<!DOCTYPE html>
<html>
<head>
<meta name="viewport" content="width=device-width, initial-scale=1.0"> <!-- mobile-friendly -->
</head>

<!--
  John Kerl 2015-01-02 with content put together from various google searches. First AJAX program.
-->
<!--
<body OnLoad="document.myarea.focus();">
<body OnLoad="document.n.focus();">
-->
<body>

<div id="myarea">
<script>
    //document.getElementById("n").focus();
    window.onload = function() {
        document.getElementById("myText").focus();
    };

    var oReq = null;
    function submit() {
        var nElement = document.getElementById("n");
        var n = nElement.value;
        //alert("n="+797);
        //alert("n="+n);
        oReq = new XMLHttpRequest(); //New request object

        var url = "factorize_inner_raw.php?n="+n;
        console.log("url="+url);
        oReq.open("POST", url, false); // 3rd arg == false: blocking
        //oReq.send();
        oReq.onreadystatechange = OnStateChange;
        //oReq.send(null);
        oReq.send();
    }
    function reqListener () {
      console.log(this.responseText);
    }

    function OnStateChange () {
        if (oReq != null) {
            if (oReq.readyState == 4) {
                var para = document.createElement("p");
                var node = null;
                if (IsRequestSuccessful (oReq)) {
                    node = document.createTextNode(this.responseText);
                }
                else {
                    //alert ("Operation failed.");
                    node = document.createTextNode("Operation failed.");
                }
                para.appendChild(node);
                var element = document.getElementById("myarea");
                element.appendChild(para);
            }
        }
    }
    // returns whether the HTTP request was successful
    function IsRequestSuccessful(httpRequest) {
        // IE: sometimes 1223 instead of 204
        var success = (httpRequest.status == 0 || 
            (httpRequest.status >= 200 && httpRequest.status < 300) || 
            httpRequest.status == 304 || httpRequest.status == 1223);
        return success;
    }

    function checkEnter(e){ //e is event object passed from function invocation
        var characterCode

        if (e && e.which) { // if 'which' property of event object is supported (NN4)
            e = e
            characterCode = e.which
        }
        else {
            e = event
            characterCode = e.keyCode
        }

        if (characterCode == 13) { //if generated character code is equal to ascii 13 (if enter key)
            submit()
            return false 
        }
        else {
            return true 
        }
    }
</script>

Integer:
<input id="n" type="text" name="n" onKeyPress="checkEnter(event)"/>
<button onclick="submit();" href="javascript:;">Submit</button>
</div>

</body>
</html>