js数字每三位加逗号代码封装

function toThousands(num) {
     var result = [ ], counter = 0;
     num = (num || 0).toString().split('');
     for (var i = num.length - 1; i >= 0; i--) {
         counter++;
         result.unshift(num[i]);
         if (!(counter % 3) && i != 0) { result.unshift(','); }
     }
     return result.join('');
 }
//输出
console.log(toThousands(9992019))
//结果 9,992,019