jQuery check if element is visible on screen

<script>
$.fn.is_on_screen = function(){
    var win = $(window);
    var viewport = {
        top : win.scrollTop(),
        left : win.scrollLeft()
    };
    viewport.right = viewport.left + win.width();
    viewport.bottom = viewport.top + win.height();
 
    var bounds = this.offset();
    bounds.right = bounds.left + this.outerWidth();
    bounds.bottom = bounds.top + this.outerHeight();
 
    return (!(viewport.right < bounds.left || viewport.left > bounds.right || viewport.bottom < bounds.top || viewport.top > bounds.bottom));
};
 
if( $('.target').length > 0 ) { // if target element exists in DOM
	if( $('.target').is_on_screen() ) { // if target element is visible on screen after DOM loaded
        $('.log').html('<div class="alert alert-success">target element is visible on screen</div>'); // log info		
	} else {
        $('.log').html('<div class="alert">target element is not visible on screen</div>'); // log info
	}
}
$(window).scroll(function(){ // bind window scroll event
	if( $('.target').length > 0 ) { // if target element exists in DOM
		if( $('.target').is_on_screen() ) { // if target element is visible on screen after DOM loaded
			$('.log').html('<div class="alert alert-success">target element is visible on screen</div>'); // log info
		} else {
			$('.log').html('<div class="alert">target element is not visible on screen</div>'); // log info
		}
	}
});
</script>

<script> $.fn.is_on_screen = function(){ var win = $(window); var viewport = { top : win.scrollTop(), left : win.scrollLeft() }; viewport.right = viewport.left + win.width(); viewport.bottom = viewport.top + win.height(); var bounds = this.offset(); bounds.right = bounds.left + this.outerWidth(); bounds.bottom = bounds.top + this.outerHeight(); return (!(viewport.right < bounds.left || viewport.left > bounds.right || viewport.bottom < bounds.top || viewport.top > bounds.bottom)); }; if( $('.target').length > 0 ) { // if target element exists in DOM if( $('.target').is_on_screen() ) { // if target element is visible on screen after DOM loaded $('.log').html('<div class="alert alert-success">target element is visible on screen</div>'); // log info } else { $('.log').html('<div class="alert">target element is not visible on screen</div>'); // log info } } $(window).scroll(function(){ // bind window scroll event if( $('.target').length > 0 ) { // if target element exists in DOM if( $('.target').is_on_screen() ) { // if target element is visible on screen after DOM loaded $('.log').html('<div class="alert alert-success">target element is visible on screen</div>'); // log info } else { $('.log').html('<div class="alert">target element is not visible on screen</div>'); // log info } } }); </script>

<div class="log">log</div>
 
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<div class="target label label-info">target element</div>

<div class="log">log</div> scroll to target element<br> scroll to target element<br> scroll to target element<br> scroll to target element<br> scroll to target element<br> scroll to target element<br> scroll to target element<br> scroll to target element<br> <div class="target label label-info">target element</div>