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 ?

It's passed at the end of the request like this:

var options = {
...
}
var data = JSON.stringify({"key":"value", ...});
require("http").request(options, function(res)  {
   ...
}).end(data);

Make sure to add the appropriate headers when sending JSON data:

var data = JSON.stringify({ key: "value" });

const options = {
    //...
    headers: {
      "Content-Type": "application/json",
      "Content-Length": json.length
    }
};

require("http").request(options, function(res)  {
   //...
}).end(data);

I recommend attaching an error handler :)

const req = require("http").request(/* ... */);
req.on("error", err => console.log("Request error: " + err.message)) ;
req.end(data);

Thanks - I'll make a note to add this to: http://www.espruino.com/Internet

Just in case there's any interest in going the other way (handling POSTed data as a server) that one does have some example code here: http://www.espruino.com/Posting+Forms