onkeydown="return isNumberKey(event,this)"

function isNumberKey(evt, obj)
{

    var charCode = (evt.which) ? evt.which : evt.keyCode
    if (charCode == 8 || charCode == 46) return false;

    return true;
}

function onkeyup(e) {
    var code;
    if (!e) var e = window.event;
    if (e.keyCode) code = e.keyCode;
    else if (e.which) code = e.which;

    if (code == 8 || code == 46)
        return false;
}

<input onkeyup="return onkeyup()" />

function checkKey(e) {
    e = e || event;
    if (!e) {return true;}
    var code = e.keyCode || e.which || null;
    if (code) {
      return (code === 8 || code === 46) ? false : true;
    }
    return true;
}

<input onkeyup="return checkKey(event)" />