js保留小数位不四舍五入

@param
@val:{type:Number}
@decimal:所保留的小数位个数
const formatDecimal = (val, decimal) => {
  val = val.toString();
  let index = val.indexOf(".");
  if (index !== -1) {
    val = val.substring(0, decimal + index + 1);
  } else {
    val = val.substring(0);
  }
  return parseFloat(val).toFixed(decimal);
};