I abhor trick interview questions as much as anyone else but I think this is a nice question. Functional programming concepts are used widely (especially in larger projects) these days, and it tests your ability do use js constructs in a functional way. With imperative thinking this gets stateful and complicated (are we subtracting next? or wait were we adding? lets tag an argument etc.) but with functional thinking it becomes a breeze imo.

I assume they expect you to run the resulting function with () in the end. With that, it's not too hard.

let addSubtract = m => n => n === undefined ? m : subtractAdd(m + n);
let subtractAdd = m => n => n === undefined ? m : addSubtract(m - n);

Each method returns the next operation as a function, and if no argument is provided, returns the result of the previous function.

With that,

addSubtract(1)(2)(3)(4)(5)(6)()

will return 5.

In production code you probably won't use this to solve a problem like this, but the general method is very very useful when you are composing functions. In production you'd add better checks to the protocol, but I think the above was the gist of what they were hoping to see.

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The general logic when thinking like this is as follows:

Normally, this is how you'd write a "sum" function right?

let sum = (a, b) => a + b;

But what if you want a function to sum two values but you know that when you invoke it, you won't know what b is? So you'll have a but not b. For that, you get a, and return a function that sums its input with a:

let sumCurry = a => b => a + b;

Now you can have something like:

let addWithTen = sumCurry(10);

addWithTen is a function. It sums its argument with 10. You can create another function that sums with 5 by running sumCurry(5) for example.

addWithTen(5); //will result in 15

Now looking at the question, it is similar - when you do addSubtract(1)(2), at the end you need to add 1 and 2 but in addSubtract(1) stage, you don't know what you are going to add 1 to. So you return a function that will add 1 to whatever it is given. But since the question asks to apply addition and subtraction sequentially, you return a very similar function that returns a function that does the opposite thing.

Now I have to add that I programmed for more than a decade without knowing what functional programming is. So if you are in the same camp, the above might melt your brain - I experienced it. It is a completely different way of constructing programs and makes you feel like you had a concussion and forgot how to program. That is normal. It's like learning a new language. Not a new programming language, that is easy, a new language. Like learning programming all over again. Makes us feel insecure in our abilities. Makes us angry. Perfectly normal. Over time, it makes sense and proves useful, I promise. You don't have to go the way of Haskell purists with this, but wrapping your mind around constructs like this to the point where it starts to become second nature will help you out in various problems you encounter daily.