千位格式化

		function format(num) {
			num += '';
			var arr = num.split('');
			arr = arr.map((item, index) => {
				if ((index + 1) % 3 == 0 && (index + 1) != arr.length) {
					item += ',';
					return item;
				}
				return item;
			});
			var str = arr.join('');
			return str;
		}
		console.log(format(12345678))