how to send json via post from javascript and read in python wsgi

I have a use case where is need to post a request from my javascript to python.wsgi, I want to send request data as json.

in my json the code looks like :

var aclient = HttpClient(); 
var pay_load = {what:"USERS", uname:username, password:pwd, index:"0"};
setTimeout(aClient.get("POST",alluserUrl, getAllUsersCallBack, JSON.stringify(pay_load)), 50);

where aclient.get is :

var HttpClient = function()
{
this.get = function(aMethod, aUrl, aCallback, payload) {
var anHttpRequest = new XMLHttpRequest();

    anHttpRequest.onreadystatechange = function() {
        if (anHttpRequest.readyState == 4 && anHttpRequest.status == 200)
        {
            //console.log("triggering callback");
            aCallback(anHttpRequest.responseText);
            //return anHttpRequest.responseText;
        }
        else
        {
            //console.log("not 200");
            //console.log(anHttpRequest.status);
            //aCallback(sample);
            //console.log(sample);
        }

    };

    if ("GET" == aMethod)
    {
        anHttpRequest.open("GET", aUrl, true);
        anHttpRequest.setRequestHeader('content-type', 'application/json');
        anHttpRequest.setRequestHeader('Access-Control-Allow-Headers', '*');
        anHttpRequest.setRequestHeader('Access-Control-Allow-Origin', '*');
        anHttpRequest.send(payload);
    }
    else if ("POST" == aMethod)
    {
        anHttpRequest.open("POST", aUrl , true);
        anHttpRequest.setRequestHeader("content-type", "application/json");
        anHttpRequest.setRequestHeader('Access-Control-Allow-Headers', '*');
        anHttpRequest.setRequestHeader('Access-Control-Allow-Origin', '*');
        anHttpRequest.send(payload);
    }
    };
   };

in python wsgi I am trying to decode the post data as :

def handlePost(environ, start_response):
    request_body_size = int(environ.get('Content-Length', 0))
    request_body = environ['wsgi.input'].read(request_body_size)
    data = json.loads(request_body)     
    what = data['what']

but it gives me error saying failed to load json.