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class="main__header-title">Scrolling</h1></div></header><div class="content"><article class="formatted" itemscope itemtype="http://schema.org/TechArticle"><meta itemprop="name" content="Scrolling"><div itemprop="author" itemscope itemtype="http://schema.org/Person"><meta itemprop="email" content="iliakan@gmail.com"><meta itemprop="name" content="Ilya Kantor"></div><div itemprop="articleBody"><p>The <code>scroll</code> event allows reacting to a page or element scrolling. There are quite a few good things we can do here.</p> <p>For instance:</p> <ul> <li>Show/hide additional controls or information depending on where in the document the user is.</li> <li>Load more data when the user scrolls down till the end of the page.</li> </ul> <p>Here’s a small function to show the current scroll:</p> <div id="9fge9ldede" data-trusted="1" class="code-example" data-autorun="true"> <div class="codebox code-example__codebox"> <div class="codebox__code" data-code="1"> <pre class="line-numbers language-javascript"><code>window.addEventListener('scroll', function() { document.getElementById('showScroll').innerHTML = window.pageYOffset + 'px'; });</code></pre> </div> </div> </div><p>In action:</p> <p>Current scroll = <b id="showScroll">scroll the window</b></p> <p>The <code>scroll</code> event works both on the <code>window</code> and on scrollable elements.</p> <h2><a class="main__anchor" name="prevent-scrolling" href="#prevent-scrolling">Prevent scrolling</a></h2><p>How do we make something unscrollable?</p> <p>We can’t prevent scrolling by using <code>event.preventDefault()</code> in <code>onscroll</code> listener, because it triggers <em>after</em> the scroll has already happened.</p> <p>But we can prevent scrolling by <code>event.preventDefault()</code> on an event that causes the scroll, for instance <code>keydown</code> event for <kbd class="shortcut">pageUp</kbd> and <kbd class="shortcut">pageDown</kbd>.</p> <p>If we add an event handler to these events and <code>event.preventDefault()</code> in it, then the scroll won’t start.</p> <p>There are many ways to initiate a scroll, so it’s more reliable to use CSS, <code>overflow</code> property.</p> <p>Here are few tasks that you can solve or look through to see applications of <code>onscroll</code>.</p> </div></article><div class="tasks formatted"><h2 class="tasks__title" id="tasks"><a class="tasks__title-anchor main__anchor main__anchor main__anchor_noicon" href="#tasks">Tasks</a></h2><div class="task tasks__task"><div class="task__header"><div class="task__title-wrap"><h3 class="task__title"><a class="main__anchor" href="#endless-page" name="endless-page">Endless page</a></h3><a class="task__open-link" href="/task/endless-page" target="_blank"></a></div><div class="task__header-note"><span class="task__importance" title="How important is the task, from 1 to 5">importance: 5</span></div><div class="task__content"><div class="task__formatted"><p>Create an endless page. When a visitor scrolls it to the end, it auto-appends current date-time to the text (so that a visitor can scroll more).</p> <p>Like this:</p> <div class="code-result"> <div class="code-result__toolbar toolbar"></div> <iframe class="code-result__iframe" data-trusted="1" style="height:200px" src="https://en.js.cx/task/endless-page/solution/"></iframe> </div><p>Please note two important features of the scroll:</p> <ol> <li><strong>The scroll is “elastic”.</strong> We can scroll a little beyond the document start or end in some browsers/devices (empty space below is shown, and then the document will automatically “bounces back” to normal).</li> <li><strong>The scroll is imprecise.</strong> When we scroll to page end, then we may be in fact like 0-50px away from the real document bottom.</li> </ol> <p>So, “scrolling to the end” should mean that the visitor is no more than 100px away from the document end.</p> <p>P.S. In real life we may want to show “more messages” or “more goods”.</p> <p><a href="https://plnkr.co/edit/YWQcVXPxWF9zG60Q?p=preview" target="_blank" data-plunk-id="YWQcVXPxWF9zG60Q">Open a sandbox for the task.</a></p></div><button class="task__solution" type="button">solution</button><div class="task__answer"><div class="task__answer-content"><div class="formatted"><p>The core of the solution is a function that adds more dates to the page (or loads more stuff in real-life) while we’re at the page end.</p> <p>We can call it immediately and add as a <code>window.onscroll</code> handler.</p> <p>The most important question is: “How do we detect that the page is scrolled to bottom?”</p> <p>Let’s use window-relative coordinates.</p> <p>The document is represented (and contained) within <code><html></code> tag, that is <code>document.documentElement</code>.</p> <p>We can get window-relative coordinates of the whole document as <code>document.documentElement.getBoundingClientRect()</code>, the <code>bottom</code> property will be window-relative coordinate of the document bottom.</p> <p>For instance, if the height of the whole HTML document is <code>2000px</code>, then:</p> <div id="tpnoxub8ao" data-trusted="1" class="code-example"> <div class="codebox code-example__codebox"> <div class="codebox__code" data-code="1"> <pre class="line-numbers language-javascript"><code>// when we're on the top of the page // window-relative top = 0 document.documentElement.getBoundingClientRect().top = 0 // window-relative bottom = 2000 // the document is long, so that is probably far beyond the window bottom document.documentElement.getBoundingClientRect().bottom = 2000</code></pre> </div> </div> </div><p>If we scroll <code>500px</code> below, then:</p> <div id="o0997ymaht" data-trusted="1" class="code-example"> <div class="codebox code-example__codebox"> <div class="codebox__code" data-code="1"> <pre class="line-numbers language-javascript"><code>// document top is above the window 500px document.documentElement.getBoundingClientRect().top = -500 // document bottom is 500px closer document.documentElement.getBoundingClientRect().bottom = 1500</code></pre> </div> </div> </div><p>When we scroll till the end, assuming that the window height is <code>600px</code>:</p> <div id="ldl7b1chrw" data-trusted="1" class="code-example"> <div class="codebox code-example__codebox"> <div class="codebox__code" data-code="1"> <pre class="line-numbers language-javascript"><code>// document top is above the window 1400px document.documentElement.getBoundingClientRect().top = -1400 // document bottom is below the window 600px document.documentElement.getBoundingClientRect().bottom = 600</code></pre> </div> </div> </div><p>Please note that the <code>bottom</code> can’t be <code>0</code>, because it never reaches the window top. The lowest limit of the <code>bottom</code> coordinate is the window height (we assumed it to be <code>600</code>), we can’t scroll it any more up.</p> <p>We can obtain the window height as <code>document.documentElement.clientHeight</code>.</p> <p>For our task, we need to know when the document bottom is not no more than <code>100px</code> away from it (that is: <code>600-700px</code>, if the height is <code>600</code>).</p> <p>So here’s the function:</p> <div id="rek2v7ytv5" data-trusted="1" class="code-example"> <div class="codebox code-example__codebox"> <div class="codebox__code" data-code="1"> <pre class="line-numbers language-javascript"><code>function populate() { while(true) { // document bottom let windowRelativeBottom = document.documentElement.getBoundingClientRect().bottom; // if the user hasn't scrolled far enough (>100px to the end) if (windowRelativeBottom > document.documentElement.clientHeight + 100) break; // let's add more data document.body.insertAdjacentHTML("beforeend", `<p>Date: ${new Date()}</p>`); } }</code></pre> </div> </div> </div><p><a href="https://plnkr.co/edit/cclA6BokVrkqb5Ls?p=preview" target="_blank" data-plunk-id="cclA6BokVrkqb5Ls">Open the solution in a sandbox.</a></p></div></div><button class="close-button task__answer-close" type="button" title="close"></button></div></div></div></div><div class="task tasks__task"><div class="task__header"><div class="task__title-wrap"><h3 class="task__title"><a class="main__anchor" href="#up-down-button" name="up-down-button">Up/down button</a></h3><a class="task__open-link" href="/task/updown-button" target="_blank"></a></div><div class="task__header-note"><span class="task__importance" title="How important is the task, from 1 to 5">importance: 5</span></div><div class="task__content"><div class="task__formatted"><p>Create a “to the top” button to help with page scrolling.</p> <p>It should work like this:</p> <ul> <li>While the page is not scrolled down at least for the window height – it’s invisible.</li> <li>When the page is scrolled down more than the window height – there appears an “upwards” arrow in the left-top corner. If the page is scrolled back, it disappears.</li> <li>When the arrow is clicked, the page scrolls to the top.</li> </ul> <p>Like this (top-left corner, scroll to see):</p> <div class="code-result"> <div class="code-result__toolbar toolbar"><div class="toolbar__tool"> <a href="https://en.js.cx/task/updown-button/solution/" target="_blank" title="open in new window" class="toolbar__button toolbar__button_external"></a> </div> </div> <iframe class="code-result__iframe" data-trusted="1" style="height:200px" src="https://en.js.cx/task/updown-button/solution/"></iframe> </div><p><a href="https://plnkr.co/edit/kbmUE99oEnyd6ASc?p=preview" target="_blank" data-plunk-id="kbmUE99oEnyd6ASc">Open a sandbox for the task.</a></p></div><button class="task__solution" type="button">solution</button><div class="task__answer"><div class="task__answer-content"><div class="formatted"><p><a href="https://plnkr.co/edit/h5fkZMlKRJOB1dcI?p=preview" target="_blank" data-plunk-id="h5fkZMlKRJOB1dcI">Open the solution in a sandbox.</a></p></div></div><button class="close-button task__answer-close" type="button" title="close"></button></div></div></div></div><div class="task tasks__task"><div class="task__header"><div class="task__title-wrap"><h3 class="task__title"><a class="main__anchor" href="#load-visible-images" name="load-visible-images">Load visible images</a></h3><a class="task__open-link" href="/task/load-visible-img" target="_blank"></a></div><div class="task__header-note"><span class="task__importance" title="How important is the task, from 1 to 5">importance: 4</span></div><div class="task__content"><div class="task__formatted"><p>Let’s say we have a slow-speed client and want to save their mobile traffic.</p> <p>For that purpose we decide not to show images immediately, but rather replace them with placeholders, like this:</p> <div id="h0bbnwt685" data-trusted="1" class="code-example" data-highlight="[{"start":0,"cols":[{"start":5,"end":26},{"start":52,"end":71}]}]"> <div class="codebox code-example__codebox"> <div class="codebox__code" data-code="1"> <pre class="line-numbers language-markup"><code><img src="placeholder.svg" width="128" height="128" data-src="real.jpg"></code></pre> </div> </div> </div><p>So, initially all images are <code>placeholder.svg</code>. When the page scrolls to the position where the user can see the image – we change <code>src</code> to the one in <code>data-src</code>, and so the image loads.</p> <p>Here’s an example in <code>iframe</code>:</p> <div class="code-result"> <div class="code-result__toolbar toolbar"></div> <iframe class="code-result__iframe" data-trusted="1" style="height:300px" src="https://en.js.cx/task/load-visible-img/solution/"></iframe> </div><p>Scroll it to see images load “on-demand”.</p> <p>Requirements:</p> <ul> <li>When the page loads, those images that are on-screen should load immediately, prior to any scrolling.</li> <li>Some images may be regular, without <code>data-src</code>. The code should not touch them.</li> <li>Once an image is loaded, it should not reload any more when scrolled in/out.</li> </ul> <p>P.S. If you can, make a more advanced solution that would “preload” images that are one page below/after the current position.</p> <p>P.P.S. Only vertical scroll is to be handled, no horizontal scrolling.</p> <p><a href="https://plnkr.co/edit/bS3h5iJHSS2xAG6O?p=preview" target="_blank" data-plunk-id="bS3h5iJHSS2xAG6O">Open a sandbox for the task.</a></p></div><button class="task__solution" type="button">solution</button><div class="task__answer"><div class="task__answer-content"><div class="formatted"><p>The <code>onscroll</code> handler should check which images are visible and show them.</p> <p>We also want to run it when the page loads, to detect immediately visible images and load them.</p> <p>The code should execute when the document is loaded, so that it has access to its content.</p> <p>Or put it at the <code><body></code> bottom:</p> <div id="bkzj3ppfso" data-trusted="1" class="code-example"> <div class="codebox code-example__codebox"> <div class="codebox__code" data-code="1"> <pre class="line-numbers language-javascript"><code>// ...the page content is above... function isVisible(elem) { let coords = elem.getBoundingClientRect(); let windowHeight = document.documentElement.clientHeight; // top elem edge is visible? let topVisible = coords.top > 0 && coords.top < windowHeight; // bottom elem edge is visible? let bottomVisible = coords.bottom < windowHeight && coords.bottom > 0; return topVisible || bottomVisible; }</code></pre> </div> </div> </div><p>The <code>showVisible()</code> function uses the visibility check, implemented by <code>isVisible()</code>, to load visible images:</p> <div id="4gwj0nlcsu" data-trusted="1" class="code-example" data-highlight="[{"start":12,"end":13}]"> <div class="codebox code-example__codebox"> <div class="codebox__code" data-code="1"> <pre class="line-numbers language-javascript"><code>function showVisible() { for (let img of document.querySelectorAll('img')) { let realSrc = img.dataset.src; if (!realSrc) continue; if (isVisible(img)) { img.src = realSrc; img.dataset.src = ''; } } } showVisible(); window.onscroll = showVisible;</code></pre> </div> </div> </div><p>P.S. The solution also has a variant of <code>isVisible</code> that “preloads” images that are within 1 page above/below the current document scroll.</p> <p><a href="https://plnkr.co/edit/ENtjdtkVRwkmKqZ4?p=preview" target="_blank" data-plunk-id="ENtjdtkVRwkmKqZ4">Open the solution in a sandbox.</a></p></div></div><button class="close-button task__answer-close" type="button" title="close"></button></div></div></div></div></div></div><div class="page__nav-wrap"><a class="page__nav page__nav_prev" href="/keyboard-events" data-tooltip="Keyboard: keydown and keyup"><span class="page__nav-text"><span class="page__nav-text-shortcut"></span></span><span class="page__nav-text-alternate">Previous lesson</span></a><a class="page__nav page__nav_next" href="/forms-controls" data-tooltip="Forms, controls"><span class="page__nav-text"><span class="page__nav-text-shortcut"></span></span><span class="page__nav-text-alternate">Next lesson</span></a></div><div class="article-tablet-foot tablet-only"><div class="article-tablet-foot__layout"><div class="share-icons"><span class="share-icons__title">Share</span><a class="share share_tw" href="https://twitter.com/share?url=https%3A%2F%2Fjavascript.info%2Fonscroll" rel="nofollow"></a><a class="share share_fb" href="https://www.facebook.com/sharer/sharer.php?s=100&p%5Burl%5D=https%3A%2F%2Fjavascript.info%2Fonscroll" rel="nofollow"></a></div><div class="article-tablet-foot__map"><a class="map" href="/tutorial/map" data-action="tutorial-map"><span class="map__text">Tutorial map</span></a></div></div></div><div class="banner-bottom-carbon"><div id="javascriptinfo_iconbar"></div></div><script src="//m.servedby-buysellads.com/monetization.custom.js" id="javascriptinfo_script" async></script><script>document.getElementById("javascriptinfo_script").onload=function(){_bsa.init("custom","CKYDEK3U","placement:javascriptinfo_iconbar",{target:"#javascriptinfo_iconbar",template:'\n <div class="iconBarFlex">\n <a href="##statlink##" class="iconBarLink" rel="sponsored noopener" target="_blank" title="##company## — ##tagline##">\n <div class="iconBarImage" style="background-color: ##backgroundColor##;">\n <img height="30" width="30" src="##image##" alt="##company## logo">\n </div>\n <div class="iconBarText">\n <div class="iconBarTagline">##companyTagline##</div>\n <div class="iconBarDescription">##description##</div>\n </div>\n </a>\n <a href="##ad_via_link##" class="iconBarVia" rel="sponsored noopener" target="_blank">ads via BuySellAds</a>\n </div>\n '})};</script><div class="comments formatted" id="comments"><div class="comments__header"><h2 class="comments__header-title"><a href="#comments" name="comments">Comments</a></h2><div class="comments__read-before"><span class="comments__read-before-link">read this before commenting…</span><div class="comments__read-before-popup"><div class="comments__read-before-popup-i"><ul><li>If you have suggestions what to improve - please <a href="https://github.com/javascript-tutorial/en.javascript.info/issues/new">submit a GitHub issue</a> or a pull request instead of commenting.</li><li>If you can't understand something in the article – please elaborate.</li><li>To insert few words of code, use the <code><code></code> tag, for several lines – wrap them in <code><pre></code> tag, for more than 10 lines – use a sandbox (<a href='https://plnkr.co/edit/?p=preview'>plnkr</a>, <a href='https://jsbin.com'>jsbin</a>, <a href='http://codepen.io'>codepen</a>…)</li></ul></div></div></div></div><div id="disqus_thread"></div><script>var disqus_config = function() { if (!this.page) this.page = {}; 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