A tiny, accessible, Promise-based, jQuery-reminiscent library for sliding elements with dynamic heights open & closed.

To see it in action, check out the following demos:

• Project Landing Page
• CodePen Example

Using JavaScript to animate an element open and closed hasn't traditionally been a straightforward task, especially if it contains dynamically sized content. You could go with something like jQuery's slideToggle(), but that path would require you to take on a lot more code than necessary. Another option is using CSS to change the max-height value of an element, but this approach is filled with arbitrariness and difficult to pull off well when you're not sure how much content you'll be animating over.

This library gets the job done using the Web Animations API, and it doesn't require elements to have fixed heights. Instead, element heights are calculated based on their contents, and then the appropriate values are applied to trigger a smooth, native transition. The animations themselves are powered by the same mechanics used within CSS transitions, making it one of the best ways to pull it off in terms of performance.

It's small, smooth, and focuses on doing one job well: sliding stuff open and closed.

Run npm install slide-element, or use a CDN like unpkg.

Make sure your target element is set to display: none;.

Use the toggle function to slide an element open & closed based on its current state.

import { toggle } from "slide-element";

document.getElementById("button").addEventListener("click", (e) => {
  toggle(document.getElementById("box"));
});

Use the down function to slide an element open.

import { down } from "slide-element";

document.getElementById("button").addEventListener("click", (e) => {
  down(document.getElementById("boxToSlideOpen"));
});

Use the up function to slide an element closed, and then set its display property to none.

import { up } from "slide-element";

document.getElementById("button").addEventListener("click", (e) => {
  up(document.getElementById("boxToSlideClosed"));
});

Each of the functions provided return promises, so you can easily wait to perform an action after an animation is complete. The resolved value will indicate if the element has just been opened (true), closed (false), or the result of an animation that interruped another (see more below).

import { toggle } from "slide-element";

document.getElementById("button").addEventListener("click", (e) => {
  toggle(document.getElementById("someElement")).then((isOpen: boolean | null) => {
    console.log("Toggling is done!");
  });
});

Depending on your settings, some users may be able to repeatedly trigger a new before a previous one has been allowed to finish, which will cause the in-progress animation instantly finish before the new one can begin.

When this occurs, the isOpen Promise that resolves after the animation is complete will return null for each animation that was triggered in interruption of the first. The initial animation, however will still resolve to the correct value. For example, pretend the following animation is clicked rapidly three times in a row.

import { toggle } from "slide-element";

document.getElementById("button").addEventListener("click", (e) => {
  toggle(document.getElementById("someElement")).then((isOpen: boolean | null) => {
    console.log(isOpen);
  });
});

When the animation has been allowed to complete, the following values will be logged -- two null values for the interrupting animations, and one boolean for the initial (and now complete) one. The point here is that it may be necessary to explicitly check for a non-null value when using the resolved "open" state.

true
null
null

If the element you're animating has any padding set to it, be sure to also apply box-sizing: border-box as well. If you don't, the resulting animation will be weird and jumpy.

<div id="myTarget" style="padding: 1rem; box-sizing: border-box; display: none;">
  My contents!
</div>

By default, slide-element uses the following transition property values:

You can override these by passing an object as the seceond parameter of any method:

up(document.getElementById("element"), {
  duration: 500,
  easing: "ease-in-out",
});

Out of the box, slide-element will set your opened element to display: block;. If you'd like to customize this, pass a display value as an option:

down(document.getElementById("element"), {
  display: "flex"
});

Out of the box, slide-element will animate your element open & closed with overflow: hidden;. If you'd like to pass your own value, use the overflow option:

down(document.getElementById("element"), {
  overflow: "auto"
});

If you'd like to use slide-element directly in the browser via CDN, simply load the code, and then reference the function you'd like to use on the global SlideElement object:

<script src="./path/to/slide-element.js"></script>
<script>
  document.getElementById('someElement').addEventListener('click', (e) => {
    SlideElement.toggle(document.getElementById("someBox"));
});
</script>

// Toggle an element based on current state.
toggle(element: HTMLElement, options?: object): Promise<boolean>

// Slide an element down.
up(element: HTMLElement, options?: object): Promise<boolean>

// Slide an element down.
down(element: HTMLElement, options?: object): Promise<boolean>

This library will respect the prefers-reduced-motion setting on a user's machine. When it's set to reduce, the sliding animation will be forced to a duration of 0, making the respective elements open and close instantly.

Additionally, it's highly recommended that you toggle the aria-expanded attribute on any element (like a button) that's responsible for triggering an animation. This can be done by adding a single line of code that fires afters an animation is complete:

document.getElementById('someButton').addEventListener('click', (e) => {
    toggle(document.getElementById('thing2')).then((opened) => {

      <!-- Set the appropriate `aria-expanded` value based on the state of the container. -->
      e.target.setAttribute("aria-expanded", opened);
    });
  });

I love to see examples of how you're using the stuff I build. If you're comfortable, please send it my way!