First steps

Express is the web framework most Node servers run on, and has been for over a decade. It is also what NestJS runs on by default: a Nest app's @Req() and @Res() are Express's request and response, and Nest middleware is Express middleware. Learning Express directly shows what those layers are built from, and it is still how a great deal of production code is written. This course uses Express 5, the current major version, running in your tab exactly as it would under Node.

An app is routes and a listen()

import express from 'express';

const app = express();

app.get('/time', (req, res) => {
  res.json({ now: new Date().toISOString() });
});

app.listen(3000, () => {
  console.log('Listening on http://localhost:3000');
});

express() creates an application. app.get(path, handler) adds a route: when a GET request's path matches, Express calls the handler with the request, req, and the response, res. There is one method per HTTP verb: app.post, app.put, app.patch, app.delete, and app.all for every verb. app.listen(port) starts the server; until it is called, the app answers nothing, and the runtime here says so.

The handler answers through res

In Express, what a handler returns is ignored. It answers by calling a method on res:

Call	Sends
res.send('text')	a string, as text/html; a Buffer as binary; an object as JSON
res.json(value)	any value as JSON, application/json
res.status(404)	nothing yet: sets the status and returns res, so it chains
res.type('text/plain')	nothing yet: sets the content type, and chains too
res.end()	nothing more: ends the response as it stands

Two consequences shape every Express handler. A handler that never calls one of the sending methods leaves the request hanging until the client gives up; nothing answers for it. And a handler that sends twice fails with Cannot set headers after they are sent to the client, the most common Express error there is. So a handler that answers early, an error for instance, returns right after:

if (!owner) {
  res.status(404).json({ error: 'No such owner' });
  return;
}
res.json(owner);

Note the default for strings: res.send('Tom') is sent as text/html, because Express was built for web pages. A plain-text answer says so with res.type('text/plain') first.

What the request carries

Part	In Express
a path parameter, /owners/:id	req.params.id
a query value, ?page=2	req.query.page
a header	req.get('accept') or req.headers.accept
the method, the path	req.method, req.path

Everything from the URL is a string: req.params.id for /owners/7 is "7", so a lookup by number converts it first. A query value is undefined when absent, and an array when the key is repeated, ?tag=a&tag=b; checking typeof value === 'string' before using it covers both.

A route path's :name segments match anything up to the next slash, so /cats/:id matches /cats/7 but not /cats/7/name, which needs a route of its own. Express 5 changed the path syntax from version 4 in one place worth knowing: a wildcard is named, /files/*path, not a bare *.

Your task

The app has one example route and calls listen(). Add, on the same cats array:

1. GET /cats answers every cat as JSON, or, with ?minAge=<n>, only the cats at least that old.
2. GET /cats/:id answers that cat as JSON, or 404 with { "error": "Cat <id> not found" }.
3. GET /cats/:id/name answers the cat's name as plain text, with content-type: text/plain, or the same 404.

When it fails

• Request timed out after 5s; the runtime was restarted: a branch of the handler never calls res.send() or res.json(), so nothing ever answered.
• Cannot set headers after they are sent to the client in the console: the 404 branch sent its answer and the handler went on to send another. return after it.
• GET /cats/2 is a 404: req.params.id is the string "2" and was compared with === to a number.
• "A name as text" fails on the content type: res.send() alone says text/html.

Remember

• express() makes an app, app.get() and friends add routes, app.listen() starts it.
• A handler answers through res; its return value is ignored.
• Answer exactly once: return after an early answer.
• Params and query values are strings; a string sent with res.send() is HTML unless res.type() says otherwise.