Javascript Ajax Cross-browser - Method POST

loadMyAjax.js

var postsList = document.querySelector('.posts_list');
var paged = 1;
var countPosts = postsList.dataset.countPosts;
var postsPerPage = parseInt(postsList.dataset.postsPerPage);

window.addEventListener( 'onInfinite', getPosts );

function hasMoreItens() {
  return postsList.children.length < countPosts;
}

function getPosts() {
  paged++;

  var data = {
    paged: paged
  };

  myAjax({
    url: ApitUrl + 'portfolio.php',
    data: data,
    success: function( response ) {
      postsList.insertAdjacentHTML( 'beforeend', response );

      if ( hasMoreItens() )
        InfiniteScroll.complete();
      else
        InfiniteScroll.stop();
    }
  });
}

myAjax.js

// MY_AJAX_FUNCTION
// ----------------
function myAjax(params) {
  var xhr;
  if (window.XMLHttpRequest) {
    xhr = new XMLHttpRequest(); // code for IE7, firefox chrome and above
  } else {
    xhr = new ActiveXObject("Microsoft.XMLHTTP"); // code for Internet Explorer
  }
  xhr.withCredentials = params.withCredentials || false;
  xhr.open('POST', params.url);
  xhr.setRequestHeader('Content-Type', 'application/x-www-form-urlencoded');

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

  xhr.send(data);

  xhr.addEventListener('readystatechange', function () {
    if (this.readyState === 4 && this.status === 200) {
      params.debug && console.log(this);
      params.success(this.responseText);
    }
  });
}

My Infinite Scroll - Pure Javascript
https://codepen.io/marcelo-ribeiro/pen/EwKKrZ