css function
`sibling-index()` CSS function
The sibling-index() CSS function returns an integer representing the position of the current element in the DOM tree relative to all its sibling elements. The returned value is the index number of the contextual child’s position among all the sibling elements within a parent element, with the first child returning 1 and the last child, returning the length of children.
[!NOTE] Like the
:nth-child()pseudo-class,sibling-index()starts from 1, not 0.
[!NOTE] The
counter()function provides a similar result but it returns a<string>(which is more suitable for generated content, whilesibling-index()returns an<integer>(which can be used for calculations).
--width: calc(sibling-index() * 30px);
--width: calc(sibling-index() * 20px);
--width: calc(sibling-index() * 10px);
--width: 100px;
<ul id="example-element">
<li>1</li>
<li>2</li>
<li>3</li>
<li>4</li>
<li>5</li>
<li>6</li>
<li>7</li>
<li>8</li>
<li>9</li>
<li>10</li>
</ul>
#example-element {
list-style-type: none;
padding: 0;
margin: 0 auto;
display: flex;
flex-direction: column;
align-items: center;
gap: 4px;
}
#example-element > li {
text-align: center;
padding: 2px;
border-radius: 8px;
width: var(--width, calc(sibling-index() * 30px));
color: white;
background-color: hsl(
calc(360deg / sibling-count() * sibling-index()) 50% 50%
);
}
Syntax
sibling-index()
Parameters
The sibling-index() function doesn’t accept parameters.
Return value
An integer; the position of the current element in the DOM tree’s sibling order.
Examples
Dynamic list width
This example demonstrates how to dynamically increase the width of each <li> item in the <ul> by 50px.
HTML
<ul>
<li>One</li>
<li>Two</li>
<li>Three</li>
<li>Four</li>
</ul>
CSS
li {
width: calc(sibling-index() * 50px);
background-color: #ffaaaa;
}
Results
Sequential animations
Combining sibling-index() with CSS animations opens new possibilities. In this example, the opacity of elements in sequential order by setting an animation-delay based on their order in the DOM.
HTML
We include a container element with four children:
<ul>
<li>One</li>
<li>Two</li>
<li>Three</li>
<li>Four</li>
</ul>
CSS
We apply the fade-in animation to each element. We use the sibling-index() function within a calc() function to set the duration of the animation-delay based on the source element’s position in the source order. The animation-fill-mode applies the animation’s 0% keyframe until the animation-duration expires.
ul {
list-style-type: none;
padding: 0;
margin: 0;
}
li {
animation-name: fade-in;
animation-duration: 1s;
animation-iteration-count: 1;
animation-timing-function: linear;
animation-fill-mode: backwards;
animation-delay: calc(1s * sibling-index());
}
@keyframes fade-in {
from {
opacity: 0;
}
to {
opacity: 1;
}
}