SOURCE

// 按指定位数截取小数部分，不四舍五入
const DEFAULT_DECIMAL_NUMBER = 3
const roundNumber = function(val, num=DEFAULT_DECIMAL_NUMBER){
  if(typeof val != 'number'){
    return val
  }
  let demencialLength = val.toString().split(".")[1].length || 0
  
  if(demencialLength <= num){
    return val
  }
  const format = Math.pow(10,num)
  const newValue = Math.round(val * format) / format
  return newValue
}