How to post json data over http as a client

From espruino htttp reference:

var options = {
    host: 'example.com', // host name
    port: 80,            // (optional) port, defaults to 80
    path: '/',           // path sent to server
    method: 'GET',       // HTTP command sent to server (must be uppercase 'GET', 'POST', etc)
    headers: { key : value, key : value } // (optional) HTTP headers
  };
require("http").request(options, function(res) {

But where do I attach some JSON data (body/payload) to the POST request ?