HTML
<!DOCTYPE html>
<html lang="en">
<head>
<meta charset="UTF-8">
<title>验证表单</title>
</head>
<body>
<form action="" name="main" onsubmit="return show()">
<label for="">账号:</label><input type="text" placeholder="请输入账号" name="nav_main">
<button type="submit">提交</button>
</form>
<script src="index.js"></script> //js代码文件
</body>
</html>Javascript
function show(){
var x=document.forms["main"]["nav_main"].value;
if(x==null||x==""){
alert("请输入账号");
}
}