modal hide not working javascript code example

Example: .show() and .hide not working for bootstrap modal

<button class="button primary" id="buy" style="text-decoration:none;" type="button">Review and confirm</button>

<div class="modal-bootstrap fade bs-example-modal-sm" id="myModal" tabindex="-1" role="dialog" aria-labelledby="smallModalLabel" aria-hidden="true">
    <!-- modal contents -->
</div>

<script type="text/javascript">
$("#buy").click(function () {
    var a = 4;
    if (a == 5) {
        alert("if");
        $('#myModal').modal('show');
    } else {
        alert("else");
        $('#myModal').modal('hide');
    }
});
</script>