shadow left/right on scroll

by tomik23

HTML

<div id="scrollbox" class="right-border">
    <ul id="myDIV">
      <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>
      <li>11</li>
      <li>12</li>
      <li>13</li>
      <li>14</li>
      <li>15</li>
      <li>16</li>
      <li>17</li>
      <li>18</li>
      <li>19</li>
      <li>20</li>
    </ul>
  </div>

SCSS

#scrollbox {
  overflow-x: auto;
  width: 500px;
  max-height: 220px;
  margin: 0 auto;

  ul {
    display: flex;
    flex-direction: row;
    white-space: nowrap;
    min-width: 100vw;
    height: 130px;
    margin: 0;
    padding: 0;
    list-style: none;

    li {
      display: flex;
      justify-content: center;
      align-items: center;
      text-align: center;
      min-width: 130px;
      border: 1px solid red;
      margin: 0 2px;
    }
  }
}

.left-border {
  box-shadow: inset 10px 0 5px -5px rgba(0, 0, 0, 0.25);
}

.right-border {
  box-shadow: inset -10px 0 5px -5px rgba(0, 0, 0, 0.25);
}

.center-border {
  box-shadow: inset 10px 0 5px -5px rgba(0, 0, 0, 0.25), inset -10px 0 5px -5px rgba(0, 0, 0, 0.25);
}

JavaScript

function myFunction() {
  let el = document.getElementById('scrollbox');
  let div = document.getElementById('myDIV');

  console.log(el.scrollLeft, div.scrollWidth, el.offsetWidth);
  if(el.scrollLeft > 40) {
    el.classList.add('center-border');
    el.classList.remove('right-border');
  } else {
    el.classList.add('right-border');
    el.classList.remove('center-border');
  }
  if(el.scrollLeft < div.scrollWidth - el.offsetWidth) {
    el.classList.remove('left-border');
  } else {
    el.classList.add('left-border');
    el.classList.remove('center-border');
  }
}

document.getElementById('scrollbox').addEventListener('scroll', myFunction);