Process JSON payloads
A client sends JSON to a POST endpoint and the server needs to read submitted fields.
Express.js · Const
req.body
req.body is the parsed key-value data submitted in an HTTP request body. Express fills it through compatible body-parsing middleware such as express.json() or express.urlencoded(); otherwise it is undefined.
How it works
req.body exposes the key-value data submitted in an HTTP request body after compatible body-parsing middleware has processed it. Without such middleware, its value is undefined; because the contents come from the user, validate them before use.
Syntax
req.bodyKey points
- The property represents submitted request-body data as key-value pairs.
- Built-in parsing middleware can populate it for JSON and URL-encoded payloads.
- It is a property on Express's request object, commonly accessed as
req.bodyinside handlers.
Common mistakes
- Do not assume the property is populated: install body-parsing middleware before routes that consume the body.
- Treat every field as untrusted input and validate both its presence and type before using it.
Example: Read a JSON request body
const express = require('express');
const app = express();
app.use(express.json());
app.post('/profile', (req, res) => {
const name = typeof req.body?.name === 'string' ? req.body.name : '';
res.json({ name });
});
app.listen(3000);JSON middleware parses matching requests before the route reads the submitted fields.
When to use it
Handle form submissions
An HTML form submits URL-encoded values that a route must consume.
Validate incoming input
An endpoint receives client-controlled data that must be checked before persistence or business logic.
Interview questions
QWhat is req.body in Express, and how is it populated?
req.body is an object containing key-value pairs submitted in the request body. Express populates it through middleware such as express.json() or express.urlencoded().
Follow-up: What happens if no parser runs? req.body remains undefined rather than containing parsed data.
QHow do you make JSON data available through req.body?
The parser must be registered before the route handler that reads req.body. For JSON requests, app.use(express.json()) is a typical choice; URL-encoded forms use express.urlencoded().
Follow-up: Why might parsing still not occur? The request Content-Type must match the parser's expected type.
QWhy must req.body be validated?
The body has a user-controlled shape, so all properties and values must be treated as untrusted. Validate expected fields and types before invoking methods or using the values.
Follow-up: What kind of failure can unchecked access cause? A missing field may make property access fail, or a supplied value may have an unexpected type.
Related
reqexpress.json()express.urlencoded()req.paramsreq.query