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The onscroll event handler
Meow! This page is far too long to scroll through. Let’s make an “Up” button!
The main page of the news site that we worked on in previous chapters is very long. If the user has read through all of the news stories and wants to return to the beginning, then it will take them a while to scroll back up. Muffin wants us to program a button that will automatically scroll the page back to the top.
The button is initially not visible, and it should appear if the user has scrolled the page by more than 200px. Why is the number 200 important? Only the cat knows why! If the button is clicked, the page should scroll to the beginning, and the button itself should disappear after that.
In order to carry out the boss’ assignment, we need to know when the user is scrolling the page. JavaScript monitors everything that happens in the browser window, including scrolling. We can tell it what to do if the page is scrolled. To do this, let’s use the onscroll event handler. It is activated every time a page is scrolled, even if the page is moved by just one pixel.
In order to track scrolling on the news site, let’s add an event handler to the mini-browser window. The browser window (or tab) is indicated in JavaScript using the word window:
window.onscroll = function () {
console.log('The page has been scrolled');
}Let’s check how the onscroll handler works.
You can add the onscroll event handler not only to the browser window, but also to separate elements on the page. For the event handler to work, the element must have its own scrollbar. You can control the scrolling of the element using the overflow CSS property.
Files
<!DOCTYPE html>
<html lang="en">
<head>
<meta charset="utf-8">
<link rel="stylesheet" href="setting.css">
<link rel="stylesheet" href="style.css">
<title>FlashNews!</title>
</head>
<body class="page light-theme">
<header class="page-header">
<div class="container">
<a class="header-logo">
<img src="img/main-logo.svg" width="67" height="29" alt="FlashNews! portal logo">
</a>
<a href="subscription.html" class="subscription-link">Subscription</a>
<button class="theme-button" type="button">Change the theme</button>
</div>
</header>
<main class="index-main">
<div class="container">
<h1 class="visually-hidden">FlashNews news portal!</h1>
<div class="news-view">
<button class="row-view view-checked" type="button">Results list</button>
<button class="tile-view" type="button">Tile</button>
</div>
<section class="news-list">
<h2 class="visually-hidden">All News</h2>
<article class="news-block" data-category="cats">
<img src="img/news-cat.jpg" alt="Cat eyes">
<div class="news-block-text">
<h3>Unique video!</h3>
<p>Discover the World Through the Eyes of Cats! Your life will never be the same…</p>
<span class="news-tag">#Kitties</span>
<time datetime="2020-03-24">March 24, 2020</time>
</div>
</article>
<article class="news-block" data-category="HR">
<img src="img/walk.jpg" alt="People go for walks">
<div class="news-block-text">
<h3>It can be healthy to go for a walk</h3>
<p>But it might not be if you take a stroll with the wrong people…</p>
<span class="news-tag">#HR</span>
<time datetime="2020-03-19">February 19, 2020</time>
</div>
</article>
<article class="news-block" data-category="games">
<img src="img/ar.jpg" alt="Person wearing virtual reality glasses">
<div class="news-block-text">
<h3>Augment your reality</h3>
<p>The editors of Flashnews! have prepared a gift for their subscribers…</p>
<span class="news-tag">#Games</span>
<time datetime="2020-02-18">February 18, 2020</time>
</div>
</article>
<article class="news-block" data-category="IT">
<img src="img/drone.jpg" alt="A small drone">
<div class="news-block-text">
<h3>Dronovirus</h3>
<p>The designers at the DogDrones.inc laboratory have assured that their drones cannot be carriers of dangerous viruses…</p>
<span class="news-tag">#IT</span>
<time datetime="2019-12-22">December 22, 2019</time>
</div>
</article>
<article class="news-block" data-category="HR">
<img src="img/new-research.jpg" alt="A man looks at a board with cards">
<div class="news-block-text">
<h3>From the world of psychology</h3>
<p>Studies show that if you do more, you can actually get more done.</p>
<span class="news-tag">#HR</span>
<time datetime="2019-10-12">October 12, 2019</time>
</div>
</article>
<article class="news-block" data-category="games">
<img src="img/new-drone.jpg" alt="Quadcopter">
<div class="news-block-text">
<h3>A quadcopter is not just a fancy toy</h3>
<p>Controlling the quadcopter allows you to develop agility, spatial thinking, and the ability to climb trees…</p>
<span class="news-tag">#Games</span>
<time datetime="2019-11-21">November 21, 2019</time>
</div>
</article>
<article class="news-block" data-category="IT">
<img src="img/new-loop.jpg" alt="Infinite loops">
<div class="news-block-text">
<h3>Infinite loops: It’s time to end them</h3>
<p>British scientists have discovered that the ability of software to function directly depends on the presence of infinite loops in it.</p>
<span class="news-tag">#IT</span>
<time datetime="2019-10-16">October 16, 2019</time>
</div>
</article>
<article class="news-block" data-category="cats">
<img src="img/news-robot2.jpg" alt="A robot vacationing at a resort">
<div class="news-block-text">
<h3>What shocking news! It’s so scandalous! Such a sensation!</h3>
<p>It turns out that you don’t have to be a cat to start programming in JavaScript!</p>
<span class="news-tag">#Kitties</span>
<time datetime="2019-12-22">December 22, 2019</time>
</div>
</article>
<article class="news-block" data-category="IT">
<img src="img/new-graph.jpg" alt="New library">
<div class="news-block-text">
<h3>New graphics library</h3>
<p>Now you can create a dashboard in just a matter of seconds.</p>
<span class="news-tag">#IT</span>
<time datetime="2019-10-14">October 14, 2019</time>
</div>
</article>
<article class="news-block" data-category="cats">
<img src="img/new-cat.jpg" alt="Cat at a laptop">
<div class="news-block-text">
<h3>This just in: SHOCKING NEWS! Secret photos from the cat nursery</h3>
<p>No one expected that THIS was going on behind the closed doors…</p>
<span class="news-tag">#Kitties</span>
<time datetime="2019-10-13">October 13, 2019</time>
</div>
</article>
</section>
</div>
</main>
<footer class="page-footer">
<div class="container">
<p>© FlashNews!</p>
<a class="footer-logo">
<img src="img/white-logo.svg" alt="FlashNews! portal logo">
</a>
</div>
</footer>
<button class="up-button" type="button">
↑
<span class="visually-hidden">Up</span>
</button>
<script src="script.js"></script>
<script src="themes.js"></script>
<script src="up-button.js"></script>
</body>
</html>
html {
scroll-behavior: smooth;
}
.hidden {
display: none;
}
.subscription-link {
position: relative;
padding-top: 8px;
padding-right: 2px;
padding-bottom: 6px;
padding-left: 26px;
margin: auto 4%;
}
.subscription-link::before {
position: absolute;
content: "";
width: 18px;
height: 12px;
top: 9px;
left: 2px;
}
.subscription-link:hover {
padding-bottom: 4px;
border-bottom: 2px solid;
}
.subscription-link:active {
opacity: 0.6;
}
.light-theme .subscription-link {
color: #6653d9;
outline-color: #b6aaff;
}
.light-theme .subscription-link:hover {
border-color: #6653d9;
}
.light-theme .subscription-link::before {
background-image: url("img/letter-light.svg");
}
.dark-theme .subscription-link {
color: #9484f2;
outline-color: #6653d9;
}
.dark-theme .subscription-link:hover {
border-color: #9484f2;
}
.dark-theme .subscription-link::before {
background-image: url("img/letter-dark.svg");
}
.up-button {
position: fixed;
z-index: 1;
padding: 0;
bottom: 75px;
right: 20px;
width: 64px;
height: 64px;
font-size: 36px;
font-weight: bold;
text-align: center;
vertical-align: middle;
border: 5px solid;
border-radius: 50%;
display: none;
cursor: pointer;
}
.light-theme .up-button {
color: #6653d9;
background-color: rgba(255, 255, 255, 0.8);
border-color: #6653d9;
outline-color: #b6aaff;
}
.light-theme .up-button:hover {
color: #473c8d;
background-color: rgba(255, 255, 255, 0.8);
border-color: #473c8d;
}
.dark-theme .up-button {
color: #9484f2;
background-color: rgba(10, 9, 16, 0.6);
border-color: #9484f2;
outline-color: #6653d9;
}
.dark-theme .up-button:hover {
color: #6653d9;
background-color: rgba(10, 9, 16, 0.6);
border-color: #6653d9;
}
.shown {
display: inline-block;
}
/* News tags styles */
.news-tag {
padding: 4px 8px;
margin: 0 auto 0 0;
border-radius: 12px;
}
.list-tiles-view .news-tag {
margin-bottom: 4px;
}
.light-theme .news-tag {
color: #aaaaaa;
background-color: #f2f2f2;
}
.dark-theme .news-tag {
color: #888888;
background-color: #0a0910;
}
let rowViewButton = document.querySelector('.row-view');
let tileViewButton = document.querySelector('.tile-view');
let newsList = document.querySelector('.news-list');
rowViewButton.onclick = function () {
rowViewButton.classList.add('view-checked');
tileViewButton.classList.remove('view-checked');
newsList.classList.remove('list-tiles-view');
};
tileViewButton.onclick = function () {
rowViewButton.classList.remove('view-checked');
tileViewButton.classList.add('view-checked');
newsList.classList.add('list-tiles-view');
};
let page = document.querySelector('.page');
let themeButton = document.querySelector('.theme-button');
themeButton.onclick = function () {
page.classList.toggle('light-theme');
page.classList.toggle('dark-theme');
};
Thanks! We’ll fix everything at once!
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Click inside the mini-browser to shift the focus onto this window.
100%
Console
- Add the
onscrollevent handler to the mini browser window using thewindow.onscroll = function () { };instruction. - Enter the
'Scroll detected!'string to the console inside the event handler. - Scroll down the page inside the mini browser.
- Open the console. Please note how the message reporting detected scrolling was output many times.
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