let sgToFixed = (num,digit=2) => {
    //num是数值；digit是保留有效数字位数
    let nums=(num||0).toString().split('.');
    return nums[0]+'.'+(nums[1]||'0').padEnd(digit, "0");    
}

sgToFixed(0,'3');  //0.000

sgToFixed(1,'2');  //1.00

sgToFixed(1);  //1.00

sgToFixed(1.1,'2');  //1.10

sgToFixed(1.11);  //1.11