Valid useCases

• Subscribing to events (window, ...)
• Releasing resources to prevent a memory leak
• Cancelling a network request/asynchronous operation

function getWindowDimensions() {
  const { innerWidth: width, innerHeight: height } = window;
  return { width, height };
}
export function MyComponent() {
  const [windowDimensions, setWindowDimensions] = useState(getWindowDimensions());
 
  const handleResize = useCallback(() => {
    setWindowDimensions(getWindowDimensions());
  }, []);
 
  useEffect(() => {
    handleResize();
    window.addEventListener('resize', handleResize);
    return () => window.removeEventListener('resize', handleResize);
  }, [handleResize]);
/* ... */
function getWindowDimensions() {
  const { innerWidth: width, innerHeight: height } = window;
  return { width, height };
}
export function MyComponent() {
  const [windowDimensions, setWindowDimensions] = useState(getWindowDimensions());
 
  const handleResize = useCallback(() => {
    setWindowDimensions(getWindowDimensions());
  }, []);
 
  useEffect(() => {
    handleResize();
    window.addEventListener('resize', handleResize);
    return () => window.removeEventListener('resize', handleResize);
  }, [handleResize]);
/* ... */
function getWindowDimensions() {
  const { innerWidth: width, innerHeight: height } = window;
  return { width, height };
}
export function MyComponent() {
  const [windowDimensions, setWindowDimensions] = useState(getWindowDimensions());
 
  const handleResize = useCallback(() => {
    setWindowDimensions(getWindowDimensions());
  }, []);
 
  useEffect(() => {
    handleResize();
    window.addEventListener('resize', handleResize);
    return () => window.removeEventListener('resize', handleResize);
  }, [handleResize]);
/* ... */
function getWindowDimensions() {
  const { innerWidth: width, innerHeight: height } = window;
  return { width, height };
}
export function MyComponent() {
  const [windowDimensions, setWindowDimensions] = useState(getWindowDimensions());
 
  const handleResize = useCallback(() => {
    setWindowDimensions(getWindowDimensions());
  }, []);
 
  useEffect(() => {
    handleResize();
    window.addEventListener('resize', handleResize);
    return () => window.removeEventListener('resize', handleResize);
  }, [handleResize]);
/* ... */

Cleanup is necessary for...

• Unsubscribing from a pub-sub interface
• Releasing resources to prevent a memory leak
• Cancelling a network request/asynchronous operation

useEffect as last-resort

• Don't use useEffect to update state based on props or state, whenever possible calculate all dependent states on the fly.
  If the calculation is expensive, use useMemo
• Don't invoke callbacks on state change, whenever possible invoke callbacks as part of event handling logic.

Add a search input and use useEffect to implement a debounce on the pokemon list as a filter.

Use setTimeout & clearTimeout to build a delay

const timeoutId = setTimeout(
  () => console.log('execute with timeout'), 300);
clearTimeout(timeoutId); 
const timeoutId = setTimeout(
  () => console.log('execute with timeout'), 300);
clearTimeout(timeoutId); 
const timeoutId = setTimeout(
  () => console.log('execute with timeout'), 300);
clearTimeout(timeoutId); 
const timeoutId = setTimeout(
  () => console.log('execute with timeout'), 300);
clearTimeout(timeoutId); 

SearchPanel.tsx

type Props = {
  searchTerm: string;
  onSearchChanged: (searchTerm: string) => void;
};
 
function SearchPanel({ searchTerm, onSearchChanged }: Props) {
  return (
    <div >
      <label htmlFor="searchTerm">Search</label>
      <input
        type="text"
        id="searchTerm"
        value={searchTerm}
        placeholder="Pokemon name"
        onChange={(e) => {
          onSearchChanged(e.target.value);
        }}
      />
    </div>
  );
}
 
export { SearchPanel };

ListPage.tsx

function ListPage() {
  const [searchTerm, setSearchTerm] = useState("");
  const [debouncedSearchTerm, setDebouncedSearchTerm] = useState("");
 
  useEffect(() => {
    const timeoutId = setTimeout(() => setDebouncedSearchTerm(searchTerm), 300);
    return () => clearTimeout(timeoutId);
  }, [searchTerm]);
  /* ... */
}
function ListPage() {
  const [searchTerm, setSearchTerm] = useState("");
  const [debouncedSearchTerm, setDebouncedSearchTerm] = useState("");
 
  useEffect(() => {
    const timeoutId = setTimeout(() => setDebouncedSearchTerm(searchTerm), 300);
    return () => clearTimeout(timeoutId);
  }, [searchTerm]);
  /* ... */
}

ListPage.tsx

function ListPage() {
  /* ... */
  return (
    <>
      <SearchPanel searchTerm={searchTerm} onSearchChanged={setSearchTerm} />
      {pokemons ? (
        pokemons
          ?.filter((p) => p.name.includes(debouncedSearchTerm))
          .map((pokemon) => (
            <PokeListEntry key={pokemon.name} name={pokemon.name} />
          ))
      ) : (
        <div>LOADING</div>
      )}
    </>
}
function ListPage() {
  /* ... */
  return (
    <>
      <SearchPanel searchTerm={searchTerm} onSearchChanged={setSearchTerm} />
      {pokemons ? (
        pokemons
          ?.filter((p) => p.name.includes(debouncedSearchTerm))
          .map((pokemon) => (
            <PokeListEntry key={pokemon.name} name={pokemon.name} />
          ))
      ) : (
        <div>LOADING</div>
      )}
    </>
}

We can reuse a concrete Hook logic by extracting it into a custom Hook .

function MyComponent() {
  const result = /* logic that uses Hooks */
  
  return <p>{result}</p>;
}

The logic to be extracted can use a single or multiple Hooks.

We simply move the logic into its own function.

function useMyCustomHook() {
  /* logic that uses Hooks */
  return result;
}
 
function MyComponent() {
  const result = useMyCustomHook();
 
  return <p>{result}</p>;
}
function useMyCustomHook() {
  /* logic that uses Hooks */
  return result;
}
 
function MyComponent() {
  const result = useMyCustomHook();
 
  return <p>{result}</p>;
}
function useMyCustomHook() {
  /* logic that uses Hooks */
  return result;
}
 
function MyComponent() {
  const result = useMyCustomHook();
 
  return <p>{result}</p>;
}

In the component we just call this function.

Stick to the Hooks naming convention: use*

usePokemonDetails.ts

/* import ... */
 
export function usePokemonDetails() {
  const { pokemonName } = useParams<"pokemonName">();
  const uri = `https://pokeapi.co/api/v2/pokemon/${pokemonName}`;
 
  const result = useQuery(["pokemon", "detail", pokemonName], () =>
    fetcher<PokemonDetailDto>(uri)
  );
 
  return result;
}
/* import ... */
 
export function usePokemonDetails() {
  const { pokemonName } = useParams<"pokemonName">();
  const uri = `https://pokeapi.co/api/v2/pokemon/${pokemonName}`;
 
  const result = useQuery(["pokemon", "detail", pokemonName], () =>
    fetcher<PokemonDetailDto>(uri)
  );
 
  return result;
}
/* import ... */
 
export function usePokemonDetails() {
  const { pokemonName } = useParams<"pokemonName">();
  const uri = `https://pokeapi.co/api/v2/pokemon/${pokemonName}`;
 
  const result = useQuery(["pokemon", "detail", pokemonName], () =>
    fetcher<PokemonDetailDto>(uri)
  );
 
  return result;
}

DetailPage.tsx

function DetailPage() {
  const { data, isLoading, isError } = usePokemonDetails();
 
  if (isLoading) return <div>LOADING</div>;
  if (isError) return <div>ERROR while loading data</div>;
 
  return (
    <div>
      <span>{data.name}</span>
      <img src={data.sprites.front_shiny} alt={data?.name} />
    </div>
  );
}

Stretch Goal: usePokemonDetails.ts

/* import ... */
 
export function usePokemonDetails(uriBuilder: (pokemonName?: string) => string) {
  const { pokemonName } = useParams<"pokemonName">();
  const uri = uriBuilder(pokemonName);
 
  const result = useQuery(["pokemon", "detail", pokemonName], () =>
    fetcher<PokemonDetailDto>(uri)
  );
 
  return result;
}
/* import ... */
 
export function usePokemonDetails(uriBuilder: (pokemonName?: string) => string) {
  const { pokemonName } = useParams<"pokemonName">();
  const uri = uriBuilder(pokemonName);
 
  const result = useQuery(["pokemon", "detail", pokemonName], () =>
    fetcher<PokemonDetailDto>(uri)
  );
 
  return result;
}

Stretch Goal: DetailPage.tsx

/* import ... */
 
const queryUrlBuilder = (pokemonName?: string) =>
  `https://pokeapi.co/api/v2/pokemon/${pokemonName}`;
 
function DetailPage() {
  const { data, isLoading, isError } = usePokemonDetails(queryUrlBuilder);
 
  if (isLoading) return <div>LOADING</div>;
  if (isError) return <div>ERROR while loading data</div>;
 
  return (
    <div>
      <span>{data.name}</span>
      <img src={data.sprites.front_shiny} alt={data?.name} />
    </div>
  );
}
/* import ... */
 
const queryUrlBuilder = (pokemonName?: string) =>
  `https://pokeapi.co/api/v2/pokemon/${pokemonName}`;
 
function DetailPage() {
  const { data, isLoading, isError } = usePokemonDetails(queryUrlBuilder);
 
  if (isLoading) return <div>LOADING</div>;
  if (isError) return <div>ERROR while loading data</div>;
 
  return (
    <div>
      <span>{data.name}</span>
      <img src={data.sprites.front_shiny} alt={data?.name} />
    </div>
  );
}

Recap

We learned…

• How to use useEffect
• When to use useEffect and when to use a different approach to reach your goal
• How to cleanup side effects in useEffect
• How to extract Hook logic from components into Custom Hooks