'undefined' printing after javascript function call

My script is loading some data from an XML file and printing a table with it.

    function draw_schedule() {
    if (window.XMLHttpRequest) {// code for IE7+, Firefox, Chrome, Opera, Safari
      xmlhttp=new XMLHttpRequest();
    }
    else {// code for IE6, IE5
      xmlhttp=new ActiveXObject("Microsoft.XMLHTTP");
    }
    xmlhttp.open("GET","schedule.xml",false);
    xmlhttp.send();
    xmlDoc=xmlhttp.responseXML;

    document.write("<table width='100%' border='1'>");
    var x=xmlDoc.getElementsByTagName("day");
    for (i=0;i<x.length;i++) {// number of days
      document.write("<tr><th colspan='2'>");
      document.write(x[i].getElementsByTagName("date")[0].childNodes[0].nodeValue);// the date for each day
      document.write("</th></tr>");
      var y=x[i].getElementsByTagName("session");// daily sessions
      for (j=0;j<y.length;j++) {
            docu开发者_运维问答ment.write("<tr><td>");
            document.write(x[i].getElementsByTagName("title")[j].childNodes[0].nodeValue);
            document.write("</td><td>");
            document.write(x[i].getElementsByTagName("time")[j].childNodes[0].nodeValue);
            document.write("</td></tr>");
    }
  }
    document.write("</table>");
}

If I call the function (separate file) from the HTML file, it prints the table and afterward prints 'undefined'. If I alternatively embed the script in the HTML, it prints the table without printing 'undefined'. I can't figure-out why having the script in a separate file would change its behaviour. I would love for someone with more wisdom than I to explain. Thanks!

---

draw_schedule() returns no value (undefined). You're possibly calling the function with document.write:

document.write(draw_schedule());

draw_schedule() returns undefined in this case, and the result would look like this:

document.write(undefined);

---

Do your document.write inside this:

xmlhttp.open("GET", "schedule.xml", false);
xmlhttp.onreadystatechange = function() {
    if(xmlhttp.readyState == 4) {
    if(xmlhttp.status == 200) {
    var result = xmlhttp.responseXML;
            // do your document.write
        }
    }
}