What happens if we put "!" charecter before a Javascript function name?

JavaScript
1

<asp:Button ID=“ButtonAdd” UseSubmitBehavior=“true” OnClick=“ButtonAddClick” OnClientClick=“return !AddClientSide();” Text=“Add” runat=“server” />

Thanks

2

! is a negation. It means that if AddClientSide() returns true, !AddClientSide() will be false. And if AddClientSide() returns false, !AddClientSide() will be true.