40 lines
1.5 KiB
Markdown
40 lines
1.5 KiB
Markdown
---
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title: Height transition
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tags: animation,intermediate
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---
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Transitions an element's height from `0` to `auto` when its height is unknown.
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- `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.
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- `overflow: hidden` prevents the contents of the hidden element from overflowing its container.
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- `max-height: 0` specifies that the element has no height initially.
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- `.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.
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- `el.scrollHeight` is the height of the element including overflow, which will change dynamically based on the content of the element.
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- `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.
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- 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.
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```html
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<div class="trigger">
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Hover me to see a height transition.
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<div class="el">Additional content</div>
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</div>
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```
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```css
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.el {
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transition: max-height 0.5s;
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overflow: hidden;
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max-height: 0;
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}
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.trigger:hover > .el {
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max-height: var(--max-height);
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}
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```
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```js
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let el = document.querySelector('.el');
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let height = el.scrollHeight;
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el.style.setProperty('--max-height', height + 'px');
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```
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