How to add additional if statement to an existing javascript function for screen width?

I have a Javascript code where I would like to add an additional rule for the if statement- if the screen width is below 767px, then keep the CSS class attribute 'av_header_transparency' Below is the code in question:

if(transparent)
                {       
                        if(st > 50)
                        {       
                                //header.removeClass('av_header_transparency');
                                av_change_class(header, 'remove', 'av_header_transparency');
                        }
                        else
                        {
                                //header.addClass('av_header_transparency');
                                av_change_class(header, 'add', 'av_header_transparency');
                        }
                }

Currently this code is removing a CSS class 'av_header_transparency' if scrolling down for 50px, but I would like to keep this on mobile screen. Is this possible?

Regards