Use the browser’s IntersectionObserver API to detect when an element enters the viewport, then add a CSS class to reveal it. JavaScript handles the trigger; CSS handles the animation. This avoids repeatedly checking element positions in a scroll handler and works well for a simple “start when visible” effect.
How do I animate an element when it scrolls into view?
Add a class to each element you want to reveal, define its starting and visible styles in CSS, then observe those elements with JavaScript.
1. Mark the elements
Place the reveal class on each element that should appear as the visitor scrolls:
<section class="reveal">This content appears as you scroll.</section>2. Define the transition in CSS
The starting style below makes the element transparent and moves it down slightly. The is-visible class restores full opacity and its original position.
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.reveal {
opacity: 0;
transform: translateY(1rem);
transition: opacity 500ms ease, transform 500ms ease;
}
.reveal.is-visible {
opacity: 1;
transform: translateY(0);
}3. Observe the elements in JavaScript
Create an observer, add the visible class when an element intersects the viewport, and start observing each matching element:
const observer = new IntersectionObserver((entries, observer) => {
for (const entry of entries) {
if (entry.isIntersecting) {
entry.target.classList.add("is-visible");
observer.unobserve(entry.target);
}
}
});
document.querySelectorAll(".reveal").forEach((element) => {
observer.observe(element);
});Put the script after the relevant HTML or run it after the document has loaded, so the elements exist when querySelectorAll() runs. In this example, unobserve() means each reveal happens once. Remove that line if you want the element to respond again when it leaves and re-enters view.
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How do I trigger an animation on scroll with JavaScript?
The key is the observer callback. It receives entries describing observed elements; when an entry’s isIntersecting value is true, the code adds the class that starts the CSS transition. The browser reports intersection changes asynchronously, including when a target enters or exits the viewport or crosses a configured threshold. See MDN’s Intersection Observer API guide.
The example uses the default threshold. You can configure one when creating the observer:
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const observer = new IntersectionObserver(callback, {
threshold: 0.25
});A threshold is a proportion of the target’s area that must intersect before notification—for example, 0.25 represents 25 percent. It is not a trigger at an exact pixel coordinate, nor does the API report exactly which pixels overlap. Use it for approximate visibility conditions, not pixel-precise effects.
Intersection Observer is useful for “start when visible” behavior because the browser manages intersection detection asynchronously. That can be simpler than a scroll handler that repeatedly calls getBoundingClientRect() for every target, but it does not guarantee that every animation will be fast or cost-free.
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What is Intersection Observer?
Intersection Observer is a browser API for asynchronously monitoring the intersection of a target element with an ancestor or the top-level document viewport. Its callback lets you respond to visibility changes without manually measuring every element on every scroll event. MDN describes deciding whether animation work is useful when content is visible as one application of the API: Intersection Observer API.
MDN describes the API as broadly available across many devices and browser versions since March 2019, while noting that some parts may vary. Check compatibility for the browsers your audience needs: MDN compatibility information.
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How do I make a scroll animation accessible?
Respect the visitor’s reduced-motion preference and ensure the content is not left hidden when motion is reduced. Add this rule to the CSS example:
@media (prefers-reduced-motion: reduce) {
.reveal {
opacity: 1;
transform: none;
transition: none;
}
}The media query reflects a device preference to minimize nonessential motion; animation can cause discomfort for some people. With this rule, the content is visible without requiring the transition. MDN explains the preference in its reduced-motion media query reference. W3C WAI also documents applying the preference in JavaScript to prevent interaction-triggered motion as a WCAG technique example, not as the only way to conform: SCR40. MDN’s Web Animations API guidance likewise recommends considering a way to pause or disable animation: Using the Web Animations API.
When should you use CSS scroll-triggered animations instead?
For a basic reveal, Intersection Observer plus a CSS class offers a straightforward JavaScript implementation. CSS also has declarative scroll-triggered animation features, which MDN presents as an alternative to JavaScript approaches such as Intersection Observer. MDN characterizes the CSS approach as simpler and more performant in its guide; that is general guidance, not a universal benchmark: CSS scroll-driven animations.
Do not confuse a scroll trigger with a scroll-driven animation. A scroll-triggered animation starts or stops a regular time-based animation when a scroll condition becomes active. A scroll-driven animation maps progress to a scroll timeline, so the animation advances with the user’s scrolling. Choose the model that matches the effect, and check browser support for the specific CSS feature independently of Intersection Observer.
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