How do I consume the JSON POST data in an Express application

I think you're conflating the use of the response object with that of the request.

The response object is for sending the HTTP response back to the calling client, whereas you are wanting to access the body of the request. See this answer which provides some guidance.

If you are using valid JSON and are POSTing it with Content-Type: application/json, then you can use the bodyParser middleware to parse the request body and place the result in request.body of your route.

Update for Express 4.16+

Starting with release 4.16.0, a new express.json() middleware is available.

var express = require('express');

var app = express();

app.use(express.json());

app.post('/', function(request, response){
  console.log(request.body);      // your JSON
   response.send(request.body);    // echo the result back
});

app.listen(3000);

Updated for Express 4.0 - 4.15

Body parser was split out into it's own npm package after v4, requires a separate install npm install body-parser

var express = require('express')
  , bodyParser = require('body-parser');

var app = express();

app.use(bodyParser.json());

app.post('/', function(request, response){
  console.log(request.body);      // your JSON
   response.send(request.body);    // echo the result back
});

app.listen(3000);

For earlier versions of Express (< 4)

var express = require('express')
  , app = express.createServer();

app.use(express.bodyParser());

app.post('/', function(request, response){
  console.log(request.body);      // your JSON
  response.send(request.body);    // echo the result back
});

app.listen(3000);

Test along the lines of:

$ curl -d '{"MyKey":"My Value"}' -H "Content-Type: application/json" http://127.0.0.1:3000/
{"MyKey":"My Value"}

@Daniel Thompson mentions that he had forgotten to add {"Content-Type": "application/json"} in the request. He was able to change the request, however, changing requests is not always possible (we are working on the server here).

In my case I needed to force content-type: text/plain to be parsed as json.

If you cannot change the content-type of the request, try using the following code:

app.use(express.json({type: '*/*'}));

Instead of using express.json() globally, I prefer to apply it only where needed, for instance in a POST request:

app.post('/mypost', express.json({type: '*/*'}), (req, res) => {
  // echo json
  res.json(req.body);
});

For those getting an empty object in req.body

I had forgotten to set headers: {"Content-Type": "application/json"} in the request. Changing it solved the problem.

For Express v4+

install body-parser from the npm.

$ npm install body-parser

https://www.npmjs.org/package/body-parser#installation

var express    = require('express')
var bodyParser = require('body-parser')

var app = express()

// parse application/json
app.use(bodyParser.json())

app.use(function (req, res, next) {
  console.log(req.body) // populated!
  next()
})