js中防止输入为空，或者为字母

function checkNum(){

    var num1=document.getElementById("num1").value;
    var num2=document.getElementById("num2").value;
    if(num1 == "" || num2== "" ){
        window.alert("not null");
        return false;
    }

  //不能带gi
    var reg=/^[1-9]\d*(\.[0-9]+)?$|^0$/i;
    if(!reg.test(num1) || !reg.test(num2)){
        window.alert("input numbers only");
        return false;
    }
}