obj { table: "users", limit: 15 };
dbParam = JSON.stringify(obj);
xmlHttp = new XMLHttpRequest();
xmlHttp.onreadystatechange = function() {
  if (this.readyState == 4 && this.status == 200) {
    myObj = JSON.parse(this.responseText);
    myTxt += "<table style='border: 1px solid #000'>"
    for (y in myObj) {
      myTxt += "<tr><td>" + myObj[y].username + "</td></tr>";
    }
    myTxt += "</table>"
    document.getElementById("demo").innerHTML = myTxt;
  }
}
xmlhttp.open("POST", "json_db_post.php", true);
xmlhttp.setRequestHeader("Content-type", "application/x-www-form-urlencoded");
xmlhttp.send("y=" + dbParam);