JavaScript - NodeJS - HTTP - req.headers vs. req.getHeaders()

Why does this work:

const http = require('http');
http.createServer((req, res) => {
  res.write(JSON.stringify(req.headers)); // req.headers ✔️
  res.end();
}).listen(3000);

but this doesn't:

const http = require('http');
http.createServer((req, res) => {
  res.write(JSON.stringify(req.getHeaders())); // req.getHeaders() ❌
  res.end();
}).listen(3000);

..producing:

$ node script.js
<go to: http://localhost:3000/>
[...]
TypeError: req.getHeaders is not a function

..when the Node.js documentation (16.x doc is identical), even under the section titled "message.headers", clearly demonstrates the use of:

console.log(request.getHeaders());