Deleting code should free resources properly

[MajeedKazemi]

• remove any assigned callbacks
• make sure that it will be freed by the garbage collector: set all pointers to null, and make sure nothing is referencing it within the reference tree

nice documentation: https://developer.mozilla.org/en-US/docs/Web/JavaScript/Memory_Management#garbage_collection

[MajeedKazemi]

should also do the same with replacements:

• replace expression
• replace statement