I had a hard time finding the documentation about the Kodi or XBMC JSONRPC API and how to make requests from a browser without running into CORS header problems. Kodi luckily supports JSONP-style requests. So if you can only make GET requests and want to circumvent CORS restrictions, use this JSONP snippet.

kodi-jsonp.js

// npm i --save jsonp
import jsonp from 'jsonp';

var host = 'http://192.168.1.140';

var data = {
    'jsonrpc': '2.0',
    'method': 'Application.GetProperties',
    'params': '[["volume","muted","version"]]',
    'id': Math.ceil(Math.random() * 10000)
};

// JSONP GET scheme: /jsonrpc?request=<encodedRequestObject>&jsonp=<jsonpCallbackName>
jsonp(
    host + '/jsonrpc?request=' + encodeURIComponent(JSON.stringify(data)),
    {},
    function(error, data) {
        console.log(error, data);
    }
);

Check if party mode is active

http://kodi.my.home/jsonrpc?request=%7B%22jsonrpc%22%3A%222.0%22%2C%22method%22%3A%22Player.GetProperties%22%2C%22params%22%3A%5B0%2C%5B%22partymode%22%5D%5D%2C%22id%22%3A127%7D

@mattmattmatt
My god, thank you for posting this up!

I was completely stuck trying to get some information from my local Kodi box just for browser React project I'm messing around on. Kept getting CORS issues, found that the CORS fix never got pulled into the Kodi master and then was totally stuck.

Thank you.