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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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Expert Comment

by:raterus
ID: 21823324
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
ID: 21823378
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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Accepted Solution

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burakiewicz earned 500 total points
ID: 21823422
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
ID: 21823428
<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
ID: 21823433
should be
("visibility", "hidden");
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LVL 33

Expert Comment

by:raterus
ID: 21823442
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
ID: 21823452
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
ID: 31468838
That worked great. All I had to do was change my javascript to set the visibility back to "visible".
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