sub wait_for_JS_function {
    my ($self, $func, $timeout) = @_;
    for (1 .. $timeout / 100) {
        my $res = $self->get_eval("typeof( window.$func )");
        return $res if $res ne 'null';
        $self->pause(100);
    }
    croak "Timed out waiting for JS func: '$func'";
}