javascript写的表单验证，当提示比如“请输入用户名”时，怎么让他自动定位到用户名表单

javascript写的表单验证，当提示比如“请输入用户名”时，怎么让他自动定位到用户名表单
<script>
    layui.use(['laydate', 'form', 'layer', 'table', 'element'], function() {
      layer = layui.layer; //弹层        $("#loginBtn").click(function () {            var username = $("#username").val();
            var password = $("#password").val();
            //var varification = $("#captcha").val();
            username = $.trim(username);
            password = $.trim(password);
            //varification = $.trim(varification);            if (username.length < 1) {                layer.msg('请输入正确格式的用户名');
                return false;            }
            else if (password.length < 6) {
                layer.msg('请输入正确格式的密码');
                return false;            }
            else //if (varification == '') {
                //layer.msg('请输入正确格式的验证码');
                //return false;            //}            $.ajax({
                type:'post',
                url:'user_Login_checklogin.html',
                data: { username: username, password: password},
                dataType:'json',
                success:function(result){
                    if(result['status'] == 0){
                        layer.msg(result['info']);
                        if(result['info'] == '验证码输入有误！') {
                            $('#captcha').val('');
                            $('#captcha').focus();
                        }
                    } else {
                        layer.msg('登录成功，正在跳转到管理中心...    ');
                        setTimeout(function() {
                            window.location.href = "<{$siteurl}>" + "user_Index_index.html";
                        },3000 );
                    }
                }
            })
        })
        $(document).keyup(function(event){
            if(event.keyCode ==13){
                $("#loginBtn").trigger("click");
            }
        });
    });
    function reimg() {
        var img = document.getElementById("vercodeImg");
        img.src = "./agent_Login_verifycode.html?csrf_token="+csrf_token+"&rnd=" + Math.random();
    }
</script>