Consider doing this via CSS

[ypantig]

To lessen the use of JavaScript, consider using :before or :after pseudo-classes to add the arrow.

Using the JavaScript method as you have now also hardcodes the arrows to these specific elements which will no longer work in case the client decides to change the menu items.

best-spring-2019/themes/best-bc/js/script.js

Line 46 in 52905a9

let arrowDown = '<button class="arrow-down"></button>';

You could do something like:

.menu-page-has-children::after {
    content: '';
    background-image: url(../../assets/vector-icons/icons/chevrons/down.svg);
}

Essentially the same styling for your .arrow-down class for the pseudo element.