Express flashcards

The small, popular web framework for Node.js.

Beginner friendly, starts from zero

CardsCheatsheetTips and tricks

ExpressStart from zero · 1 of 12

What is Express, in one plain sentence?

Think of the answer, then tap

Express is a small toolkit for building a web server: a program that waits for questions from browsers and apps and sends back answers, written in JavaScript.

Express is the front desk of your app.

Space turn over · K got it · J again · ← back

All topics

Express at a glance

Express is a small JavaScript toolkit for building web servers that answer requests from browsers and apps.

A server is a restaurant kitchen: requests are orders coming in, responses are dishes going out, and Express is the head waiter who sends each order to the right cook.

Words you will meet

Request: A question an app or browser sends to a server
Response: The answer the server sends back
Route: A method plus a path plus the function that answers it
Middleware: A function every request passes through before its handler
JSON: Plain text for sending data, like {"name": "Ravi"}
Status code: A three-digit number that says how a request went, like 404
Header: A short label on the outside of a request or response
Port: A numbered door on a computer where the server listens

The steps

1. Start from zero (12 cards): What a server is, how requests and responses work, and the words Express uses.
2. Core basics (14 cards): Your first Express 5 API: routes, params, JSON in and out, status codes and static files.
3. Next steps (16 cards): Middleware, routers, Express 5 errors and paths, validation, CORS and file uploads.
4. Go deeper (16 cards): Security headers, proxies, cookies, testing, performance, observability and production shutdown.

All 12 cards in Start from zero

What is Express, in one plain sentence?: Express is a small toolkit for building a web server: a program that waits for questions from browsers and apps and sends back answers, written in JavaScript.
What are a request and a response, and where do you meet them every day?: A request is a question an app sends, like "show my orders". The response is the answer the server sends back. Every time an app loads your messages or bills, this happens in a second.
What problem does Express solve? What was it like without it?: Plain Node.js can answer requests, but you write long if-checks for every address and read form data byte by byte. Express gives ready parts for this, so a server takes a few lines.
What are Node.js, npm and a package?: Node.js runs JavaScript outside the browser, on a computer or server. A package is ready-made code you can add to your project, and npm is the tool that downloads packages, like Express.
Name the parts of this web address.: localhost means your own computer, 3000 is the port (a numbered door on it), /orders is the path, and ?page=2 is the query string, extra details for the request.
What are methods like GET and POST, and what is a route?: A method is the verb of a request: GET reads, POST creates, PUT or PATCH changes, DELETE removes. A route is a method plus a path plus the function that answers it, called a handler.
What are JSON, a body and a header?: JSON is plain text for data, like {"name": "Ravi"}. The body is the data carried inside a request or response. Headers are short labels on the outside, like the type of content or a key.
What is a status code?: A three-digit number at the start of every response that says how it went: 200 OK, 201 created, 404 not found, 500 the server broke. Apps read it before the body.
What do middleware and async mean?: Middleware is a function every request passes through before its handler, like a security check before the gate. Async code waits for slow work, such as a database, without freezing the server.
What do you need before you start?: Node.js 18 or newer, a text editor, and a terminal: the window where you type commands. Then npm install express adds Express to your project folder.
First tiny win: what does each line of this server do?: Line 2 creates the app, line 3 adds a route that sends a reply for the home path, and listen starts waiting on port 3000. Open localhost:3000 in a browser to see it.
What will you be able to do after this topic?: Build a small API that reads requests, sends JSON with the right status codes, checks input with middleware, handles errors in one place, and is ready to run safely in production.

Express cheatsheet

App setup

const app = express() Create the app
app.use(express.json({ limit: "100kb" })) Parse JSON bodies
app.use(express.urlencoded()) Parse HTML form posts
app.use("/api", router) Mount a router on a prefix
app.set("trust proxy", 1) Trust one proxy hop
app.disable("x-powered-by") Hide the framework header
app.listen(port, (err) => {}) Start, error in callback (v5)

Request

req.params.id Path segment (string)
req.query.page Query value (read-only in v5)
req.body Parsed body, undefined without parser
req.get("authorization") Header, any case
req.ip Client IP (needs trust proxy)
req.originalUrl Full URL incl. mount prefix
req.is("application/json") Check content type

Response

res.status(201).json(obj) Status and JSON
res.sendStatus(204) Status only
res.redirect(301, url) Permanent redirect
res.set("Cache-Control", "no-store") Set a header
res.cookie(name, val, opts) Set a cookie
res.download(path, name) File as attachment
res.sendFile(name, { root }) File inline from a folder
res.locals.user Per-request shared data

Express 5 paths

/users/:id Named parameter
/files/*filepath Named wildcard, array value
/files{/*filepath} Wildcard that also matches /files
/posts{/:page} Optional segment (was :page?)
/invoices/:id.pdf Param followed by literal text
/price\(usd\) Escape reserved characters

Middleware shapes

(req, res, next) => next() Normal middleware
(err, req, res, next) => {} Error handler, after routes
next(err) Jump to the error handler
next("route") Skip to the next matching route
app.get(path, auth, validate, handler) Per-route chain
router.route(path).get().put() Many verbs, one path