JSFiddle - React, Tailwind, and code Playground

HTML

<select name="_test1" id="sidTest1">
    <option value="">()</option>
<select>

JavaScript

//optionタグの数を取得する
var cnt = $('#sidTest1').children('option').length;

if(cnt == "1"){

        //ここの条件に入ってくるのは確認済み
        alert(cnt);

        document.getElementById('sidTest1').style.display = 'none';
        document.getElementById('sidTest1').style.display = 'block';

        $('#sidTest1').children().remove();

}