Why does req.body come out undefined in Express, and what does express.json do?
SHORT ANSWER
Express does not read request bodies by itself. express.json() is middleware that parses JSON bodies with a JSON content type and places the result on req.body. Without it, req.body is undefined.
Explain it simply
A request body arrives as a stream of bytes, like water coming through a pipe. Something has to collect it, check what it says, and turn it into an object. Express leaves that to you, and express.json() is the ready-made worker for JSON bodies.
Example
js
const express = require("express");
const app = express();
app.use(express.json({ limit: "100kb" })); // JSON bodies
app.use(express.urlencoded({ extended: false })); // HTML form bodies
app.post("/register", (req, res) => {
const { name, email } = req.body;
if (!name || !email) {
return res.status(400).json({ error: "name and email are required" });
}
res.status(201).json({ name, email });
});
app.listen(3000);Details:
- It only parses requests whose
Content-Typeisapplication/json. If the client forgets the header,req.bodystays empty. - Register it before your routes; middleware order matters.
- The
limitoption stops someone from sending a huge body; the default is 100 kb. - Malformed JSON causes a 400 error that your error handler can catch.
- File uploads use
multipart/form-data, which needs a different tool such asmulter.
What to say in the interview
"express.json is built-in body-parsing middleware for JSON. It must be registered before the routes, depends on the Content-Type header, and has a size limit. Without it req.body is undefined."
Common mistakes
- Registering the parser after the route that needs it.
- Forgetting the Content-Type header in Postman or fetch.