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This Fixed my issue:<br />
<br />
use server tag &lt;%= chk_AllGameDate.ClientID %&gt;
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This Fixed my issue:

use server tag <%= chk_AllGameDate.ClientID %>

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<div class="content wysiwyg-content">
This doesn't work and I don't know why?<br />
&nbsp; &nbsp; &lt;script type=&quot;text/javascript&quot;&gt;<br />
&nbsp; &nbsp; &nbsp; &nbsp; function GameDate() {<br />
&nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; if(document.getElementById<wbr />('chk_AllG<wbr />ameDate.Cl<wbr />ientID').c<wbr />hecked)<br />
&nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; {<br />
&nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; document.getElementById('t<wbr />xt_GameDat<wbr />e.ClientID<wbr />').disable<wbr />d = 'false';<br />
&nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; }<br />
&nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; else<br />
&nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; {<br />
&nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; document.getElementById('t<wbr />xt_GameDat<wbr />e.ClientID<wbr />').disable<wbr />d = 'true';<br />
&nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; }<br />
&nbsp; &nbsp; &nbsp; &nbsp; }<br />
<br />
&nbsp; &nbsp; &lt;/script&gt;<br />
I want to enable a textbox when the checkbox is checked off!<br />
<br />
I get this error when I click the Check Box:<br />
0x800a138f - JavaScript runtime error: Unable to get property 'checked' of undefined or null reference
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This doesn't work and I don't know why?
    <script type="text/javascript">
        function GameDate() {
            if(document.getElementById('chk_AllGameDate.ClientID').checked)
            {
                document.getElementById('txt_GameDate.ClientID').disabled = 'false';
            }
            else
            {
                document.getElementById('txt_GameDate.ClientID').disabled = 'true';
            }
        }

    </script>
I want to enable a textbox when the checkbox is checked off!

I get this error when I click the Check Box:
0x800a138f - JavaScript runtime error: Unable to get property 'checked' of undefined or null reference

VS2012 / Javascript

JavaScript is a dynamic, object-based language commonly used for client-side scripting in web browsers. Recently, server side JavaScript frameworks have also emerged. JavaScript runs on nearly every operating system and  in almost every mainstream web browser.