Trying to enable a disabled button when a textfield is not empty

So im just trying to enable a button when 3 text fields are not empty,and disable the button again if they become empty.Right now I was only trying it with one field just to get it working first but cant get it to work and no idea why.Would appreciate any help

        <script type="text/javascript">
        function checkEnableSubmit() {
           if ( completePurchaseForm.address.value != "") 
          {document.getElementById("button").disabled = false;}

        <form action="RegServlet" name="completePurchaseForm" method="post">
                    <td> Address  : </td><td> <input name="address" onkeyup="checkEnableSubmit()" id="address" size=100 type="text" /> </td> 
                    <td> Card Type  : </td><td> <input name="cardType" id="cardType" size=15 type="text" /> </td> 
                    <td> Card Number  : </td><td> <input name="cardNumber" id="cardNumber" size=15 type="text" /> </td> 

            <input type="hidden" name="action" value="completePurchase" />
            <input type="button" disabled name="submit" value="Complete your purchase" />


Add id to your button <input type="button" id="button1"> to use document.getElementById("button1")

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