<details>
  <summary>Accordion title</summary>
  <div class="content">
    <p>
      Lorem ipsum dolor sit amet, consistetur apisicing elit. Modi unde, ex rem voluptates autem aliquid veniam quis temporibus repudiandae illo, nostrum, pariatur quae! At animi modi dignissimos corrupti placeat voluptatum!
    </p>
  </div>
</details>

Accordion class

To improve the reusability of the code, we should create an Accordion class. This way we can use every<details></details> Call new Accordion() on the element.

 class Accordion {
  constructor(el) {}
  onClick(e) {}
  shrink() {}
  open() {}
  expand() {}
  onAnimationFinish(open) {}
}

constructor()

The constructor is used to store the data required for each accordion.

 constructor(el) {
  this.el = el;
  this.summary = el.querySelector('summary');
  this.content = el.querySelector('.content');
  this.animation = null;
  this.isClosing = false;
  this.isExpanding = false;
  this.summary.addEventListener('click', (e) => this.onClick(e));
}

onClick()

In the onClick() function, we check if the element is being animate (close or expand). If the user clicks on the accordion while the element is being animate, we need to do this check. If the click speed is too fast, we don't want the accordion to jump from fully expanded to completely closed.<details></details> When an element is opened, the browser will add an open property to it. We can get the value of this property through this.el.open .

 onClick(e) {
  e.preventDefault();
  this.el.style.overflow = 'hidden';
  if (this.isClosing || !this.el.open) {
    this.open();
  } else if (this.isExpanding || this.el.open) {
    this.shrink();
  }
}

shrink()

This shrink() function uses WAAPI's .animate() function. You can read the MDN documentation for more information. WAAPI is very similar to CSS @keyframes . We need to define the start and end keyframes of the animation. In this example, we only need two keyframes, the first is the current height of the element and the second is the height after the accordion is closed. The current height is stored in the startHeight variable. Close height is stored in the endHeight variable, equal to<summary></summary> height.

 shrink() {
  this.isClosing = true;
  const startHeight = `${this.el.offsetHeight}px`;
  const endHeight = `${this.summary.offsetHeight}px`;
  if (this.animation) {
    this.animation.cancel();
  }
  this.animation = this.el.animate({
    height: [startHeight, endHeight]
  }, {
    duration: 400,
    ease: 'ease-out'
  });
  this.animation.onfinish = () => this.onAnimationFinish(false);
  this.animation.oncancel = () => this.isClosing = false;
}

open()

The open() function is called when we want to expand the accordion. This function currently does not control the accordion animation. First, we calculate<details></details> The height of the element and apply it to the element using inline style. Once done, we can set its open property to make the content visible, but since we set overflow: hidden and fixed height for the element, the content is still hidden. Then, we wait for the next frame to call the expand() function and create an animation for the element.

 open() {
  this.el.style.height = `${this.el.offsetHeight}px`;
  this.el.open = true;
  window.requestAnimationFrame(() => this.expand());
}

expand()

The expand() function is similar to the shrink() function, but it does not animation from the current height to the close height, but from the height of the element to the end height. The end height is equal to<summary></summary> height plus the height of the internal content.

 expand() {
  this.isExpanding = true;
  const startHeight = `${this.el.offsetHeight}px`;
  const endHeight = `${this.summary.offsetHeight this.content.offsetHeight}px`;
  if (this.animation) {
    this.animation.cancel();
  }
  this.animation = this.el.animate({
    height: [startHeight, endHeight]
  }, {
    duration: 400,
    ease: 'ease-out'
  });
  this.animation.onfinish = () => this.onAnimationFinish(true);
  this.animation.oncancel = () => this.isExpanding = false;
}

onAnimationFinish()

This function is called when the animation is collapsed or expanded. As you can see, there is a parameter open , set to true when the accordion is turned on, allowing us to set the open HTML attribute on the element, as it is no longer handled by the browser.

 onAnimationFinish(open) {
  this.el.open = open;
  this.animation = null;
  this.isClosing = false;
  this.isExpanding = false;
  this.el.style.height = this.el.style.overflow = '';
}

Set up accordion

We did most of the code!

All that's left is for each of the HTML<details></details> Elements use our Accordion class. To do this, we use<details></details> querySelectorAll on the tag and create a new Accordion instance for each element.

 document.querySelectorAll('details').forEach((el) => {
  new Accordion(el);
});

Notice

To calculate the close height and the open height, we need to ensure<summary></summary> Always have the same height as the content.

For example, don't try<summary></summary> Add padding when turned on, as this may cause jumps during the animation process. The same goes for internal content – ??it should have a fixed height, and we should avoid content with height changes during the animation opening.

Also, don't<summary></summary> Add margins between contents, as it does not calculate height keyframes. Instead, use padding directly on the content to add some spacing.

Summarize

That's it, we created a beautiful accordion animation using JavaScript without any libraries required!

Please note that the image path /uploads/20250331/174338369667e9ec90190a3.jpg needs to be replaced with the actual image path. I can't access or process the image file, so I can only keep the original format.