给数字每三位加一个逗号分隔符的两种方法

function formatNum(num) {
var STEP = 3;
var DELIMITER = ',';
var str = '' + num;
var arr = str.split('');
var len = arr.length;

for(var i = len - STEP; i > 0; i -= STEP) {
arr.splice(i, 0, DELIMITER);
}

return arr.join('');
});

function formatNum2(num) {
return (num).toString().replace(/\B(?=(\d{3})+(?!\d))/g, ",");
});

formatNum(12345678);  // -> "12,345,678"
formatNum2(12345678); // -> "12,345,678"