4.7

function validateForm() { var x=document.forms["myForm"]["fname"].value; if (x==null || x=="") { alert("姓必须填写"); return false; } }

实例：

<form name="myForm" action="demo-form.php" onsubmit="return validateForm()" method="post"> 姓: <input type="text" name="fname"> <input type="submit" value="提交"> </form>