---
title: Height transition
tags: animation
---

Transitions an element's height from `0` to `auto` when its height is unknown.

```html
<div class="trigger">
  Hover me to see a height transition.
  <div class="el">content</div>
</div>
```

```css
.el {
  transition: max-height 0.5s;
  overflow: hidden;
  max-height: 0;
}

.trigger:hover > .el {
  max-height: var(--max-height);
}
```

```js
var el = document.querySelector('.el')
var height = el.scrollHeight
el.style.setProperty('--max-height', height + 'px')
```

#### Explanation

1. `transition: max-height: 0.5s cubic-bezier(...)` specifies that changes to `max-height` should be transitioned over 0.5 seconds, using an `ease-out-quint` timing function.
2. `overflow: hidden` prevents the contents of the hidden element from overflowing its container.
3. `max-height: 0` specifies that the element has no height initially.
4. `.target:hover > .el` specifies that when the parent is hovered over, target a child `.el` within it and use the `--max-height` variable which was defined by JavaScript.

---

1. `el.scrollHeight` is the height of the element including overflow, which will change dynamically based on the content of the element.
2. `el.style.setProperty(...)` sets the `--max-height` CSS variable which is used to specify the `max-height` of the element the target is hovered over, allowing it to transition smoothly from 0 to auto.

#### Browser Support

<div class="snippet__requires-javascript">Requires JavaScript</div>
<span class="snippet__support-note">
  ⚠️ Causes reflow on each animation frame, which will be laggy if there are a large number of elements
  beneath the element that is transitioning in height.
</span>

- https://caniuse.com/#feat=css-variables
- https://caniuse.com/#feat=css-transitions