js做一个简易计算器

运用js做一个简易的计算器效果，实现加减乘除取余

<input type="text" id="a">
        <select name="" id="fh">
        <option value="+">+</option>
        <option value="-">-</option>
        <option value="*">x</option>
        <option value="/">÷</option>
        <option value="%">%</option>
    </select>
        <input type="text" id="b">
        <input type="button" name="" id="btn" value="=">
        <input type="text" disabled name="" id="c">
    </body>
    <script>
        var a = document.getElementById("a");
        var b = document.getElementById("b");
        var fh = document.getElementById("fh");
        var c = document.getElementById("c");
        var btn=document.getElementById("btn")
        btn.onclick=function(){
            var num1=Number(a.value);
            var num2=Number(b.value);
            add(num1,num2,fh.value);
           
        }
        function add(num1, num2 ,fh){
            var sum=0;
            switch (fh){
                case "+":
                sum=num1+num2;break;
                case "-":
                sum=num1-num2;break;
                case "*":
                sum=num1*num2;break;
                case "/":
                sum=num1/num2;break;
                case "%":
                sum=num1%num2
            }
            c.value=sum
        }
    
    </script>