How to send javascript object with POST XMLHttpRequest

Question

I want to send the following data in POST XMLHttpRequest method.

Data to be sent: var data = {xx:"value",user:{password:"pass",username:"xyz"}};

Code used:

var xmlhttp = new XMLHttpRequest();
var data = {xx:"value",user:{password:"pass",username:"xyz"}};
data = JSON.stringify(data);
xmlhttp.open("POST",Url,true);
xmlhttp.onreadystatechange = function() {
    if(xmlhttp.readyState == 4 && xmlhttp.status == 200) {
        alert(xmlhttp.responseText);
    }
}
xmlhttp.send(data );

Result: Getting "xx" is missing.

Kindly help me to resolve this issue.