更新ajax send（）内容

I'm trying to receive data from ajax call and then send this data back using ajax.
javascript code:
setInterval (function () { var xmlhttp = new XMLHttpRequest(); var content = "data=1"; xmlhttp.onreadystatechange = function() { if (xmlhttp.readyState == 4 && xmlhttp.status == 200) { content = "data=" + xmlhttp.responseText; alert (xmlhttp.responseText); } } xmlhttp.open("POST" , "execute-page.php" , true); xmlhttp.setRequestHeader("Content-type","application/x-www-form-urlencoded"); xmlhttp.send(content); },5000);
the problem now is that it keeps sending the old content. how can I update content variable with ajax response text?

Move the line var content = "data=1"; out of the function:

var content = "data=1";
setInterval (function ()
    {
      var xmlhttp = new XMLHttpRequest();
      xmlhttp.onreadystatechange = function()
        {
            if (xmlhttp.readyState == 4 && xmlhttp.status == 200)
              {
                content = "data=" + xmlhttp.responseText;
                alert (xmlhttp.responseText);
              }
        }
      xmlhttp.open("POST" , "execute-page.php" , true);
      xmlhttp.setRequestHeader("Content-type","application/x-www-form-urlencoded");
      xmlhttp.send(content);
    },5000);