// JS 自带的 toLocaleString
function formatNumber(num) {
  return Number(num).toLocaleString()
}
console.log(formatNumber(123456789.123)) // 123,456,789.123

// 正则表达式
function formatNumber(num) {
  return num.toString().replace(/\d+/, function (n) {
    return n.replace(/(\d)(?=(?:\d{3})+$)/g, '$1,')
  })
}
console.log(formatNumber(123456789.123)) // 123,456,789.123

// 字符串递归方法
function formatNumber(num, chart=',', length=3) {
  let result = ''
  let nums = num.toString().split('.')
  let int = nums[0]
  let decmial = nums[1] ? '.' + nums[1] : ''
  let index = 0
  for (let n = int.length - 1; n >= 0; n--) {
    index ++
    result = int[n] + result
	if (index % length === 0 && n !== 0) { result = chart + result }
  }
  return result + decmial
}
console.log(formatNumber(123456789.123)) // 123,456,789.123

// slice 截取分割
function formatNumber(num, char=',', length=3) {
  let result = ''
  let nums = num.toString().split('.')
  let int = nums[0];
  let decmial = nums[1] ? '.' + nums[1] : ''
  while (int.length > length) {
    result = char + int.slice(-length) + result
    int = int.slice(0, int.length - length)
  }
  if (int) { result = int + result }
  return result + decmial
}
console.log(formatNumber(123456789.123)) // 123,456,789.123