I liked the economy in this, and it builds the row of Pascal's triangle faster than using Lucas's method (which was my first solution). So I wrote my own version of this method which is a bit more annotated.

The penny is beginning to drop. There is an inferred 'return' after an arrow, so your arrow functions return an arrow function. To begin with I was trying to read it as a function of two variables, which of course is different.

Hi, szib
I've got to say that I struggle to understand this because of the way functions are being used as variables. I will have to look this up in the context of arrow functions.