document.querySelector('.button').onmousemove = (e) => {
  const x = e.pageX - e.target.offsetLeft
  const y = e.pageY - e.target.offsetTop

  e.target.style.setProperty('--x', `${ x }px`)
  e.target.style.setProperty('--y', `${ y }px`)

}

.button {
  position: relative;
  appearance: none;
  background: #f72359;
  padding: 1em 2em;
  border: none;
  color: white;
  font-size: 1.2em;
  cursor: pointer;
  outline: none;
  overflow: hidden;
  border-radius: 100px;

  span {
    position: relative;
  }

  &::before {
    --size: 0;  

    content: '';
    position: absolute;
    left: var(--x);
    top: var(--y);
    width: var(--size);
    height: var(--size);
    background: radial-gradient(circle closest-side, #4405f7, transparent);
    transform: translate(-50%, -50%);
    transition: width .2s ease, height .2s ease;
  }

  &:hover::before {
    --size: 400px;
  }
}