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

Clearly there are always multiple solutions, but prioritising the possible answers as follows seems to succeed:
(1) Solutions that don't involve fractions.
(2) Solutions that refer to the "first seen" variables in the list of examples.

OP solved it, closing

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".

I would love to know too. Two people already tried to explain it to me and failed ;D There must be a reason why this kata is tagged as puzzle.

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.

good

That is just an artifact of how the test results are printed, it's not an error in your solution.

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.