Bootstrap window closes after alert statement

I've below bootstrap modal. I'm just showing only 2 fields here. But I've more than 10 fields which are all mandatory. I'm doing javascript validation on click of Add button in Add_click(); and alerting user that All fields are mandatory. After click on Alert, the Modal window just goes off. I need this window to be retained until user gives all the details.

<div id="modalUserAddition" class="modal fade">
            <div class="modal-dialog" style="width: 600px">
                <div class="modal-content">
                    <div class="modal-header">
                        <button type="button" class="close" data-dismiss="modal">
                            &times;<span class="sr-only">Close</span>
                        </button>
                        <h4 class="modal-title">User Details</h4>
                    </div>
                    <div class="modal-body">

                        <div class="form-group">
                            <input type="text" id="txtUserName" class="form-control" runat="server" placeholder="User Name"   />
                        </div>
                        <div class="form-group">
                            <input type="text" id="txtPassword" class="form-control" runat="server" placeholder="Password" />
                        </div>

                    </div>
                    <div class="modal-footer">
                        <button type="button" id="Close" class="btn btn-default" data-dismiss="modal">Close</button>

                        <button type="button" id="Add" onclick="Add_click();" class="btn btn-primary" data-dismiss="modal">Add User</button>

                    </div>

                </div>
            </div>
        </div>

I tried below code, but it didn't work for me.

alert('All fields are mandatory!');
    //$('#modalUserAddition').modal('toggle');
    $('#modalUserAddition').modal({
        show: true
    });

Please help!