Javascript scroll event direction

Mastering Scroll Direction Detection in JavaScript: Best Practices and Examples

Learn how to detect scroll direction using JavaScript and improve your website’s performance. Our guide covers best practices, code examples, and more.

• Detecting Scroll Direction in JavaScript
• React Hook for Detecting Scroll Direction
• The “scroll” event JavaScript Tutorial
• Adding Event Listener to Scroll in JavaScript
• Using Intersection Observer Instead of Scroll Event in JavaScript
• Implementing Infinite Scroll with Scroll Event
• Other simple examples of code for scroll up or down event listener
• Conclusion
• How do I know if my scroll is up or down?
• Which direction is scrolling up?
• How to get scroll up or down in JavaScript?
• How do I add an event listener to scroll?

As online user experience becomes more and more important, having a smooth and intuitive interface is crucial for the success of your website or application. One aspect of this is detecting the scroll direction of the user, which can be used to trigger animations, load more content, or simply provide a better browsing experience. In this blog post, we will explore the best practices and examples for mastering scroll direction detection using JavaScript.

Detecting Scroll Direction in JavaScript

Detecting the scroll direction of the user is a relatively simple task in JavaScript. The window object provides two built-in methods that can be used for this purpose: addEventListener() and scroll() . The addEventListener() method can be used to listen for scroll events, while the scroll() method returns the current scroll position of the page.

To detect the scroll direction, we need to compare the previous scroll position with the current one. This can be achieved by storing the previous scroll position in a variable and comparing it to the current position in the scroll event listener. Here’s an example code snippet:

let previousScrollY = 0;window.addEventListener('scroll', () =>  const currentScrollY = window.scrollY; if (currentScrollY > previousScrollY)  console.log('User is scrolling down'); > else  console.log('User is scrolling up'); > previousScrollY = currentScrollY; >); 

It’s important to note that adding an event listener to the scroll event can negatively impact performance, especially on mobile devices. To avoid this, we can use the passive event listener option, which tells the browser that our event handler will not prevent scrolling. Here’s an updated example with the passive option:

let previousScrollY = 0;window.addEventListener('scroll', (event) =>  const currentScrollY = window.scrollY; if (currentScrollY > previousScrollY)  console.log('User is scrolling down'); > else  console.log('User is scrolling up'); > previousScrollY = currentScrollY; >,  passive: true >); 

React Hook for Detecting Scroll Direction

If you’re using React, you can use the useState() hook to detect the scroll direction. Here’s an example code snippet:

import React,  useState, useEffect > from 'react';function App()  const [scrollDirection, setScrollDirection] = useState(null); const handleNavigation = () =>  const currentScrollY = window.scrollY; if (currentScrollY > previousScrollY)  setScrollDirection('down'); > else  setScrollDirection('up'); > previousScrollY = currentScrollY; > useEffect(() =>  window.addEventListener('scroll', handleNavigation,  passive: true >); return () =>  window.removeEventListener('scroll', handleNavigation); >; >, []); return ( div> scrollDirection && p>User is scrolling scrollDirection>/p>> /* rest of the app */> /div> ); > 

This code sets up the handleNavigation() function as the event handler for the scroll event, and uses the useState() hook to update the scroll direction state. The useEffect() hook is used to add and remove the event listener.

The “scroll” event JavaScript Tutorial

Learn about JavaScript’s scroll event in this coding tutorial. I show you two ways to listen to Duration: 7:58

Adding Event Listener to Scroll in JavaScript

If you prefer to use vanilla JavaScript, you can add an event listener to the scroll event using the addEventListener() method. Here’s an example code snippet:

let previousScrollY = 0;document.addEventListener('scroll', (event) =>  const currentScrollY = window.scrollY; if (currentScrollY > previousScrollY)  console.log('User is scrolling down'); > else  console.log('User is scrolling up'); > previousScrollY = currentScrollY; >,  passive: true >); 

Note that we’re using the document object instead of the window object, which captures all scroll events on the page. It’s also important to set up the event listener only once, as adding multiple listeners can also negatively impact performance. To do this, we can simply move the addEventListener() call outside of any function or event.

Using Intersection Observer Instead of Scroll Event in JavaScript

The Intersection Observer API is a relatively new feature in JavaScript that can be used to detect when an element enters or leaves the viewport. It can be used instead of the scroll event to improve performance and reduce jank. Here’s an example code snippet:

const options =  root: null, rootMargin: '0px', threshold: 0.5 >;const callback = (entries, observer) =>  entries.forEach(entry =>  if (entry.isIntersecting)  console.log('Element is in viewport'); > else  console.log('Element is out of viewport'); > >); >;const observer = new IntersectionObserver(callback, options); observer.observe(document.querySelector('#target')); 

This code sets up an IntersectionObserver object and uses it to observe an element with the #target ID. The threshold option specifies at what percentage of the target’s visibility the observer’s callback should be executed. In this case, it will be executed when the target is at least 50% visible.

Implementing Infinite Scroll with Scroll Event

Infinite scroll is a popular feature in web applications that automatically loads more content as the user scrolls down. It can be implemented using the scroll event and AJAX requests. Here’s an example code snippet:

let page = 1;document.addEventListener('scroll', (event) =>  const currentScrollY = window.scrollY; const maxScrollY = document.body.scrollHeight - window.innerHeight; if (currentScrollY >= maxScrollY)  page += 1; fetch(`/api/posts?page=$page>`) .then(response => response.json()) .then(data =>  // append new posts to the DOM >); > >,  passive: true >); 

This code sets up the scroll event listener and uses an AJAX request to fetch more posts when the user reaches the bottom of the page. Note that we’re keeping track of the current page number using a variable.

Other simple examples of code for scroll up or down event listener

In Javascript , for instance, scroll down or up event listener code example

var lastScrollTop = 0;// element should be replaced with the actual target element on which you have applied scroll, use window in case of no target element. element.addEventListener("scroll", function() < // or window.addEventListener("scroll". var st = window.pageYOffset || document.documentElement.scrollTop; // Credits: "https://github.com/qeremy/so/blob/master/so.dom.js#L426" if (st >lastScrollTop) < // downscroll code >else < // upscroll code >lastScrollTop = st , false);

Conclusion

In this blog post, we’ve explored the best practices and examples for mastering scroll direction detection using JavaScript. We’ve covered different ways to detect scroll direction, including using built-in JavaScript methods, React hooks, and the Intersection Observer API. We’ve also discussed best practices for implementing a scroll event listener, such as using the passive event listener option and the throttle function. Finally, we’ve shown how to implement the popular infinite scroll feature using the scroll event and AJAX requests. By following these best practices and examples, you can provide a better browsing experience for your users and improve the performance of your web applications.

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How to Detect Scroll Direction with JavaScript?

Sometimes, we want to detect scroll direction with JavaScript.

In this article, we’ll look at how to detect scroll direction with JavaScript.

Listen to the scroll Event

We can listen to the scroll event of window to check the scroll direction.

For instance, we can write:

window.onscroll = function(e) < console.log(this.oldScroll >this.scrollY); this.oldScroll = this.scrollY; > 

to set a scroll event listener to the window.onscroll property.

Then we get the scrollY value and check if it’s bigger than the oldScroll value.

If it’s bigger, that means we’re scrolling down.

Then we set the scrollY value to the oldScroll property so that we can keep the old scroll value.

Both values are in pixels so we can compare them directly.

Listen to the scroll Event and Track the pageYOffset Property

We can also use the pageYOffset property to track the location that we’ve scrolled to vertically.

For instance, we can write:

let oldValue = 0 let newValue = 0 window.addEventListener('scroll', (e) => < newValue = window.pageYOffset; if (oldValue < newValue) < console.log("Up"); >else if (oldValue > newValue) < console.log("Down"); >oldValue = newValue; >); 

to add a scroll listener to window with addEventListener .

Then we set newValue to window.pageYOffset .

If newValue is bigger than oldValue , then we scrolled up.

If newValue is smaller than oldValue , then we scrolled down.

In the end, we set newValue to oldValue so we can compare the 2.

Conclusion

We can detect scroll direction with the pageYOffset or the scrollY properties.

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By John Au-Yeung

Web developer specializing in React, Vue, and front end development.

2 replies on “How to Detect Scroll Direction with JavaScript?”

This is not a complaint, I came across your page when googling for something else, but…

…it’s interesting that you use two different methods of event assignment, literal .onscroll as well as the addEventListener without offering any reasons as to which you’d use and why. You capture the event object (e) in both situations, yet don’t even bother to use it, instead preferring to directly set the window object. Finally you swap between using .scrollY and pageYOffset but provide no information as to why there are two (seemingly) identical properties and which you should use dependent on your delivery platform.

None of your code is incorrect, that’s not the point of this comment. It’s that your article could be far more comprehensive with very little effort.

Nothing in your code is wrong, I’m not suggesting that.

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How to Detect Scroll Direction in JavaScript

Typically, web developers add event handlers to listen to scrolling event. But sometimes you may need to call functions or features based on scroll direction. In this article, we will learn how to detect scroll direction using JavaScript. There are many third-part libraries for this purpose, but we can easily do this using plain JavaScript.

How to Detect Scroll Direction in JavaScript

Let us say you have a div myDiv whose scrolling direction you need to detect. The first step is to select the element.

var d = document.getElementById("myDiv);

We define a variable to store the position of last scrollTop value and initialize it to zero.

Next we add an event handler for scroll event. When the user scrolls up or down, the scroll event is frequently called.

element.addEventListener("scroll", function() < var st = window.pageYOffset || document.documentElement.scrollTop; if (st >lastScrollTop) < // downscroll code >else < // upscroll code >lastScrollTop = st , false);

Every time the event handler for scrolling is called, we retrieve the scrolltop value, compare it with the lastScrollTop value to determine if the scrolling is upwards or downwards. If current scrolltop value is greater than the lastScrollTop value it means user is scrolling down, else scrolling up. We also store the current ScrollTop value as the lastScrollTop value for later use.

Instead of adding the event handler to an element, you can also add it to the entire window if you want, as shown below.

// or window.addEventListener("scroll".

In this article, we have learn how to detect scroll direction in JavaScript. You can customize it as per your requirement.

It is useful to add customized functionality depending on direction of scrolling for users.

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