JavaScript: call C# codebehind function

Hi i wanted to know is it possible to call a codebehind function in my Javascript? The javascript is in the head of my asp.net webpage page.

I have tried many methods but none seem to have worked. Its the first time iv had to do this so an example would be a nice learning curve fix.

My codebehind function in C#

protected void GetAverageRent_TextChanged(object sender, EventArgs e)
{
    string Postcode = _postCodeInput.Value.ToString();
    var webGet = new HtmlWeb();
    var doc = webGet.Load("http://www.webadress.com/" + Postcode);


    HtmlNode AvgPrice = doc.DocumentNode.SelectSingleNode("//div[@class='split2r right']//strong[@class='price big']");

    if (AvgPrice != null)
    {
        AverageRentLbl.Text = AvgPrice.InnerHtml.ToString();
    }
    else
    {
        AverageRentLbl.Text = "Invalid Postcode!";
        AverageRentLbl.ForeColor = Color.Red;
        AverageRentLbl.Font.Bold = true;
    }
}

My Javascript

<script>
function codeAddress() {

        document.getElementById('map-canvas').setAttribute("style", "height:200px");

        geocoder = new google.maps.Geocoder();
        var latlng = new google.maps.LatLng(-34.397, 150.644);
        var mapOptions = {
            zoom: 16,
            center: latlng,
            mapTypeId: google.maps.MapTypeId.ROADMAP
        }
        map = new google.maps.Map(document.getElementById('map-canvas'), mapOptions);

        //var address = document.getElementById('_mainContentHolder__postCodeValue').value;
        var address = document.getElementById('ContentPlaceHolder1__postCodeInput').value;

        geocoder.geocode({ 'address': address }, function (results, status) {
            if (status == google.maps.GeocoderStatus.OK) {
                map.setCenter(results[0].geometry.location);
                var marker = new google.maps.Marker({
                    map: map,
                    position: results[0].geometry.location
                });

                document.getElementById('ContentPlaceHolder1__latValue').value = results[0].geometry.location.lat();
                document.getElementById('ContentPlaceHolder1__longValue').value = results[0].geometry.location.lng();
            } else {
                alert('Sorry the postcode you entered in invalid. Please try again: ' + status);
                document.getElementById('ContentPlaceHolder1__postCodeInput').value = "";
            }
        });</Script>