Custom Hooks – Complete Intro to React v7

For now, we're going to make a custom hook of our own. Just like useState is a hook, there are a few others like useEffect (which we'll use in this lesson), useReducer (for doing Redux-like reducers), useRefs (for when you need to have programmatic access to a DOM node), and useContext (for using React's context which we'll do shortly as well.) But like React hooks, we can use these hooks to make our re-usable hooks.

We need a list of breeds based on which animal is selected. In general this would be nice to request once and if a user returns later to the same animal, that we would have some cache of that. We could implement it in the component (and in general I probably would, this is overengineering it for just one use) but let's make a custom hook for it.

Make a new file called useBreedList.js in src and put this in it.

import { useState, useEffect } from "react";

const localCache = {};

export default function useBreedList(animal) {
  const [breedList, setBreedList] = useState([]);
  const [status, setStatus] = useState("unloaded");

  useEffect(() => {
    if (!animal) {
      setBreedList([]);
    } else if (localCache[animal]) {
      setBreedList(localCache[animal]);
    } else {
      requestBreedList();
    }

    async function requestBreedList() {
      setBreedList([]);
      setStatus("loading");
      const res = await fetch(
        `http://pets-v2.dev-apis.com/breeds?animal=${animal}`
      );
      const json = await res.json();
      localCache[animal] = json.breeds || [];
      setBreedList(localCache[animal]);
      setStatus("loaded");
    }
  }, [animal]);

  return [breedList, status];
}

We're using hooks inside of our custom hook. I can't think of a custom hook you would make that wouldn't make use of other hooks.
We're returning two things back to the consumer of this custom hook: a list of breeds (including an empty list when it doesn't have anything in it) and an enumerated type of the status of the hook: unloaded, loading, or loaded. We won't be using the enum today but this is how I'd design it later if I wanted to throw up a nice loading graphic while breeds were being loaded.
We're tossing in localCache so if it loads once, it won't have to reload the same API call in the same session. You could take this further by sticking it in local storage or we could be more intelligent about ETags.

Head over to SearchParam.js and put this in there.

import useBreedList from "./useBreedList";

// replace `const breeds = [];`
const [breeds] = useBreedList(animal);

That should be enough! Now you should have breeds being populated every time you change animal! (Do note we haven't implemented the submit button yet though.)

🏁 Click here to see the state of the project up until now: 06-custom-hooks

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