simple-ajax.js

function encodeAsUrl_(params){
	return Object.keys(params).map(key => key + '=' + params[key]).join('&')
}

function handleResponce_(xhr, cb, errcb){
    return function () {
        if (4 === xhr.readyState) {
            if(xhr.status >= 200 && xhr.status < 400){
                cb(xhr);
            } else {
                errcb(xhr);
            }
        }
    }
}

function get(url, cb, errcb) {
    var xhr = new XMLHttpRequest();
    xhr.open('GET', url, true);
    xhr.onreadystatechange = handleResponce_(xhr, cb, errcb);
    xhr.send();
}

function post(url, params, cb, errcb){
	var xhr = new XMLHttpRequest();
	xhr.open('POST', url, true);
	xhr.onreadystatechange = handleResponce_(xhr, cb, errcb);
	xhr.setRequestHeader("Content-type", "application/x-www-form-urlencoded");
	xhr.send(encodeAsUrl(params));
}