// 感觉你的需求就是去0，使用parseFloat就行
parseFloat(num) + '%'

var num =12.10;
if(num%1==0){
    num =~~num;
}else{
if(num%10==0){
num =~~num+'.'+num/10;
}
}
num;

Math.round(num*100)/100 + '%'

小数点第三位开始会四舍五入

var testArr = ['11.0000', '57,12', '12.00', '12.10', '12.01'];
testArr.forEach(val => console.log(val.replace(/(\.?0*$|$)/, '%')))

function changeToPercent(num){
 if(!/\d+\.?\d+/.test(num)){
  alert("必须为数字");
 }
 var result = (num * 100).toString(),
  index = result.indexOf(".");
 if(index == -1 || result.substr(index+1).length <= 2){
  return result + "%";
 }
 return result.substr(0, index + 3) + "%";
}