Writing React Effects

Writing React Effects Skill

Guides writing React components that avoid unnecessary useEffect calls.

Core Principle

Effects are an escape hatch for synchronizing with external systems (network, DOM, third-party widgets). If there's no external system, you don't need an Effect.

Calculate Derived State During Rendering

If a value can be computed from current props/state, do not store it in state or update it in an effect. Derive it during render to avoid extra renders and state drift. Do not set state in effects solely in response to prop changes; prefer derived values or keyed resets instead.

Incorrect (redundant state and effect):

function Form() {
  const [firstName, setFirstName] = useState('First')
  const [lastName, setLastName] = useState('Last')
  const [fullName, setFullName] = useState('')

  useEffect(() => {
    setFullName(firstName + ' ' + lastName)
  }, [firstName, lastName])

  return <p>{fullName}</p>
}

Correct (derive during render):

function Form() {
  const [firstName, setFirstName] = useState('First')
  const [lastName, setLastName] = useState('Last')
  const fullName = firstName + ' ' + lastName

  return <p>{fullName}</p>
}

References: You Might Not Need an Effect

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