choose the validation and login packages.
• The downside: you must make these choices yourself.

Simple lesson: Express gives you a small base and lets you choose the rest.

Express.js vs Node.js

People often mix these two up. They are not the same kind of thing.

• Node.js = the runtime. It runs JavaScript on a computer.
• Express = a package. It runs inside Node.js.
• You install Node.js once on your computer.
• You install Express into each project with npm.
• Express cannot run without Node.js. Node.js runs fine without Express.

Layers: your code on top of Express, on top of Node.js, on top of the computer

Your code: routes like app.get("/menu")

Express: routing, middleware, req and res helpers

Node.js: runs JavaScript, http module, files

Your computer or a cloud server

Each layer stands on the one below it.

Simple lesson: Node.js runs your program. Express is a helper package inside it.

Express.js vs the Node.js HTTP Module

The best way to see the difference is to build the same server twice.

Both servers answer / with a welcome and /menu with a list.

The Plain Node.js Way

This version uses the built-in node:http module only.

Run it and press all three buttons.

• / and /menu work.
• /menu?veg=true gives 404, because req.url is the whole text /menu?veg=true.
• To fix it, you would have to split the URL yourself.

The Express Way

Run it and press all three buttons.

• All three work, including /menu?veg=true.
• Express matches the path only, and keeps the query separately.
• res.send saw an array, so it sent JSON and set the content type for you.
• The 404 page for unknown paths comes free.

Side by Side

Simple lesson: Express is built on the http module. It just does the repeated work for you.

Express 4 vs Express 5

Express 5 is the current version. Many older tutorials still show Express 4.

• You do not need to learn this whole table now. Later lessons teach each row.
• For now, remember two things: copy paths from old code carefully, and check your Node.js version.

You can see one change right now. Run the lab below.

• The server stops with TypeError: Unexpected ?.
• Change "/menu/:id?" to "/menu{/:id}" and press Run again.
• Now both /menu and /menu/2 work.

Simple lesson: Learn Express 5 syntax. When you copy old code, check its paths first.

How an Express App Works

Every request goes on the same short trip through your app.

1. Request: a client sends a method and a path, such as GET /menu.
2. Middleware: helper functions run first, in the order you added them.
3. Route matching: Express looks for a rule with the same method and path.
4. Route handler: the matching function runs your code.
5. Response: the handler sends an answer with res.send or res.json.
6. Done: the trip ends. No match means a 404 answer.

Animation: a request passes through middleware, route matching and the handler, then a response goes back

Request

Middleware

Route matching

Handler

Response

1. GET /menu arrives 2. Middleware runs first 3. Express finds the GET /menu rule 4. Your handler runs 5. The answer goes back to the client

One request, one trip, one response.

You will learn each step in detail later in this course.

Simple lesson: A request goes in, passes through your functions, and exactly one response comes out.

What You Can Build with Express

• Web applications: full websites with pages and forms.
• REST APIs: send JSON data to apps, such as the canteen menu.
• Backend services: do work for other programs, such as sending emails.
• Server-rendered websites: build HTML pages on the server.
• Login services: check passwords and keep users signed in.
• File upload services: accept photos and documents.
• Mobile app backends: give Android and iPhone apps their data.
• Single-page app backends: serve a React or Vue app and its API.
• Microservices: many small servers, each doing one job.

Simple lesson: If it talks over HTTP, you can probably build it with Express.

Setting Up an Express Project

Open a terminal in the folder where you keep your projects. Run these one by one.

mkdir canteen-api
cd canteen-api
npm init -y
npm install express
npm list express

What each command does:

• mkdir canteen-api makes a new project folder.
• cd canteen-api moves into it.
• npm init -y creates package.json with default answers.
• npm install express downloads Express into node_modules.
• npm list express prints the version. It should start with express@5.

Turn On ES Modules

Open package.json. Add "type": "module" and two scripts.

{
  "name": "canteen-api",
  "version": "1.0.0",
  "type": "module",
  "scripts": {
    "dev": "node --watch server.js",
    "start": "node server.js"
  },
  "dependencies": {
    "express": "^5.1.0"
  }
}

• "type": "module" lets you use import and export in .js files.
• npm run dev starts the server and restarts it every time you save a file.
• npm start starts the server once. Real servers in the cloud use this one.

Simple lesson: npm init, npm install express, then add "type": "module".

Splitting app.js and server.js

Real projects keep two small files instead of one.

• app.js = builds the app: the routes and settings. It does not start anything.
• server.js = starts the app with app.listen.
• Tests can import app.js without starting a real server.
• Later, you can start the same app in different ways.

• The terminal runs npm start, which runs node server.js.
• server.js imports the app from ./app.js. Note the .js at the end.
• Node.js needs the full file name when you use import.

Simple lesson: app.js describes the app. server.js switches it on.

Environment Configuration

Some values change from computer to computer, such as the port or a database password.

• Do not type these values into your code.
• Keep them in environment variables instead.
• Environment variable = a named value the computer gives your program.
• Node.js puts them all in process.env.
• Every value in process.env is a string, such as "4000".

The .env File

During development, you keep these values in a file named .env.

PORT=4000
CANTEEN_NAME=Green Valley School Canteen

Loading .env

Node.js does not read .env by itself. Pick one of these two ways.

• Node.js 20.6 or later: run node --env-file=.env server.js.
• The dotenv package: run npm install dotenv, then add import "dotenv/config"; at the top of server.js.
• Do not use both. Two loaders make it hard to know where a value came from.

• The server listens on 4000, the value from .env.
• Open the .env tab, change PORT to 5000, and press Run again.
• Then delete the PORT line. The server falls back to 3000.

Keeping .env Secret

• Add .env to your .gitignore file, so Git never uploads it.
• Create .env.example with the same names but no real values.
• Teammates copy .env.example to .env and fill in their own values.

# .env.example
PORT=3000
CANTEEN_NAME=

Simple lesson: Settings live in .env, your code reads them from process.env, and Git never sees .env.

Development Tools

You need a way to send requests to your server while you build it.

• node --watch: restarts the server when you save a file. npm run dev uses it.
• Browser: good for quick GET requests. Type http://localhost:3000/menu.
• curl: a terminal command that sends any request. It is built into Windows, macOS and Linux.
• Postman: a popular app for sending and saving requests.
• Bruno: a free, open-source app like Postman. It saves requests as files in your project.

Try these curl commands while your server runs:

curl http://localhost:3000/
curl http://localhost:3000/menu
curl -i http://localhost:3000/hello

• curl URL prints the response body.
• curl -i URL also prints the status line and headers.

In this course, the Send panel in each lab works like a tiny Postman.

Simple lesson: Use the browser for quick checks and curl, Postman or Bruno for everything else.

Starter Project Files

Your finished starter project looks like this:

canteen-api/
├── node_modules/      packages downloaded by npm (never edit)
├── .env               your settings and secrets (never commit)
├── .env.example       the same names, no real values (commit this)
├── .gitignore         files Git should ignore
├── app.js             builds the Express app and its routes
├── package.json       name, scripts and packages
├── package-lock.json  exact package versions (made by npm)
└── server.js          starts the server

The .gitignore file needs only two lines for now:

node_modules
.env

Simple lesson: Five small files are enough to start any Express project.

Your First Express Application, Step by Step

Here is the full app.js and server.js again. Every line is explained below.

The Pieces

• express() = creates the application object, usually called app.
• app.get(path, handler) = adds a route for GET requests to that path.
• Route handler = the function that runs when the route matches.
• req = the request object. req.query.name reads ?name=Priya.
• res = the response object. res.send() sends text, HTML or JSON.
• Port = a door number on the computer. Your server listens at door 3000.
• app.listen(port, callback) = starts the server. The callback runs once it is ready.

Try /greet?name=Priya, then /greet with no name. The || gives a default value.

Stopping and Restarting

• Press Ctrl + C in the terminal to stop the server.
• Run npm run dev (or node server.js) to start it again.
• With --watch, saving a file restarts the server for you.
• Without --watch, you must stop and start the server to see changes.

In this lab, ↻ Restart does the same as stopping and starting.

Simple lesson: Create the app, add routes, then listen on a port.

Fixing Common Startup Errors

When a server will not start, the terminal tells you why. Read the first error line.

Press Run. The terminal shows ERR_MODULE_NOT_FOUND.

• The import says "./app", but the file is app.js.
• Change it to "./app.js" and run again.

The Errors You Will Meet Most

• Cannot find module './app': add the .js ending to the import.
• Cannot find package 'express': run npm install express in the project folder.
• Warning: MODULE_TYPELESS_PACKAGE_JSON (older Node.js shows Cannot use import statement outside a module): add "type": "module" to package.json.
• EADDRINUSE: address already in use :::3000: another server already uses port 3000. Stop it, or pick another port.
• The server starts on the wrong port: .env was not loaded. Use --env-file=.env or import "dotenv/config".

Simple lesson: Read the first line of the error. It usually names the exact problem.

Common mistakes

Check Yourself

Exercises

What to Remember

• Express is a small, unopinionated framework for web servers on Node.js.
• It adds routing, middleware and easy req and res helpers to the http module.
• This course uses Express 5, which needs Node.js 18 or later and newer path syntax.
• Start a project with npm init -y, npm install express and "type": "module".
• Read settings from process.env, load .env one way only, and never commit it.
• A first server is three steps: express(), app.get(...), app.listen(...).

Next: HTTP Fundamentals. You will learn what requests, responses, methods and status codes really are.