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javascript error when trying to use getElementById: 'null' is null or not an object?

Posted on 2008-06-19
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Last Modified: 2012-05-05
I thought this was pretty simple. I have an ASP:Image named img_load. I have a javascript set up to run onclick. All I want to do is set the visibility to true. Simple right? But I'm getting a 'null' is null or not an object error when trying to do anything with my javascript object.  The image is in an update panel. Does that make a difference? Is that what's causing my error?
<script type="text/javascript">

        function show_wait()

        {

            var img;

            img = document.getElementById("img_load");

            img.visible = true;   

        }

    </script>

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Question by:ohmErnie
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8 Comments
 
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Expert Comment

by:raterus
Comment Utility
Look at the source of the page as the browser sees it, find the image and use the ID it has.  My guess is it's been rewritten by asp.net.

To get this "renamed" id on the server, you need to use the ClientID function of the control.  I've used this in the past to easily make the switch.

<script type="text/javascript">
        function show_wait()
        {
            var img;
            img = document.getElementById('<%= img_load.ClientID %>');
            img.visible = true;  
        }
    </script>
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Author Comment

by:ohmErnie
Comment Utility
Well, when I look at the source code, I don't even see my image. Is that because I set it invisible in ASP?
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burakiewicz earned 500 total points
Comment Utility
if you set it to visible=false in the code behind it is never output in the html,
if you add the style attribute  img.Style.Add("visibilty", "hidden");
it will be output to the page so you can access it
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Expert Comment

by:ali_kayahan
Comment Utility
<script type="text/javascript">
        function show_wait()
        {
            var img;
            img = document.getElementById("img_load");
            img.style.display = 'block';  
        }
    </script>
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Expert Comment

by:burakiewicz
Comment Utility
should be
("visibility", "hidden");
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Expert Comment

by:raterus
Comment Utility
Yeah, if you set Visible=false on the server control, it won't even be rendered.  What you need to do is set CSS to hide the image, but the HTML will remain.

The CSS attribute to use is display:none or display:block
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Expert Comment

by:ruscomp
Comment Utility
It may also not be shown if you are using update panels... AJAX loaded html won't show in the source...
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Author Closing Comment

by:ohmErnie
Comment Utility
That worked great. All I had to do was change my javascript to set the visibility back to "visible".
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