将数字每千分位用逗号隔开

function format(n){
  let num = n.toString()
  let decimals = ''
  let int = ''
  if(num.includes('.')) {
    [int, decimals] = num.split('.')
  }
  const end = decimals ? '.'+decimals : ''
  if(int.length < 3){
    return n
  }else{
    let left = int.length % 3
    if(left > 0){
      return int.slice(0, left) + ',' + int.slice(left).match(/\d{3}/g).join(',') + end
    }else{
      return int.slice(left).match(/\d{3}/g).join(',') + end
    }
  }
}
format(12345.88)