What problems do Hooks solve?

Before I detail my current frustrations with Hooks I do want to state for the record that I am, for the most part, a fan of Hooks.

I often hear that the main reason for the existence of Hooks is to replace class components. Sadly, the main heading in the official React site’s post introducing Hooks really undersells Hooks with this not-so-bold statement:

Hooks are a new addition in React 16.8. They let you use state, and other React features without writing a class.

This explanation does not give me a lot of motivation to use Hooks apart from “classes are not cool, man!” For my money, Hooks allow us to address cross-cutting concerns in a much more elegant way than the previous patterns such as mixins, higher-order components and render props.

Functionality like logging and authentication are not component-specific, and Hooks allow us to attach this type of reusable behavior to components.

What was wrong with class components?

There is something beautiful and pure about the notion of a stateless component that takes some props and returns a React element. It is a pure function and as such, side effect free.

export const Heading: React.FC<HeadingProps> = ({ level, className, tabIndex, children, ...rest }) => { const Tag = `h${level}` as Taggable; return ( <Tag className={cs(className)} {...rest} tabIndex={tabIndex}> {children} </Tag> ); };

Unfortunately, the lack of side effects makes these stateless components a bit limited, and in the end, something somewhere must manipulate state. In React, this generally meant that side effects are added to stateful class components. These class components, often called container components, execute the side effects and pass props down to these pure stateless component functions.

There are several well-documented problems with the class-based lifecycle events. One of the biggest complaints is that you often have to repeat logic in componentDidMount and componentDidUpdate .

async componentDidMount() { const response = await get(`/users`); this.setState({ users: response.data }); }; async componentDidUpdate(prevProps) { if (prevProps.resource !== this.props.resource) { const response = await get(`/users`); this.setState({ users: response.data }); } };

If you have used React for any length of time, you will have encountered this problem.

With Hooks, this side effect code can be handled in one place using the effect Hook.

const UsersContainer: React.FC = () => { const [ users, setUsers ] = useState([]); const [ showDetails, setShowDetails ] = useState(false); const fetchUsers = async () => { const response = await get('/users'); setUsers(response.data); }; useEffect( () => { fetchUsers(users) }, [ users ] ); // etc.

The useEffect Hook is a considerable improvement, but this is a big step away from the pure stateless functions we previously had. Which brings me to my first frustration.

This is yet another JavaScript paradigm to learn

For the record, I am a 49-year-old React fanboy. The one-way data flow will always have a place in my heart after working on an ember application with the insanity of observers and computed properties.

The problem with useEffect and friends is that it exists nowhere else in the JavaScript landscape. It is unusual and has quirks, and the only way for me to become proficient and iron out these quirks is to use it in the real world and experience some pain. No tutorial using counters is going to get me into the flow. I am a freelancer and use other frameworks apart from React, and this gives me fatigue. The fact that I need to set up the eslint-plugin-react-hooks to keep me on the straight and narrow for this specific paradigm does make me feel a bit wary.

To hell and back with the dependencies array

The useEffect Hook can take an optional second argument called the dependencies array which allows you to optimize when React would execute the effect callback. React will make a comparison between each of the values via Object.is to determine whether anything has changed. If any of the elements are different than the last render cycle, then the effect will be run against the new values.

The comparison works great for primitive JavaScript types, but the problems can arise if one of the elements is an object or an array. Object.is will compare objects and arrays by reference, and there is no way to override this functionality and supply a custom comparator.

Reference checking objects by reference is a common gotcha, and I can illustrate this with the following scaled-down version of a problem I encountered:

const useFetch = (config: ApiOptions) => { const [data, setData] = useState(null); useEffect(() => { const { url, skip, take } = config; const resource = `${url}?$skip=${skip}&take=${take}`; axios({ url: resource }).then(response => setData(response.data)); }, [config]); // <-- will fetch on each render return data; }; const App: React.FC = () => { const data = useFetch({ url: "/users", take: 10, skip: 0 }); return <div>{data.map(d => <div>{d})}</div>; };

On line 14, a new object is passed into useFetch on each render if we do not do something to ensure the same object is used each time. In this scenario, it would be preferable to check this object’s fields and not the object reference.

I do understand why React has not gone down the route of doing deep object compares like we may see on things like use-deep-object-compare. You can get into some serious performance problems if not careful. I do seem to revisit this problem a lot, and there are a number of fixes for this. The more dynamic your objects are the more workarounds you start adding.

There is an eslint plugin that you really should be using with the automatic –fix setup in your text editor of choice to apply eslint fixes automatically. I do worry about any new feature that requires an external plugin to check correctness.

The fact that use-deep-object-compare, use-memo-one and others exist is a testimony to this being a common enough problem or at the very least, a point of confusion.

React relies on the order in which Hooks are called

Some of the first custom Hooks to hit the shelves were several useFetch implementations that use Hooks to query a remote API. Most skirt around the issue of calling the remote API from an event handler because Hooks can only be called from the start of a functional component.

What if the data we have has pagination links and we want to re-run the effect when the user clicks a link? Below is a simple useFetch example:

const useFetch = (config: ApiOptions): [User[], boolean] => { const [data, setData] = useState<User[]>([]); const [loading, setLoading] = useState(true); useEffect(() => { const { skip, take } = config; api({ skip, take }).then(response => { setData(response); setLoading(false); }); }, [config]); return [data, loading]; }; const App: React.FC = () => { const [currentPage, setCurrentPage] = useState<ApiOptions>({ take: 10, skip: 0 }); const [users, loading] = useFetch(currentPage); if (loading) { return <div>loading....</div>; } return ( <> {users.map((u: User) => ( <div>{u.name}</div> ))} <ul> {[...Array(4).keys()].map((n: number) => ( <li> <button onClick={() => console.log('what do we do now?')}>{n + 1}</button> </li> ))} </ul> </> ); };

On line 23, the useFetch Hook will be called once on the first render. On lines 35 – 38, pagination buttons are rendered but how would we call the useFetch Hook from the event handlers of these buttons?

The rules of Hooks clearly state:

Don’t call Hooks inside loops, conditions, or nested functions. Instead, always use Hooks at the top level of your React function.

Hooks need to be called in the same order each time the component renders. There are several reasons why this is the case which is beautifully articulated in this post.

You definitely cannot do this:

<button onClick={() => useFetch({ skip: n + 1 * 10, take: 10 })}> {n + 1} </button>

Calling the useFetch Hook from an even handler breaks the rules of Hooks because you would break the order in which the Hooks are called on each render.

Return an executable function from the Hook

I have seen two solutions (that I like) to this problem which both follow the same theme. There is react-async-hook which returns an execute function from the Hook:

import { useAsyncCallback } from 'react-async-hook'; const AppButton = ({ onClick, children }) => { const asyncOnClick = useAsyncCallback(onClick); return ( <button onClick={asyncOnClick.execute} disabled={asyncOnClick.loading}> {asyncOnClick.loading ? '...' : children} </button> ); }; const CreateTodoButton = () => ( <AppButton onClick={async () => { await createTodoAPI('new todo text'); }} > Create Todo </AppButton> );

The call to the useAsyncCallback Hook will return an object that has the expected loading, error and result properties along with an execute function that we can call in the event handler.

react-hooks-async takes a slightly similar approach with its useAsyncTask function.

Here is a complete example with a scaled-down version of useAsyncTask below:

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const createTask = (func, forceUpdateRef) => { const task = { start: async (...args) => { task.loading = true; task.result = null; forceUpdateRef.current(func); try { task.result = await func(...args); } catch (e) { task.error = e; } task.loading = false; forceUpdateRef.current(func); }, loading: false, result: null, error: undefined }; return task; }; export const useAsyncTask = (func) => { const forceUpdate = useForceUpdate(); const forceUpdateRef = useRef(forceUpdate); const task = useMemo(() => createTask(func, forceUpdateRef), [func]); useEffect(() => { forceUpdateRef.current = f => { if (f === func) { forceUpdate({}); } }; const cleanup = () => { forceUpdateRef.current = () => null; }; return cleanup; }, [func, forceUpdate]); return useMemo( () => ({ start: task.start, loading: task.loading, error: task.error, result: task.result }), [task.start, task.loading, task.error, task.result] ); };

The createTask function returns a task object with this interface:

interface Task { start: (...args: any[]) => Promise<void>; loading: boolean; result: null; error: undefined; }

The task has the loading , error and result states that we would expect but it also returns a start function that we can call at a later date.

A task created by createTask does not trigger an update so forceUpdate and forceUpdateRef in useAsyncTask trigger the update instead.

We now have a start function that we can call from an event handler or at least from somewhere else apart from the very start of a functional component.

But now we have lost the ability to call our Hook when the functional component has first run. Thankfully react-hooks-async comes with a useAsyncRun function to facilitate this:

export const useAsyncRun = ( asyncTask: ReturnType<typeof useAsyncTask>, ...args: any[] ) => { const { start } = asyncTask; useEffect(() => { start(...args); // eslint-disable-next-line react-hooks/exhaustive-deps }, [asyncTask.start, ...args]); useEffect(() => { const cleanup = () => { // clean up code here }; return cleanup; }); };

The start function will be executed when any of the args arguments change.

The usage of the Hook now looks like this:

const App: React.FC = () => { const asyncTask = useFetch(initialPage); useAsyncRun(asyncTask); const { start, loading, result: users } = asyncTask; if (loading) { return <div>loading....</div>; } return ( <> {(users || []).map((u: User) => ( <div>{u.name}</div> ))} <ul> {[...Array(4).keys()].map((n: number) => ( <li key={n}> <button onClick={() => start({ skip: 10 * n, take: 10 })}> {n + 1} </button> </li> ))} </ul> </> ); };

The useFetch Hook is called at the start of the functional component in keeping with the laws of Hooks. The useAsyncRun function takes care of calling the API initially and the start function can be used in the onClick handler of the pagination buttons.

The useFetch Hook is now fit for purpose, but unfortunately, the complexity has risen. We have also introduced a closure which makes me slightly scared.

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Conclusion

I think this useFetch example is an excellent example of my current frustrations with Hooks.

I do feel we are jumping through a few unexpected hoops for an elegant result, and I do understand why the call order of Hooks is essential. Unfortunately only having Hooks callable at the start of a functional component is limiting and I think we will still be scratching for ways around this. The useFetch solution is complex and Hooks also force you into working with closures, and I have many scars from unexpected things happening when working with closures.

Closures (like the ones passed to useEffect and useCallback) can capture old versions of props and state values. In particular, this happens if the “inputs” array is inadvertently missing one of the captured variables; this can be confusing.

Stale state due to code executing in a closure is one of the problems the Hooks linter sets out to cure. Stack Overflow has many questions about stale state in useEffect and friends. I seem to have spent an excessive amount of time twiddling various dependency arrays and wrapping functions in useCallback to avoid the stale state or infinite re-rendering. I understand why it is necessary, but it feels annoying, and there is no substitute for just going through a real-world problem to earn your stripes.

I mentioned at the beginning of the post that I am mostly in favor of Hooks, but they are deceptively complex. You will not find anything similar anywhere else on the JavaScript landscape. Having Hooks callable in every render of a functional component introduces issues that regular mixins do not. The need for a linter to enforce the pattern does make me wary, and the need for working with closures is always something that will introduce issues.

I would love to be proven wrong about this so please tell me how I have got Hooks wrong in the comments below.