</select>
     =
     <input type="text" readonly>
    <br>
    <button onclick="calculate();"> 計算 </button>
</div>

<script>
    function calculate() {
        const input = document.querySelectorAll('input');
       // console.log(input[0].value);
        const 華氏 = Number(input[0].value);
        const 攝氏 = Number(input[1].value);
        const select = document.querySelector('select').value;
        // console.log(select);
        let result;
        if (select === '+') {
            result =  Number * (9 / 5) + 32;
        } else if (select === '-') {
            result = (Number - 32) * 5 / 9;
        } 
        
    }
</script>