Handling any POST data in Express

Request body data will be placed to req.body. A few examples for decoding popular request data formats:

application/x-www-form-urlencoded
application/json
multipart/form-data

Form URL Encoded

application/x-www-form-urlencoded is a default data format for sending forms:

<form method="POST" action="/">
  <input type="text" name="username" placeholder="username">
  <input type="submit">
</form>

Use express.urlencoded middleware:

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

/** Decode Form URL Encoded data */
app.use(express.urlencoded());

/** Show page with a form */
app.get('/', (req, res, next) => {
  res.send(`<form method="POST" action="/">
  <input type="text" name="username" placeholder="username">
  <input type="submit">
</form>`);
});

/** Process POST request */
app.post('/', function (req, res, next) {
  res.send(JSON.stringify(req.body));
});

/** Run the app */
app.listen(3000);

Run this code and open localhost:3000.

Fill the form and send. You will see a response:

JSON

application/json mostly used for AJAX (XMLHttpRequest) requests.

Use express.json:

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

/** Decode JSON data */
app.use(express.json());

/** Show page with a input field, button and javascript */
app.get('/', (req, res, next) => {
  res.send(`<script src="https://cdn.jsdelivr.net/npm/@codexteam/ajax"></script>

<script>
var send = function() {
  var username = document.getElementById('username').value;
  
  ajax.post({
    url: '/',
    data: {
      username: username
    },
  })
    .then(response => {
      console.log(response.body)
    })
    .catch(console.error);
}
</script>

<input type="text" name="username" placeholder="username" id="username">
<button onclick="send()">Send</button>`);
});

/** Process POST request */
app.post('/', function (req, res, next) {
  res.send(JSON.stringify(req.body));
});

/** Run the app */
app.listen(3000);

Run this code, open localhost:3000 and toggle web console.

Fill the form and send it. In the console you will see a server response with data you have sent.

Form data

multipart/form-data used for sending files or form-data objects.

Express by default has no ability to parse this type of encoding. So you can use multer's middlewares.

For example try multer().none() middleware for decoding text-only multipart form.

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

/** Require multer */
const multer = require('multer');

/** Show page with a form with a specific enctype */
app.get('/', (req, res, next) => {
  res.send(`<form method="POST" action="/" enctype="multipart/form-data">
  <input type="text" name="username" placeholder="username">
  <input type="submit">
</form>`);
});

/** Process POST request with a mutter's middleware */
app.post('/', multer().none(), function (req, res, next) {
  res.send(JSON.stringify(req.body));
});

/** Run the app */
app.listen(3000);

Example page looks the same as a first one.