使用jquery从窗口顶部到达200px时如何更改div的类？

Question

I have 5 boxes on a webpage that you can see here:

http://demo.axistheme.com/methodology/Wakeup-Method.html .

The 1st box has the text 01 with a yellow background color and 2nd and the others have the text 02, 03 etc. with a gray background color.

When scrolling the window, how can I change the background color of each of the other boxes (one by one) at the point where they reach 200px beneath the top of the window?

Answer 1

You need to check - whenever the window is scrolled - if any of the elements are less than 200px from the top of the viewport.

To determine each element's vertical offset position relative to the viewport, you can subtract the window vertical scroll position ( $(window).scrollTop ) from each element's vertical offset position relative to the document ( .offset().top ).

ie. var offsetTopPosition = $(ELEMENT).offset().top - $(window).scrollTop();

Working Example:

 $(document).ready(function(){ $(window).scroll(function(){ $('div').each(function(){ var offsetTopPosition = $(this).offset().top - $(window).scrollTop(); if (offsetTopPosition < 100) { $(this).addClass('active'); } if (offsetTopPosition > 100) { $(this).removeClass('active'); } }); }); });

 div { width: 36px; height: 36px; line-height: 36px; margin-bottom: 200px; color: rgb(0,0,0); background-color: rgb(191,191,191); font-size: 24px; font-weight: bold; text-align: center; } .active { background-color: yellow; }

 <script src="https://ajax.googleapis.com/ajax/libs/jquery/2.1.1/jquery.min.js"></script> <div class="active">1</div> <div>2</div> <div>3</div> <div>4</div> <div>5</div>

Answer 2

You can use offset() in combination with $(window).scrollTop() to determine the position of your elements.

    $('.box').each(function( index ) {
        var threshold = 200;
        var topOffset = $( this ).offset().top;
        if( topOffset - $(window).scrollTop() < threshold){
            $( this ).addClass('active');
       }else{
           $( this ).removeClass('active');
        }
    });

See a working example here: https://jsfiddle.net/jkrielaars/yt584kLz/2/