This is quite absurd.

1. I think undefined and null are just opinionated and they have different use cases anyways, because you can hardly get null out of javascript itself. It needs to be set from you. They are just different.

2. If you don't know reduce or reduceRight, there's a low chance that you are a practical, experienced developer.

3.const [, , foo] = parts and const [foo] = parts are just different. The first expression gets the third element from the array, while the second expression gets the first element out of the array. I am lost as to your intention on what you are trying to say here.

4. I stopped reading then.