try throw catch 語句檢測input值

        <form action="">
            <div>請輸入0-20的數字</div>
            <input type="text" id="npt">
            <span id="btn" onclick=" result()">檢測</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 "不能為空";
                if (npt > 20) throw "不能大於20";
                if (isNaN(npt)) throw "不是數字";
                if (npt < 0) throw "不能小於0"
            }
            catch (err) {
                mes.innerHTML = "輸入的值" + err;
            }
        }
        
        //throw語句檢測輸入變數的值。如果值是錯誤的，會丟擲一個異常（err）;
        //異常 (err) 通過 catch 語句捕獲並自定義輸出錯誤資訊：
        err是throw丟擲的資訊
    </script>
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