js保留n位小数 不四舍五入

var retain=function (num,decimal){
        num = num.toString();
        let index = num.indexOf('.');
                if(index !== -1){
                    num = num.substring(0,decimal + index + 1)
                }else{
                    num = num.substring(0)
                }
                return parseFloat(num).toFixed(decimal) 
    }