AJAX on pure JS

ajax.js

function getXmlHttp(){
    var xmlhttp;
    try {
        xmlhttp = new ActiveXObject("Msxml2.XMLHTTP");
    } catch (e) {
        try {
            xmlhttp = new ActiveXObject("Microsoft.XMLHTTP");
        } catch (E) {
            xmlhttp = false;
        }
    }
    if (!xmlhttp && typeof XMLHttpRequest!='undefined') {
        xmlhttp = new XMLHttpRequest();
    }
    return xmlhttp;
}

function doAjax(url, method, params) {
    var ajax = getXmlHttp();
    ajax.open(method, url, true);
    ajax.setRequestHeader("Content-type", "application/json; charset=utf-8");
    ajax.onreadystatechange = function() {
        if (ajax.readyState == 4) {
            if(ajax.status == 200) {
                console.log( ajax.responseText );
                // if JSON in responce
                // console.log( JSON.parse( ajax.responseText ) );
            } else {
                alert('Ошибка связи с сервером');
            }
        }
    };
    ajax.send(params);
}

// example to use
var url = "path/to/backend/handler",
    method = "POST",
    params = {
        "param1": 'value1',    
        "param2": 'value2'
    }
doAjax(url, method, params);