JS判断输入的字符串是否为数字

onkeydown="javascript:return checkNumber(event)"
function checkNumber(e) {
    var num = document.getElementById("tbNumber");
    if ((e.keyCode >= 48 && e.keyCode <= 57) || (e.keyCode >= 96 && e.keyCode <= 105)) {
        return true;
    } 
    return false;
}