在您的Javascript中，您正在通过ID进行检查，但输入字段没有ID。
<input type="text" name="a" id ="a"/><input type="text" name="b" id="b" />
检查输入是否为字符串，
isNAN()如以下方式使用
if(isNaN(a) || isNaN(b)){alert("enter a number");}以下是完整的工作示例
<html><head> <meta http-equiv="Content-Type" content="text/html; charset=UTF-8"> <title>JSP Page</title> <style type="text/css"> *{ font-family: cursive; } </style> <script type="text/javascript"> function validate() { var a = document.getElementById("a"); var b = document.getElementById("b"); var c = document.getElementById("a").value; var d = document.getElementById("b").value; var valid = true; if(a.value.length<=0 || b.value.length<=0) { alert("Don't leave the field empty!"); valid = false; } else{ if(isNaN(c) || isNaN(d)){ alert("Enter a number"); valid = false;} } return valid; }; </script></head><body bgcolor="#CDCDCC" font-family="cursive" font-size="20px;" font-weight="bold"> <h2>Welcome <%=request.getParameter("uname")%>! Enter the numbers and the operation that you want to perform: </h2> <form font-size="75px;" action ="serv" method="get" onsubmit="return validate();" > <hr/> Enter the 1st number: <input type="text" name="a" id="a" /><br/> Enter the 2st number: <input type="text" name="b" id="b" /><br/><br/> <label>Add</label><input type="radio" name="option" value="Add" /><br/> <label>Subtract</label><input type="radio" name="option" value="Subtract"/><br/> <label>Multiply</label><input type="radio" name="option" value="Multiply"/><br/> <label>Divide</label><input type="radio" name="option" value="Divide" /><br/> <input type="submit" value="Submit" /> </form></body>
