ajax.js

var url = "";

var params = {
    name: 'John',
    email: 'webmaster@mysite.com'
};


var xhr = new XMLHttpRequest();
xhr.open('POST', url + '?' + buildQueryString(params));
xhr.responseType = 'json';
xhr.send(null);
xhr.onreadystatechange = function () {
    var DONE = 4; // readyState 4 means the request is done.
    var OK = 200; // status 200 is a successful return.
    if (xhr.readyState === DONE) {
        if (xhr.status === OK) {
            console.log(xhr.response); // 'This is the returned text.'
        } else {
            console.log('Error: ' + xhr.status); // An error occurred during the request.
        }
    }
}

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