This is nice except that every single one of 4 ifs will be tested even though at most 1 can be truthy.

Please add some comment to your solution. It is giberrish IMO

Why? You are reducing to a single number according to the data ops. The only time there is no operation on that number is for operation o , which just pushes the number to the main function's output array. Seems pretty straightforward.
I've done it a bit differently in that my reduce function builds the main output array but I like this top solution, focusing on the number itself.

I'm sorry. How careless of me.

The board is always 9 cells by 9 cells, and every cell only contains integers from 0 to 9.

Wrong solution.How about board with -1 and 10 ? just like this(all 1 is replaced by -1 and all 9 is replaced by 10)
[
[5, 3, 4, 6, 7, 8, 10, -1, 2],
[6, 7, 2, -1, 10, 5, 3, 4, 8],
[-1, 10, 8, 3, 4, 2, 5, 6, 7],
[8, 5, 10, 7, 6, -1, 4, 2, 3],
[4, 2, 6, 8, 5, 3, 7, 10, -1],
[7, -1, 3, 10, 2, 4, 8, 5, 6],
[10, 6, -1, 5, 3, 7, 2, 8, 4],
[2, 8, 7, 4, -1, 10, 6, 3, 5],
[3, 4, 5, 2, 8, 6, -1, 7, 10],
]

Using reduce as a quasi loop without even returning any reduced value is kind of ugly to my taste.

Прекрасное решение через Set! Однако, с точки зрения алгоритма, код можно улучшить.
Цикл for следовало бы включить в функцию validSet, и затем проверять валидность поэтапно, а не единожды в конце...
Ведь если, к примеру, строки (rowSet) не валидны, то составлять столбцы (columnSet) и квадраты (boxSet, boxCorner) дальше уже не имеет смысла.
Да, при этом код стал бы внешне чуть менее лаконичным, но думаю это было бы логичнее и производительнее.

Although it checks for uniqueness, the numbers could still be greater than 9

good

This is the kind of functional programming i am striving to learn how to write.
Super clever

boxCorner is so impressive how did u come with this idea i mean how good are u at math ? i want to become better i u can give me some tips

Very elegant use of the Set here. Congratulations.

Can somebody explain this answer please?