JavaScript Error Event definition

Good afternoon, I made a popup with jquery, and when I put it to run on element inspecte it shows an error to the following:

  

events: 1357 Uncaught ReferenceError: open_popup is not defined

I took a look at it is talk about definition, so I have no idea what this is and how to do it.

code:

<div id="banner_popup" style="display:none">
    <a href="javascript: void(0);" onclick="fecha_banner();" class="linksFechar">
        <img class="imgfechar" src="~/Images/Popup/fechar.png" border="0">
    </a>
    <a href="@Url.Action("TornarPagina", "Home")">
        <div class=" info">
        <h1><span class="linha1">Faça o </span></br><span class="linha2a"> ABC </span><span class="linha2b">do</span><span class="linha2c"> ABC </span></br><span class="linha3">sua página Inicial!</span></h1>
        <div class="button">
            Clique Aqui!
        </div>
</div>
    </a>
</div>

<script language="JavaScript">
        if(document.getElementById('banner_popup'))
            abre_popup();
</script>

<script>
    $(document).ready(function () {

        var banner = sessionStorage.getItem('banner_popup');
        if (banner != "1") {
            abre_popup();
            sessionStorage.setItem('banner_popup', "1");
        }
    });

    function fecha_banner_timeout() {
        setTimeout(function () {
            fecha_banner()
        }, 10000);
    }

    function abre_popup() {
        var banner_obj = document.getElementById('banner_popup');

        banner_obj.style.left = '';
        banner_obj.style.top = '';  

        banner_obj.style.display = '';

        fecha_banner_timeout();
    }

    function fecha_banner() {
        var banner_obj = document.getElementById('banner_popup');
        banner_obj.style.display = 'none';
    }
</script>