支持整数和浮点数

function toThousands(num) {
  num = (num || 0).toString();
  let number = 0,
      floatNum = '',
      intNum = '';
  // 判断是否有小数位，有则截取小数点后的数字
  if (num.indexOf('.') > 0) {
    number = num.indexOf('.'); // 获取小数点出现的位置
    floatNum = num.substr(number); // 截取arr.substr(form, length)
    intNum = num.substring(0,number); // 截取arr.substring(start, end)
  } else {
    intNum = num;
  }
  let result = [],
      counter = 0;
  intNum = intNum.split('');
  // 利用3的倍数，向数组插入','
  for (let i = intNum.length - 1; i >= 0; i--) {
    counter++;
    result.unshift(intNum[i]);
    if (!(counter % 3) && i != 0) { result.unshift(','); }
  }
  return result.join('') + floatNum || '';
}

const A1 = 1234567890122;
const A2 = 1234567890122.122;
console.log(toThousands(A1)); //1,234,567,890,122
console.log(toThousands(A2)); // 1,234,567,890,122.122

• toLocaleString

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

• 正则

  // 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
  

• 递归

  // 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

  // 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
  

数字每三位加逗号/三位分节号

	function numberFormat(str) {

		/**
		* a123 => null
		* "" => null
		*/
		if (isNaN(str) || !str) {
			return null;
		}

		/**
		* 00123 => 123
		* 123. => 123
		* 123.1230 => 123.123
		*/
		var s = (str + '').replace(/(^0+)|(\.+$)|(0+$)/g, "");

		var regExp = new RegExp('^(-?[0-9]+)([0-9]{3})');
		while(regExp.test(s)) {
			s = s.replace(regExp, '$1,$2');
		}

		return s;
	}

	console.log(numberFormat(""));
	//null

	console.log(numberFormat("a00012345"));
	//null

	console.log(numberFormat("00012345"));
	//12,345

	console.log(numberFormat("00012345."));
	//12,345

	console.log(numberFormat("00012345.123"));
	//12,345.123

	console.log(numberFormat("00012345678910.123000"));
	//12,345,678,910.123
	

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