For now, the most difficult task that I have been solving.

Nice very hard kata, as 2kyu.

Therefore, in my JavaScript solution, I had to supply "1x" in the answer for the test, where the formula is "c".

Why simplify(["x = b","b = c","c = d","d = e"], "c") == "1x" ? Why isn't the answer "1b"? ? Where is the logic ?

Without RegExp, strong.

Select Node 10.

JS: expected: '[12, 750]', instead got: '[6, 750]'. But only '[6, 750]' is correct? Or expected: '[7, 444]', instead got: '[1, 444]'...

The difficulty is to subtract combinations with a 0 at the beginning from all combinations, it is not clear how to do this.

It would be necessary to remove the parentheses in the problem statement, they are not in the tests.

Before that, I was not aware of the problems of the % operator with large numbers in JavaScript.

Strong.

Testing for:
numbers = Array too big to be displayed ( don't understand

Nice solution for swap.