请实现一个js脚本，要求做到将数字转化为千分位表示如：1234567转化为1,234,567

//法一

function parseNum(num){

var list = new String(num).split('').reverse();

for(var i = 0; i < list.length; i++){

if(i % 4 == 3){

list.splice(i, 0, ',');

}

}

return list.reverse().join('');

}

console.log(parseNum(10000121213));

//法二

function parseNum(num){

var reg=/(?=(?!\b)(\d{3})+$)/g;

return String(num).replace(reg, ',');

}

console.log(parseNum(10000121213));

//法三

String.prototype.strReverse = function(){

return this.split('').reverse().join('');

}

function parseNum(num){

var str_num = String(num);

var len = str_num.length;

var tail = str_num.slice(0, len % 3);

tail = tail.strReverse();

var reg=/\d{3}/g;

var list = str_num.strReverse().match(reg);

list.push(tail);

var res = list.join(',').strReverse();

return res;

}

console.log(parseNum(10000121213));

//法四

function parseNum(num){

var list = String(num).split('').reverse();

var temp = [];

for(var i = 0, len = list.length; i < len; i = i + 3){

temp.push(list.slice(i, i + 3).join(''));

}

return temp.join(',').split('').reverse().join('');

}

console.log(parseNum(10000121213));