Nice example of a post request to another page and the returned data.
xhttp.onreadystatechange = function()
{
if (this.readyState == 4 && this.status == 200)
{
alert(this.responseText);
}
};
xhttp.open("POST", "doworkpage.php", true);
xhttp.setRequestHeader("Content-type", "application/x-www-form-urlencoded");
xhttp.send("parameter1=example¶meter2=moreexample");