数字格式化

// 数字格式化 splice(-3) 1,234,567



const formatNumber = num => {

  const str = num.toString()
  const res = []
  const strAry = [...str]
  while(strAry.length) {
    res.unshift(',', ...strAry.splice(-3))
  }
  res.shift()
  return res.join('')
  
}

num.toLocaleString()