In the add() function inlinked-list.js I see the following line of code:
if (current.next) { current.next.previous = newNode; } // <7>
What is the reason behind this line of code? If we are inserting behind current, I don't know why would you set the current.next.previous to the node we wanna insert.

https://github.com/amejiarosario/dsa.js/blob/cc1ccc0183ea5d4675815413b7d1d4cc14da348c/src/data-structures/linked-lists/linked-list.js#L83-L105