Http POST in Ajax using pure Javascript

http-post.js

var url = 'https://udemy.com.br/OpenClass';
var obj = { schoolID: 77 , classId: 3};

var callBackParameter = {};
var callBackFunction = function(ajax, callBackParameter) {
    let objResponse = JSON.parse(ajax.responseText);
    console.log('Response Object', objResponse);
};

var callBackErrorParamater = {};
var callBackErrorFunction = function(ajax, callBackErrorParamater) {
    console.log('Fail', ajax.responseText);
};

function HttpPOST(url, obj, callBackFunction, callBackParameter, callBackErrorFunction, callBackErrorParamater) {
    let json = JSON.stringify(obj);
    let ajax = new XMLHttpRequest();
    ajax.open('POST', url, true);
    ajax.setRequestHeader("Content-type", "application/json");
    ajax.send(json);
    ajax.onload = function () {
        if (ajax.readyState == 4 && (ajax.status == 200 || ajax.status == 201)) {
            if (typeof callBackFunction !== typeof undefined && callBackFunction !== null)
                callBackFunction(ajax, callBackParameter);
        }
        else {
            if (typeof callBackErrorFunction !== typeof undefined && callBackErrorFunction !== null)
                callBackErrorFunction(ajax, callBackErrorParamater);
        }
    };
};