Ajax در جاواسکریپت

کدی که نوشتم

function myf(){

    var input = document.getElementById('input');

    document.getElementById('show').innerHTML = input;

};

function myFunction() {
  // input, filter, ul, li, a, i, txtValue;
  var input = document.getElementById("");
  if ( window.XMLHttpRequest) {
    var request = new XMLHttpRequest();
    request.onreadystatechange = function () {
      if (xmlhttp.readyState == 4 && xmlhttp.status == 200) {
        document.getElementById("mainp").innerHTML = xmlhttp.responseText;
      }
    };
    request.open("GET", "" + str, true);
    request.send();
  }
}