Enable/Disable Button Based On Empty TextBoxes Using JavaScript

Add this script to the page

<script language="javascript" type="text/javascript">

function SetButtonStatus(sender, target)

{

if ( sender.value.length >= 12 )

document.getElementById(target).disabled = false;

else

document.getElementById(target).disabled = true;

}

script>

<asp:TextBox ID="txtText" runat="server" onkeyup="SetButtonStatus(this, 'btnButton')">asp:TextBox>

<asp:Button ID="btnButton" runat="server" Text="Button" Enabled="false" />

This JavaScript validates the textbox and enables the button only if the textbox contains 12 characters.