I need to validate a textbox as numbers

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ASP.NET

I used a javascript for only type numbers. Textboxname as txtFromDay.
But now I can't type any. Because of script returns false.
U used a alert() method for display the typed data,But It result as blank message. What is the reason and solution.
script as below

function Checkprice(e)
    {
    var ups=document.getElementById('<%=txtFromDay.ClientID%>').value;
                     if (e.keyCode >= 48 & e.keyCode <= 57)
                        {
                        alert(ups);
                           return true;
                        }
                    else
                        {
                        alert(ups);
                        return false;
                        }
    }