how to convert jquery ajax response to base 64

<script>

        $(document).ready(function () {

            $('#btnLogin').click(function () {

              

                $.ajax({
                    url: 'testurl',
                    type: 'Get',
                    beforeSend: function (xhr) {
                        xhr.setRequestHeader("Authorization", "Basic " + btoa("username" + ":" + "Password"));
                    },                    
                    success: function (response) {
                        
                        console.log(response)
            $('#imga').html('<img src="data:image/gif;base64,' + response + '" />');


                    },
                    error: function (error) {
                        alert("error")
                        console.log(error)
                    }
                });

            });
        });
    </script>

<input type="button" id="btnLogin" value="Login" />

   <div id="imga"></div>