try throw catch 语句检测input值

        <form action="">
            <div>请输入0-20的数字</div>
            <input type="text" id="npt">
            <span id="btn" onclick=" result()">检测</span>
            <p></p>
        </form>
    <script>
        function result() {
            let btn = document.getElementById('btn');
            let mes = document.getElementsByTagName('p')[0];
            mes.innerHTML = "";
            mes.style.color = 'red';
            let npt = document.getElementById('npt').value;
            try {
                if (npt == '') throw "不能为空";
                if (npt > 20) throw "不能大于20";
                if (isNaN(npt)) throw "不是数字";
                if (npt < 0) throw "不能小于0"
            }
            catch (err) {
                mes.innerHTML = "输入的值" + err;
            }
        }
        
        //throw语句检测输入变量的值。如果值是错误的，会抛出一个异常（err）;
        //异常 (err) 通过 catch 语句捕获并自定义输出错误信息：
        err是throw抛出的信息
    </script>