Express Response JSON

Sep 13, 2019

Express response objects have a json() function. The res.json() function takes a single parameter, an object obj, serializes it to JSON, and sends it in the HTTP response body.

const axios = require('axios');
const express = require('express');
const app = express();

app.get('*', function(req, res) {
  res.json({ answer: 42 });
});

const server = await app.listen(3000);

const response = await axios.get('http://localhost:3000');
response.data; // { answer: 42 }
response.headers['content-type']; // 'application/json; charset=utf-8'

Express also sets the content-type header to application/json. Most HTTP clients, like Axios, handle automatically transforming JSON strings into JavaScript objects using JSON.parse() when the content type is application/json.

The res.json() uses JSON.stringify() under the hood to serialize objects into JSON. You can configure the arguments that Express passes to JSON.stringify() using app.use(). For example, to make Express pretty print JSON, you can use app.set('json spaces', 2) as shown below.

const axios = require('axios');
const express = require('express');
const app = express();

// Make Express pass '2' as the 3rd argument to `JSON.stringify()`
app.set('json spaces', 2);

app.get('*', function(req, res) {
  res.json({ answer: 42, hello: 'world' });
});

const server = await app.listen(3000);

const response = await axios.get('http://localhost:3000', {
  transformResponse: res => res // Disable automatic JSON parsing
});
// {
//   "answer": 42,
//   "hello": "world"
// }
response.data;

Want to become your team's Express expert? There's no better way to really grok a framework than to write your own clone from scratch. In 15 concise pages, this tutorial walks you through how to write a simplified clone of Express called Espresso. Get your copy!

Espresso supports:
  • Route handlers, like `app.get()` and `app.post()`
  • Express-compatible middleware, like `app.use(require('cors')())`
  • Express 4.0 style subrouters
As a bonus, Espresso also supports async functions, unlike Express.

Get the tutorial and master Express today!

Did you find this tutorial useful? Say thanks by starring our repo on GitHub!

More Express Tutorials

×
Mastering JS
Hi, I'm a JavaScript programming bot. Ask me something about JavaScript!