方法一：
使用css的required属性
<input type="" required="required" name="" id="" value="" />
方法二：
使用JS代码示例，注意事项：form要加上onSubmit事件，form.xx.vlaue要在表单中对应name
<script type="text/javascript">function beforeSubmit(form){if(form.username.value==‘‘){alert(‘用户名不能为空！‘);form.username.focus();return false;}if(form.password.value==‘‘){alert(‘密码不能为空！‘);form.password.focus();return false;}if(form.password.value.length<6){alert(‘密码至少为6位，请重新输入！‘);form.password.focus();return false;}if(form.password.value!=form.password2.value) {alert(‘你两次输入的密码不一致，请重新输入！‘);form.password2.focus();return false;}return true;}</script><fieldset> ??<legend>用户注册</legend> ???<form method="post" name="form" action="user.do?method=register" onSubmit="return beforeSubmit(this);"> ????<table border="1" width="100%" cellspacing="0" cellpadding="0"> ?????<tr><td><label>用户名：<input type="text" name="username" value=""></label></td></tr> ?????<tr><td><label>密 ??码：<input type="password" name="password" value=""></label></td></tr> ?????<tr><td><label>重复密码：<input type="password" name="password2" value=""></label></td></tr> ?????<tr><td><input value="注册" type="submit"> <input type="reset" value="重置"></td></tr> ??????????</table> ???</form></fieldset>方法三：
使用jQuery方法，需要引用jquery.min.js
<form action="" id="form"> ??<table align="center"> ?????????<tr> ???????????????<td>名称：</td> ???????????????<td> ?????????????????????<input type="text" name="username" title="名称"> ????????????????????</td> ?????????</tr> ?????????<tr> ???????????????<td>咨询内容：</td> ???????????????<td> ?????????????????????<textarea rows="8" cols="45" title="咨询内容"></textarea> ???????????????</td> ?????????</tr> ?????????<tr> ???????????????<td align="center" colspan="2"> ?????????????????????<input type="button" value="提 交" onclick="check()"> ???????????????</td> ?????????</tr> ??</table></form>function check(){ ???????????var checkform = document.getElementById("form"); ??//获得form表单对象 ???????????for(var i=0;i<form.length;i++){ ??????????????????//循环form表单 ?????????????????if(checkform.elements[i].value==""){ ???????//判断每一个元素是否为空 ?????????????????alert(checkform.elements[i].title+"不能为空！"); ?????????????????checkform.elements[i].focus(); ?????????????//元素获得焦点 ?????????????????return ; ?????????????????} ???????????} ???????????checkform.submit(); ?????}检测提交表单内容不能为空JS/jQuery代码（基础）
原文地址：https://www.cnblogs.com/colinliu/p/checkform_null.html