Refactor functions to use currying

Question

I have three functions that I would like to refactor using currying but I am struggling get my head around functions that return functions. I am trying to break down and design the curried functions as a starting point.

I found this medium article that gives a very simple design.

function discount(discount) {
    return (price) => {
        return price * discount;
    }
}

const tenPercentDiscount = discount(0.1);

tenPercentDiscount(500);

At a high level, my functions get called like so:

getCatHelper(prioritisedBreeds, catDefintions, debugToggle)
  .then((breed) => setConfigData(breed, config))
  .catch(() => false);

• if: debugToggle - it passes it mapCatValueToKey to get the cat breed
• else: makes a network request, passes an array of catBreeds to getPriorityBreed
• sets some data with the returned breed and a config within scope

getPriorityBreed(prioritisedBreeds, catBreeds, catDefintions)

• calls mapCatValueToKey to map over the catBreeds and make a new array with the catDefintions
• loops through two lists - the prioritisedBreeds and the mappedCats from above to return the priority breed

mapCatValueToKey(breed, catDefintions)

• mapping function to map a breed (value) to a cat definition object

---

Here's how getCatHelper is being called:

function useCatHook(
  config: Config,
  catDefintions: any = CAT_DEFS,
  getCat: (
      prioritisedBreeds,
      catDefintions,
      debugToggle,
  ) = getCatHelper,
  debugToggle,
): any {
  const [data, setData] = useState(undefined);
  const prioritisedBreeds = Object.keys(config);

  // ...

  getCatHelper(prioritisedBreeds, catDefintions, debugToggle)
      .then(breed => setData(breed))
      .catch(() => false);

  // ...
}

My questions are:

1. How do I design this?
2. Should I care about the internals or just design the function and the function it returns?
3. When refactoring, should I be working from the outer or the inner function first?

Any help would be appreciated :)

Answer 1

I wanted the hook not to know about CAT_DEFS and to pass it straight to the getCatHelper.

I made this work:

export function getCatHelper {
    return (
        prioritisedBreeds,
        catDefintions,
        debugToggle,
        getDataLayer = getDataFromRequest
    ) {

    // ...

    };
}

function useCatHook(
  config,
  getCat (
      prioritisedBreeds,
      catDefintions,
      debugToggle
  ) = getCatHelper(CAT_DEFS),
  debugToggle
) {

  // ...

}