<form action="" method="post" onsubmit="return checkname()">

<input type="text" class="form-control" id="exampleInputEmail1" name="username" placeholder="请输入你的名字">

<button type="submit" class="btn-primary" >我要留言</button>

</form>

<script>

　　function checkname(){

　　var username=document.querySelector(".form-control").value;

　　if(username==""||username==null){alert('请输入用户名');return false;}

}

</script>