React 컴포넌트의 7가지 코드 냄새

Passing too many props into a single component may be a sign that the component should be split up.

How many are too many you ask? Well.. "it depends". You might find yourself in a situation where a component have 20 props or more, and still be satisfied that it only does one thing. But when you do stumble upon a component that has many props or you get the urge to add just one more to the already long list of props there's a couple of things to consider:

Avoid passing props that are incompatible with each other.

For instance, we might start by creating a common <Input /> component that is intended to just handle text, but after a while we also add the possibility to use it for phone numbers as well. The implementation could look something like this:

function Input({ value, isPhoneNumberInput, autoCapitalize }) {
  if (autoCapitalize) capitalize(value)

  return <input value={value} type={isPhoneNumberInput ? 'tel' : 'text'} />
}

The problem with this is that the props isPhoneNumberInput and autoCapitalize don't make sense together. We can't really capitalize phone numbers.

Don't stop the data flow by copying props into state.

Consider this component:

function Button({ text }) {
  const [buttonText] = useState(text)

  return <button>{buttonText}</button>
}

By passing the text prop as the initial value of useState the component now practically ignores all updated values of text . If the text prop was updated the component would still render its first value. For most props this is unexpected behavior which in turn makes the component more bug-prone.

A more practical example of this happening is when we want to derive some new value from a prop and especially if this requires some slow calculation. In the example below, we run the slowlyFormatText function to format our text -prop, which takes a lot of time to execute.

function Button({ text }) {
  const [formattedText] = useState(() => slowlyFormatText(text))

  return <button>{formattedText}</button>
}

Don't return JSX from functions inside a component.

This is a pattern that has largely disappeared when function components became more popular, but I still run into it from time to time. Just to give an example of what I mean:

function Component() {
  const topSection = () => {
    return (
      <header>
        <h1>Component header</h1>
      </header>
    )
  }

  const middleSection = () => {
    return (
      <main>
        <p>Some text</p>
      </main>
    )
  }

  const bottomSection = () => {
    return (
      <footer>
        <p>Some footer text</p>
      </footer>
    )
  }

  return (
    <div>
      {topSection()}
      {middleSection()}
      {bottomSection()}
    </div>
  )
}

While this might feel okay at first it makes it hard to reason about the code, discourages good patterns, and should be avoided. To solve it I either inline the JSX because a large return isn't that big of a problem, but more often this is a reason to break these sections into separate components instead.

Remember that just because you create a new component you don't have to move it to a new file as well. Sometimes it makes sense to keep multiple components in the same file if they are tightly coupled.

Avoid using multiple booleans to represent a components state.

When writing a component and subsequently extending the functionality of the component it's easy to end up in a situation where you have multiple booleans to indicate which state the component is in. For a small component that does a web request when you click a button you might have something like this:

function Component() {
  const [isLoading, setIsLoading] = useState(false)
  const [isFinished, setIsFinished] = useState(false)
  const [hasError, setHasError] = useState(false)

  const fetchSomething = () => {
    setIsLoading(true)

    fetch(url)
      .then(() => {
        setIsLoading(false)
        setIsFinished(true)
      })
      .catch(() => {
        setHasError(true)
      })
  }

  if (isLoading) return <Loader />
  if (hasError) return <Error />
  if (isFinished) return <Success />

  return <button onClick={fetchSomething} />
}

When the button is clicked we set isLoading to true and do a web request with fetch. If the request is successful we set isLoading to false and isFinished to true and otherwise set hasError to true if there was an error.

While this technically works fine it's hard to reason about what state the component is in and it's more error-prone than alternatives. We could also end up in an "impossible state", such as if we accidentally set both isLoading and isFinished to true at the same time.

A better way to handle this is to manage the state with an "enum" instead. In other languages enums are a way to define a variable that is only allowed to be set to a predefined collection of constant values, and while enums don't technically exist in Javascript we can use a string as an enum and still get a lot of benefits:

function Component() {
  const [state, setState] = useState('idle')

  const fetchSomething = () => {
    setState('loading')

    fetch(url)
      .then(() => {
        setState('finished')
      })
      .catch(() => {
        setState('error')
      })
  }

  if (state === 'loading') return <Loader />
  if (state === 'error') return <Error />
  if (state === 'finished') return <Success />

  return <button onClick={fetchSomething} />
}

By doing it this way we've removed the possibility for impossible states and made it much easier to reason about this component. Finally, if you're using some sort of type system like TypeScript it's even better since you can specify the possible states:

const [state, setState] = useState<'idle' | 'loading' | 'error' | 'finished'>('idle')

Avoid using too many useState hooks in the same component.

A component with many useState hooks is likely doing Too Many Things™️ and probably a good candidate for breaking into multiple components, but there are also some complex cases where we need to manage some complex state in a single component.

Here's an example of how some state and a couple of functions in an autocomplete input component could look like:

function AutocompleteInput() {
  const [isOpen, setIsOpen] = useState(false)
  const [inputValue, setInputValue] = useState('')
  const [items, setItems] = useState([])
  const [selectedItem, setSelectedItem] = useState(null)
  const [activeIndex, setActiveIndex] = useState(-1)

  const reset = () => {
    setIsOpen(false)
    setInputValue('')
    setItems([])
    setSelectedItem(null)
    setActiveIndex(-1)
  }

  const selectItem = (item) => {
    setIsOpen(false)
    setInputValue(item.name)
    setSelectedItem(item)
  }

  ...
}

We have a reset function that resets all of the state and a selectItem function that updates some of our state. These functions both have to use quite a few state setters from all of our useState s to do their intended task. Now imagine that we have a lot of more actions that have to update the state and it's easy to see that this becomes hard to keep bug free in the long run. In these cases it can be beneficial to manage our state with a useReducer hook instead:

const initialState = {
  isOpen: false,
  inputValue: "",
  items: [],
  selectedItem: null,
  activeIndex: -1
}
function reducer(state, action) {
  switch (action.type) {
    case "reset":
      return {
        ...initialState
      }
    case "selectItem":
      return {
        ...state,
        isOpen: false,
        inputValue: action.payload.name,
        selectedItem: action.payload
      }
    default:
      throw Error()
  }
}

function AutocompleteInput() {
  const [state, dispatch] = useReducer(reducer, initialState)

  const reset = () => {
    dispatch({ type: 'reset' })
  }

  const selectItem = (item) => {
    dispatch({ type: 'selectItem', payload: item })
  }

  ...
}

By using a reducer we've encapsulated the logic for managing our state and moved the complexity out of our component. This makes it much easier to understand what is going on now that we can think about our state and our component separately.

Avoid large useEffect s that do multiple things. They make your code error-prone and harder to reason about.

A mistake that I made a lot when hooks were released was putting too many things into a single useEffect . To illustrate, here's a component with a single useEffect :

function Post({ id, unlisted }) {
  ...

  useEffect(() => {
    fetch(`/posts/${id}`).then(/* do something */)

    setVisibility(unlisted)
  }, [id, unlisted])

  ...
}

While this effect isn't that large it still do multiple things. When the unlisted prop changes we will fetch the post even if id hasn't changed.

To catch errors like this I try to describe the effects I write by saying "when [dependencies] change do this" to myself. Applying that to the effect above we get "when id or unlisted changes, fetch the post and update visibility". If this sentence contains the words "or" or "and" it usually points to a problem.

Breaking this effect up into two effects instead:

function Post({ id, unlisted }) {
  ...

  useEffect(() => { // when id changes fetch the post
    fetch(`/posts/${id}`).then(/* ... */)
  }, [id])

  useEffect(() => { // when unlisted changes update visibility
    setVisibility(unlisted)
  }, [unlisted])

  ...
}

By doing this we've reduced the complexity of our component, made it easier to reason about and lowered the risk of creating bugs.