Http GET in Ajax using pure Javascript

http-get.js

var schoolID = 77;
var classId = 3;
var url = 'https://udemy.com.br/Cursos?schoolID='+schoolID+'&classId='+classId;

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

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

var FuncHttpGet = function HttpGET(url, callBackFunction, callBackParameter, callBackErrorFunction, callBackErrorParamater) {
    let ajax = new XMLHttpRequest();
    ajax.open("GET", url, true);
    ajax.send();
    ajax.onload = function () {
        if (ajax.readyState == 4 && ajax.status == 200)
        {
            callBackFunction(callBackParameter, ajax); //PUT INSTRUCTION USING THE RESPONSE OBJ HERE
        }
        else
        {
            callBackErrorFunction(callBackErrorParamater, ajax); //PLACE ERROR TREATMENT HERE
        }
    };
};

FuncHttpGet(url, callBackFunction, callBackParameter, callBackErrorFunction, callBackErrorParamater);