html代碼

 <form id="myForm" method="post" action="" onsubmit="return check()">
     <input id="num" type="number" onkeyup="this.value=this.value.replace(/\D/g,‘‘)"
                   onafterpaste="this.value=this.value.replace(/\D/g,‘‘)" >
    <input type="submit" id="submit" value="提交">
</form>

JS代碼

<script>
    function check() {
        var num= document.getElementById("num").value;
        if (num== null || num === ‘‘) {
            alert("請填寫數字");
            return false;
        }
        return true;
    }
</script>

提交form前使用JS先進行驗證