====== 数値を3桁でカンマ区切り（number_format） ======
===== Intl.NumberFormat を使う方法 =====
<code>
<script>
let numberFormat = new Intl.NumberFormat('ja')
let price = 123456789;
console.log(numberFormat.format(price));
</script>
</code>
===== 自作関数を使う方法 =====
<code>
<script>
let price = 123456789;
console.log(number_format(price));

function number_format(num) {
  return num.toString().replace(/([0-9]+?)(?=(?:[0-9]{3})+$)/g, '$1,');
}
</script>
</code>