'use strict';

const http = require('http');

const PORT = 8080;
const server = http.createServer((request, response) => {
  let buffer = '';
  request.on('data', chunk => {
    buffer += chunk;
  });
  request.on('end', () => {
    const responseString = `Received string ${buffer}`;
    console.log(`Responding with: ${responseString}`);
    response.writeHead(200, "Content-Type: text/plain");
    response.end(responseString);
  });
}).listen(PORT, () => {
  console.log(`Listening on ${PORT}`);
});

Remarks

Node.js uses streams (opens new window) to handle incoming data.

Quoting from the docs,

A stream is an abstract interface for working with streaming data in Node.js. The stream module provides a base API that makes it easy to build objects that implement the stream interface.

To handle in request body of a POST request, use the request object, which is a readable stream. Data streams are emitted as data events on the request object.

 request.on('data', chunk => {
    buffer += chunk;
  });
  request.on('end', () => {
    // POST request body is now available as `buffer`
  });

Simply create an empty buffer string and append the buffer data as it received via data events.

NOTE

1. Buffer data received on data events is of type Buffer (opens new window)
2. Create new buffer string to collect buffered data from data events for every request i.e. create buffer string inside the request handler.