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css/snippets/mouse-cursor-gradient-tracking.md

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+---
+title: Mouse cursor gradient tracking
+type: snippet
+tags: [visual,interactivity]
+cover: tram-car
+dateModified: 2021-01-07T23:52:15+02:00
+---
+
+A hover effect where the gradient follows the mouse cursor.
+
+- Declare two CSS variables, `--x` and `--y`, used to track the position of the mouse on the button.
+- Declare a CSS variable, `--size`, used to modify the gradient's dimensions.
+- Use `background: radial-gradient(circle closest-side, pink, transparent)` to create the gradient at the correct position.
+- Use `Document.querySelector()` and `EventTarget.addEventListener()` to register a handler for the `'mousemove'` event.
+- Use `Element.getBoundingClientRect()` and `CSSStyleDeclaration.setProperty()` to update the values of the `--x` and `--y` CSS variables.
+
+```html
+<button class="mouse-cursor-gradient-tracking">
+  <span>Hover me</span>
+</button>
+```
+
+```css
+.mouse-cursor-gradient-tracking {
+  position: relative;
+  background: #7983ff;
+  padding: 0.5rem 1rem;
+  font-size: 1.2rem;
+  border: none;
+  color: white;
+  cursor: pointer;
+  outline: none;
+  overflow: hidden;
+}
+
+.mouse-cursor-gradient-tracking span {
+  position: relative;
+}
+
+.mouse-cursor-gradient-tracking::before {
+  --size: 0;
+  content: '';
+  position: absolute;
+  left: var(--x);
+  top: var(--y);
+  width: var(--size);
+  height: var(--size);
+  background: radial-gradient(circle closest-side, pink, transparent);
+  transform: translate(-50%, -50%);
+  transition: width 0.2s ease, height 0.2s ease;
+}
+
+.mouse-cursor-gradient-tracking:hover::before {
+  --size: 200px;
+}
+```
+
+```js
+let btn = document.querySelector('.mouse-cursor-gradient-tracking');
+btn.addEventListener('mousemove', e => {
+  let rect = e.target.getBoundingClientRect();
+  let x = e.clientX - rect.left;
+  let y = e.clientY - rect.top;
+  btn.style.setProperty('--x', x + 'px');
+  btn.style.setProperty('--y', y + 'px');
+});
+```