Javascript getElementById gives error 'null' is null or not an object

Question

I want to call a c# function using javascript, so I added an invisible asp.net button to my Default.aspx, and now I am tring to call it inside javascript so that it will trigger the c# function, but it gives 'null' is null or not an object. I tried to move my javascript part to many places to solve the problem but it didn't work.

<asp:Content ID="HeaderContent" runat="server"
ContentPlaceHolderID="HeadContent">
</asp:Content>

    <asp:Content ID="BodyContent" runat="server" ContentPlaceHolderID="MainContent">
        <h2>        WELCOME     </h2>
     <div style="display: none;">
            <asp:Button ID="button1" runat="server"
             OnClick="btn_SearchLibrary_Click" />
     </div>

        <script type="text/javascript">
            function displaymessage() {
                var button = document.getElementById('button1');
                button.click();
            }
        </script>


<!-- Some codes in here, below there is a gridview templatefield -->



    <asp:TemplateField HeaderText="BookName"
    SortExpression="BookName" ItemStyle-Width="150px">
                                <ItemTemplate>
                                    <asp:HyperLink ID="HyperLink1"
    runat="server" Text='<%# Bind("BookName") %>'
    NavigateUrl="javascript:displaymessage()"></asp:HyperLink>
                                </ItemTemplate>
                            </asp:TemplateField>

Are you sure the rendered page uses the literal id format instead of the old ctl00 prefix format? — Grant Thomas
– Grant Thomas, Commented Mar 12, 2012 at 14:29
I am not sure, but I am using .NET 4.0, so possibly it will not be an old format, am I right — HOY
– HOY, Commented Mar 12, 2012 at 14:30
JavaScript is on the client. c# is on the server. You cannot "call" a c# function from JavaScript - they're on two different machines! — Diodeus - James MacFarlane
– Diodeus - James MacFarlane, Commented Mar 12, 2012 at 14:30
Diodeus, please see the second answer that has 18 up votes in this topic, that is what I am tring to do stackoverflow.com/questions/3713/…, I am calling asp.net function with javascript, asp.net function will be firing the c# function — HOY
– HOY, Commented Mar 12, 2012 at 14:32
assuming the javascript error is client-side error in the browser, it will be better to post the markup returned to the browser as well. — yas
– yas, Commented Mar 12, 2012 at 14:37

DotNetUser · Accepted Answer · 2012-03-12 14:37:18Z

2

try this

     document.getElementById('<%=button1.ClientID%>')

answered Mar 12, 2012 at 14:37

DotNetUser

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Comments

Rob Angelier · Accepted Answer · 2012-03-12 14:35:03Z

2

You should solve this with an Ajax implementation. This way you can call a webservice wich will contain some business logic and sends a response back to the client.

Create a web service using eq. (WCF, ServiceStack)
Create a button
Subscribe to the click event of the button with jQuery and call your webservice.
In the success callback function, do some client side logic like updating your list of books.

Best regards,

Rob

answered Mar 12, 2012 at 14:35

Rob Angelier

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Comments

jbabey · Accepted Answer · 2012-03-12 14:41:20Z

1

While I would highly suggest pursuing an ajax/WCF solution, the problem with your current approach is most likely the ID of the button. ASP controls are usually not rendered with the ID you specify in markup, but prefixed by "ct100" or "container$id" or "container_id", depending on the version of ASP. I would suggest you view source on your page to see the rendered ID and make sure that you are passing that to your document.getElementById.

answered Mar 12, 2012 at 14:41

jbabey

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