'get' works but 'post' do not work

<script>
        var xmlhttp;
        function loadXmlDoc() {
            if (window.XMLHttpRequest) {
                xmlhttp = new XMLHttpRequest();
            }
            else {
                xmlhttp = new ActiveXObject('Microsoft.XMLHTTP');
            }

            xmlhttp.onreadystatechange = function () {
                if (xmlhttp.readyState == 4 && xmlhttp.status == 200)
                {
                    document.getElementById('div').innerHTML = xmlhttp.responseText;
                }
            }

            xmlhttp.open('get', '../TextFile1.txt', true);  //change get to post not work, why????
            xmlhttp.send();
        };
     
    </script>