How to Send JSON Response in Node.js: Simple Guide

In Node.js, you can send a JSON response by setting the Content-Type header to application/json and sending a JSON string using res.end() with the built-in http module. When using Express, simply use res.json() to send a JSON response easily.

Syntax

To send a JSON response in Node.js, you need to set the Content-Type header to application/json and send the JSON data as a string. In the built-in http module, use res.writeHead() to set headers and res.end() to send the response. In Express, use res.json() which handles headers and stringifying automatically.

javascript

/* Using built-in http module */
res.writeHead(200, { 'Content-Type': 'application/json' });
res.end(JSON.stringify({ key: 'value' }));

/* Using Express framework */
res.json({ key: 'value' });

Example

This example shows a simple Node.js server using the built-in http module that sends a JSON response. It also shows how to do the same with Express for easier handling.

javascript

/* Using built-in http module */
const http = require('http');

const server = http.createServer((req, res) => {
  if (req.url === '/data') {
    const data = { message: 'Hello, JSON!' };
    res.writeHead(200, { 'Content-Type': 'application/json' });
    res.end(JSON.stringify(data));
  } else {
    res.writeHead(404, { 'Content-Type': 'text/plain' });
    res.end('Not Found');
  }
});

server.listen(3000, () => {
  console.log('Server running at http://localhost:3000/');
});

/* Using Express framework */
const express = require('express');
const app = express();

app.get('/data', (req, res) => {
  res.json({ message: 'Hello, JSON!' });
});

app.listen(3000, () => {
  console.log('Express server running at http://localhost:3000/');
});

Output

Server running at http://localhost:3000/
Express server running at http://localhost:3000/

Common Pitfalls

• Forgetting to set the Content-Type header to application/json can cause clients to misinterpret the response.
• Sending a JavaScript object directly without converting it to a JSON string using JSON.stringify() will cause errors in the built-in http module.
• Using res.send() in Express without JSON data may send plain text instead of JSON.

javascript

/* Wrong way with http module (missing JSON.stringify) */
res.writeHead(200, { 'Content-Type': 'application/json' });
res.end({ message: 'Hello' }); // This will cause an error

/* Right way */
res.writeHead(200, { 'Content-Type': 'application/json' });
res.end(JSON.stringify({ message: 'Hello' }));

Quick Reference

Method	Usage	Notes
http module	res.writeHead(200, {'Content-Type': 'application/json'}); res.end(JSON.stringify(data));	Manual header and string conversion required
Express	res.json(data);	Automatically sets header and stringifies object
Common mistake	res.end(object);	Causes error, must stringify first

Key Takeaways

Always set the Content-Type header to application/json when sending JSON responses.

Use JSON.stringify() to convert objects to JSON strings when using the built-in http module.

Express's res.json() method simplifies sending JSON by handling headers and stringifying automatically.

Avoid sending raw objects directly in res.end() as it causes errors.

Test your API responses in a browser or tool like Postman to verify JSON format.