Dopostback on Key Press using javascript

<script type="text/javascript">
document.onkeyup = KeyCheck;
function KeyCheck(e) {
    var KeyID = (window.event) ? event.keyCode : e.keyCode; 

//alert("KeyId="+KeyID);

switch (KeyID) {
        case 118:
            __doPostBack('__Page', 'F7');
            break;
        case 119:
            __doPostBack('__Page', 'F8');
            break;
        case 120:
            __doPostBack('__Page', 'F9');
            break;
        case 121:
            __doPostBack('__Page', 'F10');
            break;
    }
}
</script>

Code Behind write:

protected void Page_Load(object sender, EventArgs e)
    {
        Page.ClientScript.GetPostBackEventReference(this, "");
        string eventArgs = Request["__EVENTARGUMENT"]; if (!string.IsNullOrEmpty(eventArgs))
        {
            switch (eventArgs)
            {
                case "F7":
                    a("F7");
                    break;
                case "F8":
                    a("F8");
                    break;
                case "F9":
                    a("F9");
                    break;
                case "F10":
                    a("F10");
                    break;
            }
        }
    }

   public void a(string str)
    {
         Response.Write("You pressed : " + str);
    }