JSON request and response patterns in Express

Introduction

JSON is a simple way to send and receive data between a client and a server. Express helps handle JSON easily in web apps.

When building APIs that send data to web or mobile apps

When a client sends data like form inputs to the server

When you want to respond with structured data instead of HTML pages

When you need to exchange data between different systems in a simple format

Syntax

Express

app.use(express.json())

app.post('/path', (req, res) => {
  const data = req.body
  res.json({ message: 'Received', yourData: data })
})

Use express.json() middleware to parse incoming JSON requests automatically.

Use res.json() to send JSON responses easily.

Examples

This example receives user data in JSON and sends it back with a success status.

Express

app.use(express.json())

app.post('/user', (req, res) => {
  const user = req.body
  res.json({ status: 'success', user })
})

This example sends a JSON response with app info when a GET request is made.

Express

app.get('/info', (req, res) => {
  res.json({ app: 'MyApp', version: '1.0' })
})

Sample Program

This program creates a simple Express server that listens on port 3000. It uses JSON middleware to parse incoming JSON data. When a POST request is sent to /echo with JSON data, it responds by sending back a JSON object confirming receipt and showing the data.

Express

import express from 'express'

const app = express()
const port = 3000

app.use(express.json())

app.post('/echo', (req, res) => {
  const receivedData = req.body
  res.json({ message: 'Data received', data: receivedData })
})

app.listen(port, () => {
  console.log(`Server running on http://localhost:${port}`)
})

Important Notes

Always use express.json() before routes that expect JSON data.

Clients must set the Content-Type header to application/json when sending JSON.

Use res.json() instead of res.send() for automatic JSON formatting and headers.

Summary

Express makes handling JSON requests and responses simple with built-in middleware.

Use express.json() to parse incoming JSON data automatically.

Use res.json() to send JSON responses with correct headers.