Js判断是不是数字并保留小数位数

// 判断是不是数字 字符数字也算
function isNum(num){
    let ss =''+num;
    return (ss!==''&&ss!=='null'&&ss!=='Infinity'&&ss!=='-Infinity'&&!isNaN(num));
}
// 有小数就保留 length 位小数；没有就返回原数字
function reserveNum(num,length){
    if(isNum(num)){
        let str = ''+num;
        if(str.includes('.')){
            let decimalNum = str.split('.')[1], integerNum = str.split('.')[0], obj ={1:'0',2:'00',3:'000'};
            decimalNum = decimalNum.length >= length ? decimalNum.substring(0,length) : decimalNum+obj[(length-decimalNum.length)];
            return (integerNum+'.'+decimalNum);
        }else{
            return num;
        }
    }
}