infinite scroll jQuery

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Javascript

jQuery

hi.
Im implementing infinite scrolling,i want the pages to be loaded before scroll reaches the end of page,like 50 pixels remaining to end.
i know i can do something like below,but its not working.
how should i do that?

$(window).scroll(function(){
        if  ($(window).scrollTop() == $(document).height() - $(window).height()-50){
           lastPostFunc();
        }
});