使用 JavaScript 在滚动上显示 Div

Question

I am attempting to reveal divs as the user scrolls through the page. I am attempting to do this using javascript and css without any additional libraries.

Currently, I have a setup using jQuery that gets the job done but I am having issues finding a javascript only solution. I do not want the div to appear in a certain position, just simply as it appears in the user's viewport.

I am using opacity: 0 to hide the div, and then using $(this).animate({'opacity':'1'},500); to display.

I have been unable to find a similar javascript option that does not use a library or jQuery.

Can I achieve the code below using javascript?

A working jquery solution example is:

 $(document).ready(function() { /* Every time the window is scrolled... */ $(window).scroll( function(){ /* Check the location of each desired element */ $('.scroll-in').each( function(i){ var bottom_of_object = $(this).offset().top + $(this).outerHeight(); var bottom_of_window = $(window).scrollTop() + $(window).height(); /* If the object is completely visible in the window, fade it it */ if( bottom_of_window > bottom_of_object ){ $(this).animate({'opacity':'1'},500); } }); }); });

 #container { height:2000px; } #container div { margin:50px; padding:50px; background-color:blue; }.scroll-in { opacity:0; }

 <script src="https://cdnjs.cloudflare.com/ajax/libs/jquery/3.3.1/jquery.min.js"></script> <div id="container"> <div>Show</div> <div>Show</div> <div class="scroll-in">Fade In</div> <div class="scroll-in">Fade In</div> <div class="scroll-in">Fade In</div> </div>

Answer 1

What you want to use is an intersection observer - this is a html5 method of determinig when an element is in the vieport. This can be used for funky effects - eg autoplaying a video - only when the video in in the viewport - (eg: Facebook)... or lazy loading images.

Note that you can specifiy the amount by which the element must be in the viewport for the effect to take place - this is the "threshold" in the config options.

Also note that intersection observers ARE NOT supported in IE - so you will either need to use a polyfill or have a fallback like I have provided in the example.

 let paragraphs = document.querySelectorAll('p'); if ('IntersectionObserver' in window) { // IntersectionObserver Supported let config = { root: null, rootMargin: '0px', threshold: 1 }; let observer = new IntersectionObserver(onChange, config); paragraphs.forEach(paragraph => observer.observe(paragraph)); function onChange(changes, observer) { changes.forEach(change => { if (change.intersectionRatio > 0) { // Stop watching and load the image showParagraph(change.target); observer.unobserve(change.target); } }); } } else { // IntersectionObserver NOT Supported paragraphs.forEach(paragraph => showParagraph(paragraph)); } function showParagraph(paragraph) { paragraph.classList.add('fade-in'); }

 p { opacity: 0 }.fade-in { animation-name: fadeIn; animation-duration: 1000ms; animation-timing-function: cubic-bezier(0, 0, 0.4, 1); animation-fill-mode: forwards; } @keyframes fadeIn { from { opacity: 0; } to { opacity: 1; } }

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Answer 2

You could accomplish this with plain javascript using an IntersectionObserver . Here's a quick skeletal example:

 const callback = (entries, observer) => { entries.forEach(entry => { if (entry.isIntersecting) { if (entry.intersectionRatio >.5) { entry.target.classList.add('active'); } else { entry.target.classList.remove('active'); } } }); } const observer = new IntersectionObserver(callback, {threshold: 1}); document.querySelectorAll('div').forEach(d => observer.observe(d));

 div { background: bisque; min-height: 100px; width: 50%; margin-bottom: 0.5rem; opacity: 0; transition: all 0.5s ease-out; } div.active { background: tomato; width: 100%; opacity: 1; }

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