If you are writing ES6 syntax in designer's script library, you will get an error - ""const is a reserve identified".

I think designer default to ES5 syntax checking.

To work around, you can write your javascript at "WebContent" folder using Package Explorer. For this example, I will put my javascript in "javascripts" folder.

This is how you point the javascript:

  <script src"javascript/like_button.js"></script>

Reference:
https://hclpnpsupport.hcltech.com/csm?id=community_question&sys_id=3bc76f951bbc481877761fc58d4bcb5e