Body parser
- Node.js course
- Intermediate
- Lesson 34/116
- 5 min
Learn body parser middleware with express.json().
Body parser: express.json()
express.json() is a built-in body parser middleware in Express. It automatically parses incoming JSON bodies and puts them in req.body. Usage: app.use(express.json());.
After this middleware is registered, POST routes access the parsed JSON via req.body. Example: app.post("/data", (req, res) => { res.json({ received: req.body.name }); });. If the client sends { "name": "Alex" } then req.body.name is "Alex". If express.json() is not registered, req.body stays undefined. That is why the body parser must be registered before app.post routes.
Reading data with req.body
After express.json() is registered, every POST request's body is available as parsed JSON in req.body. Example: if the client sends { "name": "Alex", "age": 30 }, then req.body.name is "Alex" and req.body.age is 30.
This is the basic way to receive data in APIs. The client sends JSON with a Content-Type: application/json header; Express parses it with express.json(). For the response, res.json(data) automatically adds the Content-Type: application/json header and converts the object to JSON. So request and response flow as JSON in the same format.
Practice Write a POST route with body parser
Add body parser with app.use(express.json()). Add the route app.post("/data", (req, res) => { res.json({ received: req.body.name }); }). The client sends { name: "Alex" }. The server closes itself.
Starter code
const express = require("express");
const app = express();
// Add the JSON parser middleware
// Read data from the request body in a POST route, respond
// write here
const server = app.listen(3000, () => {
const http = require("http");
const opts = { hostname: "localhost", port: 3000, path: "/data", method: "POST", headers: { "Content-Type": "application/json" } };
const r = http.request(opts, (res) => {
let body = "";
res.on("data", (c) => body += c);
res.on("end", () => { console.log("Reply: " + body); server.close(); });
});
r.write(JSON.stringify({ name: "Alex" }));
r.end();
});Sample solution
const express = require("express");
const app = express();
app.use(express.json());
app.post("/data", (req, res) => {
res.json({ received: req.body.name });
});
const server = app.listen(3000, () => {
const http = require("http");
const opts = { hostname: "localhost", port: 3000, path: "/data", method: "POST", headers: { "Content-Type": "application/json" } };
const r = http.request(opts, (res) => {
let body = "";
res.on("data", (c) => body += c);
res.on("end", () => { console.log("Reply: " + body); server.close(); });
});
r.write(JSON.stringify({ name: "Alex" }));
r.end();
});Expected output
Yanıt: {"alindi":"Ayşe"}
Questions and answers
Review the lesson questions together with their answers. To test yourself, solve the lesson interactively.
1 Complete the express.json call
app.use(express.____());app.use(express.json());Explanation: express.json() parses JSON bodies.
2 Match each piece to its role
| Term | Meaning |
|---|---|
express.json() |
Parses JSON body |
req.body |
Parsed data |
app.post |
Defines a POST route |
res.json() |
Sends a JSON response |
3 What does express.json() do?
app.use(express.json())- Renders an HTML template
- Parses JSON bodies into req.body Correct answer
- Statik dosya sunar
- Redirects the URL
Explanation: It parses JSON bodies into req.body.