js 浮点数四舍五入保留小数后指定位数
  precision保留位数
  function abandonNum(num,precision){
   
          var source_precision=parseInt(precision);
  var sourcenum=num.toString();
    if(sourcenum.indexOf(".") != -1){
                  if(sourcenum.split(".")[1].length >source_precision ){
                     sourcenum = parseFloat(sourcenum).toFixed(source_precision);
return sourcenum;
                  }
     } else{
   return sourcenum;
     }
     return  sourcenum;
}