Adding one textbox value to another textbox

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jQuery

Hi all of you,

I'm a beginner in JQuery and I want to assign one textbox value to another textbox. I have written a code for that like:

<script type="text/javascript">
    $("#btnTest").click(function(event) {
        event.preventDefault();
        $("#txtLastName").val() = $("input:txtFirstName").val();
    });
</script>

but it's not working.
Can anybody please help me through this?
I'm waiting for your answer.