Here is how you can extract the data that was sent as JSON in the request body.

If you are using Express, that’s quite simple: use the body-parser Node module.

For example, to get the body of this request:

const axios = require('axios')

axios.post('https://flaviocopes.com/todos', {
  todo: 'Buy the milk'
})

This is the matching server-side code:

const bodyParser = require('body-parser')

app.use(bodyParser.urlencoded({
  extended: true
}))

app.use(bodyParser.json())

app.post('/endpoint', (req, res) => {
  console.log(req.body.todo)
})

If you’re not using Express and you want to do this in vanilla Node, you need to do a bit more work, of course, as Express abstracts a lot of this for you.

The key thing to understand is that when you initialize the HTTP server using http.createServer(), the callback is called when the server got all the HTTP headers, but not the request body.

The request object passed in the connection callback is a stream.

So, we must listen for the body content to be processed, and it’s processed in chunks.

We first get the data by listening to the stream data events, and when the data ends, the stream end event is called, once:

const server = http.createServer((req, res) => {
  // we can access HTTP headers
  req.on('data', chunk => {
    console.log(`Data chunk available: ${chunk}`)
  });
  req.on('end', () => {
    //end of data
  })
})

So to access the data, assuming we expect to receive a string, we must put it into an array:

const server = http.createServer((req, res) => {
  let data = []
  req.on('data', chunk => {
    data.push(chunk)
  });
  req.on('end', () => {
    JSON.parse(data).todo // 'Buy the milk'
  })
})