Node.js Foundations·From script to service

Express routes and middleware

Express gives structure to routing, parsing and error handling.

Intermediate 14 min read Lesson 8 of 300 Challenge included

What you will be able to do

Use express.json() with a size limit before routes that read JSON.
Keep route handlers small; move business logic into modules.
Register an error handler with four arguments at the end.

In plain English

Middleware is a function that can inspect a request, end the response or call next. Put cross-cutting concerns such as authentication, logging and validation in middleware instead of repeating them in every route.

app.jsJavaScript

import express from 'express'

const app = express()
app.use(express.json({ limit: '16kb' }))

function requireKey(request, response, next) {
  if (request.get('x-api-key') !== process.env.API_KEY) {
    return response.status(401).json({ error: 'UNAUTHORIZED' })
  }
  next()
}

app.get('/health', (_request, response) => {
  response.json({ ok: true })
})

app.post('/api/notes', requireKey, (request, response) => {
  response.status(201).json({ id: crypto.randomUUID(), text: request.body.text })
})

app.use((error, _request, response, _next) => {
  console.error(error)
  response.status(500).json({ error: 'INTERNAL' })
})

Your turn

Add an API key guard

Medium

Write middleware that returns 401 when x-api-key does not match process.env.API_KEY, otherwise calls next.

JS · starter

function requireKey(request, response, next) {
  next()
}

Check your understanding before moving on.

Knowledge checkQuestion 1

How does Express identify an error-handling middleware?

Select an answer to continue

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Next: Async errors without crashes

Lesson 8 of 300 · 0% of course