To get the scrollbar width, we can create an element with the scroll, but without borders and paddings.

Then the difference between its full width `offsetWidth` and the inner content area width `clientWidth` will be exactly the scrollbar:

```js run
// create a div with the scroll
let div = document.createElement('div');

div.style.overflowY = 'scroll';
div.style.width = '50px';
div.style.height = '50px';

// must put it in the document, otherwise sizes will be 0
document.body.append(div);
let scrollWidth = div.offsetWidth - div.clientWidth;

div.remove();

alert(scrollWidth);
```