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I just have a general question about debouncing. I have three menus at different positions on the page that when they reach a position 85px from the top of the window on scroll they bee fixed. They are layered to overlap as they reach the top. I currently have a function for each and am looking to optimise as much as possible. My reading indicates the .offset.top calculation is quite taxing.

My question is: Am I overthinking it and is it necessary to debounce in this case? If my interpretation is right the three offset calculations are performed constantly on scroll. Can anyone suggest an optimisation or alterntively explain why it is not neccesary.

Thank you.

$(function(){
    // Check the initial Position of the fixed_nav_container
    var stickyHeaderTop0 = $('.fixed_heading_shop').offset().top;

    $(window).scroll(function(){
            if( $(window).scrollTop() > stickyHeaderTop0-85) {
                    $('.fixed_heading_shop').css({position: 'fixed', top: '85px'});  
                    $('.ghost_div0').css({display: 'block'});
            } else {
                    $('.fixed_heading_shop').css({position: 'relative', top: '0px'});
                    $('.ghost_div0').css({display: 'none'});
            }   

    });

  }); 


$(function(){
    // Check the initial Position of the fixed_nav_container
    var stickyHeaderTop1 = $('.fixed_heading_pricetable').offset().top;

     $(window).scroll(function(){
            if( $(window).scrollTop() > stickyHeaderTop1-85 ) {
                    $('.fixed_heading_pricetable').css({position: 'fixed', top: '85px'});  
                    $('.ghost_div1').css({display: 'block'});       
            } else {
                    $('.fixed_heading_pricetable').css({position: 'relative', top: '0px'});
                    $('.ghost_div1').css({display: 'none'});    
            }
         });

}); 



$(function(){
    // Check the initial Position of the fixed_nav_container
    var stickyHeaderTop2 = $('.fixed_heading_layout').offset().top;
     $(window).scroll(function(){    
            if( $(window).scrollTop() > stickyHeaderTop2-85) {
                    $('.fixed_heading_layout').css({position: 'fixed', top: '85px'});  
                    $('.ghost_div2').css({display: 'block'});
            } else {
                    $('.fixed_heading_layout').css({position: 'relative', top: '0px'});
                    $('.ghost_div2').css({display: 'none'});
            }
          });

}); 

I just have a general question about debouncing. I have three menus at different positions on the page that when they reach a position 85px from the top of the window on scroll they bee fixed. They are layered to overlap as they reach the top. I currently have a function for each and am looking to optimise as much as possible. My reading indicates the .offset.top calculation is quite taxing.

My question is: Am I overthinking it and is it necessary to debounce in this case? If my interpretation is right the three offset calculations are performed constantly on scroll. Can anyone suggest an optimisation or alterntively explain why it is not neccesary.

Thank you.

$(function(){
    // Check the initial Position of the fixed_nav_container
    var stickyHeaderTop0 = $('.fixed_heading_shop').offset().top;

    $(window).scroll(function(){
            if( $(window).scrollTop() > stickyHeaderTop0-85) {
                    $('.fixed_heading_shop').css({position: 'fixed', top: '85px'});  
                    $('.ghost_div0').css({display: 'block'});
            } else {
                    $('.fixed_heading_shop').css({position: 'relative', top: '0px'});
                    $('.ghost_div0').css({display: 'none'});
            }   

    });

  }); 


$(function(){
    // Check the initial Position of the fixed_nav_container
    var stickyHeaderTop1 = $('.fixed_heading_pricetable').offset().top;

     $(window).scroll(function(){
            if( $(window).scrollTop() > stickyHeaderTop1-85 ) {
                    $('.fixed_heading_pricetable').css({position: 'fixed', top: '85px'});  
                    $('.ghost_div1').css({display: 'block'});       
            } else {
                    $('.fixed_heading_pricetable').css({position: 'relative', top: '0px'});
                    $('.ghost_div1').css({display: 'none'});    
            }
         });

}); 



$(function(){
    // Check the initial Position of the fixed_nav_container
    var stickyHeaderTop2 = $('.fixed_heading_layout').offset().top;
     $(window).scroll(function(){    
            if( $(window).scrollTop() > stickyHeaderTop2-85) {
                    $('.fixed_heading_layout').css({position: 'fixed', top: '85px'});  
                    $('.ghost_div2').css({display: 'block'});
            } else {
                    $('.fixed_heading_layout').css({position: 'relative', top: '0px'});
                    $('.ghost_div2').css({display: 'none'});
            }
          });

}); 

• javascript
• jquery
• optimization
• debouncing

Share Improve this question Follow asked Apr 5, 2016 at 2:17 JPBJPB 60022 gold badges66 silver badges3131 bronze badges

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3 Answers 3

Sorted by: Reset to default 3

For this case, I think it's really a matter of preference. Take a look at how the site responds in your given situation, and make adjustments if you feel the user experience is negatively affected. I tend to throttle/debounce scroll events.

There are a few things you can do to speed up your scroll handlers (slightly). If you can use id's, for example, as in jQuery's guide to optimizing selectors, e.g. $('#myElement') is fast because it uses document.getElementById.

More minor adjustments if you're worried about performance: Don't make any calls to adjust css if no call is needed. i.e. if nothing changed since the last time the scroll handler was fired. (See isFixed boolean)

$(function(){
  var OFFSET = 85;
  var WAIT = 10;

  // Check the initial Position of the fixed_nav_container
  var stickyHeaderTop0 = $('.fixed_heading_shop').offset().top;
  var isFixed = false; // assuming that's the right default value

  $(window).scroll(_.throttle(function(){
    if($(window).scrollTop() > stickyHeaderTop0 - OFFSET) {
      if(!isFixed) {
        $('#fixed_heading_shop').css({position: 'fixed', top: OFFSET+'px'});
        $('#ghost_div0').css({display: 'block'});
        isFixed = true;
      }
    }
    else {
      if(isFixed) {
        $('#fixed_heading_shop').css({position: 'relative', top: '0px'});
        $('#ghost_div0').css({display: 'none'});
        isFixed = false;
      }
    }
  }, WAIT));
});

The only repeated call is $(window).scrollTop() and if you can bine all your scroll handlers (3?) then you would only need to make that call once per [throttled] scroll event.

Without making any changes in HTML mark up, you can optimize your code a little:

$(function(){
// Check the initial Position of the fixed_nav_container
 var stickyHeaderTop0 = $('.fixed_heading_shop').offset().top;
 var stickyHeaderTop1 = $('.fixed_heading_pricetable').offset().top;
 var stickyHeaderTop2 = $('.fixed_heading_layout').offset().top;
 $(window).scroll(function(){
        if( $(window).scrollTop() > stickyHeaderTop0-85) {
                $('.fixed_heading_shop').css({position: 'fixed', top: '85px'});  
                $('.ghost_div0').css({display: 'block'});
        } else {
                $('.fixed_heading_shop').css({position: 'relative', top: '0px'});
                $('.ghost_div0').css({display: 'none'});
        }
        if( $(window).scrollTop() > stickyHeaderTop1-85 ) {
                $('.fixed_heading_pricetable').css({position: 'fixed', top: '85px'});  
                $('.ghost_div1').css({display: 'block'});       
        } else {
                $('.fixed_heading_pricetable').css({position: 'relative', top: '0px'});
                $('.ghost_div1').css({display: 'none'});    
        }
        if( $(window).scrollTop() > stickyHeaderTop2-85) {
                $('.fixed_heading_layout').css({position: 'fixed', top: '85px'});  
                $('.ghost_div2').css({display: 'block'});
        } else {
                $('.fixed_heading_layout').css({position: 'relative', top: '0px'});
                $('.ghost_div2').css({display: 'none'});
        }
 });
});

And if you give a mon class to the three elements for which the if..else functionality is same, then it will be more optimized. I am adding a class 'mon' here in JS, you can add it in html itself.

$(function(){
// add mon class to elements
 $('.fixed_heading_shop, .fixed_heading_pricetable, .fixed_heading_layout').addClass('mon');
 var elemArr = [];
 // Check the initial Position of the fixed_nav_container
 $('.mon').each(function(index){
        elemArr.push($(this).offset().top);
 })
 $(window).scroll(function(){
     $('.mon').each(function(index){
        if( $(window).scrollTop() > elemArr[index]) {
                $(this).css({position: 'fixed', top: '85px'});  
                $('.ghost_div'+index).css({display: 'block'});
        } else {
                $(this).css({position: 'relative', top: '0px'});
                $('.ghost_div'+index).css({display: 'none'});
        }
     });
 });
});

I would say, rather than trying to set the css directly, take advantage of classes.

.fixed_heading_shop,
.fixed_heading_pricetable {
  position:relative;
  top:0;
}
.ghost_div0,
.ghost_div1 {
  display:block;
}
.scroll_trick .fixed_heading_shop,
.scroll_trick .fixed_heading_pricetable {
  position:fixed;
  top:85px;
}
.scroll_trick .ghost_div0,
.scroll_trick .ghost_div1 {
  display:none;
}

$(function(){
  var $window = $(window);
  var $body = $('body');
  var top = $('.fixed_heading_shop').offset().top-85;

  $window.scroll(function(){
    if( $window.scrollTop() > top) {
      $body.addClass('scroll_trick');
    } else {
      $body.removeClass('scroll_trick');
    }   
  });
});

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