表单提交时，只提交了密码，而没有提交用户名是怎么回事呢？

HTML文件中相关代码

<script>

            $('#login').click(function(){

                $.ajax({

                    url:"/login",

                    data:{

                        username:$('#username').val(),

                        password:$('#password').val()

                    },

                    dataType:"json",

                    success(res){

                        if(res.err){

                        alert(res.msg)

                        }else{

                            alert("登陆成功")

                       }

                    }

                })

            })

        </script>