Retrieve value from javascript funcion on client click

I have a function on JS that must execute before the codebehind, but i’m having trouble retrieving it.
It will return a variable that i should use on the codebehind. The function i created is this one:

            <script type="text/javascript" >
                function returnHash() {
                    var checkout = new DirectCheckout('086F7D02A071267FEBF102EB759D9845B8B3333DFA65BC45B807F41A8525AD8D'); /* Em sandbox utilizar o construtor new DirectCheckout('SEU TOKEN PUBLICO', false); */
                    var month = document.getElementById("txtDataVencUser").value;
                    var year = document.getElementById("txtDataVencUser").value;

                    var cardData = {
                        cardNumber: document.getElementById("txtNumeroCartaoUser").value,
                        holderName: document.getElementById("txtNomeCartaoUser").value,
                        securityCode: document.getElementById("txtCodSegurancaUser").value,
                        expirationMonth: month.substring(0, 2),
                        expirationYear: "20" + year.substring(3, 5)

                    checkout.getCardHash(cardData, function (cardHash) {
----------------------------> I NEED TO RETRIEVE "cardHash" VALUE IN CODEBEHIND
                    }, function (error) {

I don’t know if this step is correct, but i am using the onClientClick to try to execute the JS before the codebehind:

<asp:Button ID="btnFinalizarCompraJunoUser" runat="server" Text="FINALIZAR COMPRA" OnClientClick="returnHash()" OnClick="btnFinalizarCompraJunoUser_Click" Visible="true" />

I am having trouble figuring out what to do next, can someone help me please?

7 thoughts on “Retrieve value from javascript funcion on client click”

  1. There are a number of ways – perhaps as many flavors of ice cream.

    But, since you are going to run a "code behind" stub based on the click?

    I would just drop a hidden field on the page, and then just set that value in js

    So right below your button – add a hidden field say like this:

    <form id="form1" runat="server">
            <br />
        <asp:Button ID="Button1" runat="server" Text="Button" 
            OnClientClick="myfun();return true" Width="68px"
         <asp:HiddenField ID="HiddenField1" runat="server" ClientIDMode="Static" />
    <script type="text/javascript">
    function myfun() {
        // do whatever - walk the dog - have fun!!!
        var f1 = document.getElementById("HiddenField1");
        f1.value = "2222";

    So in above I just shove 2222 into the hidden1 field.

    Now, in the the code behind button stub, you can do this:

     Debug.Print("my click f1 = " & HiddenField1.Value);

    So the code behind will now have a good old regular plane jane control to look at and get the value in question.

    So, it depends on how many values – but using a code behind friendly ( control and setting the value with the js client side code probably is the least effort here.

    I also of course used "staticID" for the above. If you for some reason don’t like (or want) to use staticID for above, then the js selection would be:

     var f1 = document.getElementById("<%=HiddenField1.Clientid%>");
     f1.value = cardHash;

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