React Foundations·Components, state and data

Effects and data fetching

Synchronise with the outside world without leaking requests or state.

Intermediate 15 min read Lesson 8 of 300 Challenge included

What you will be able to do

Declare every reactive dependency in the dependency array.
Use AbortController to cancel fetches when the effect is cleaned up.
Do not use an effect for values you can compute during render.

In plain English

Effects run after render. An effect that fetches data needs a cleanup path and must ignore late responses after the component unmounts or dependencies change. Prefer a data library when the application grows.

LessonLoader.jsxJSX

import { useEffect, useState } from 'react'

export default function LessonLoader({ slug }) {
  const [lesson, setLesson] = useState(null)

  useEffect(() => {
    const controller = new AbortController()
    fetch('/api/lessons/' + slug, { signal: controller.signal })
      .then((response) => response.json())
      .then(setLesson)
      .catch((error) => {
        if (error.name !== 'AbortError') console.error(error)
      })
    return () => controller.abort()
  }, [slug])

  return lesson ? <h2>{lesson.title}</h2> : <p>Loading…</p>
}

Your turn

Cancel a fetch

Medium

Add an AbortController to a fetch effect and abort the request in the cleanup function.

JSX · starter

useEffect(() => {
  fetch('/api/lessons').then((r) => r.json())
}, [])

Check your understanding before moving on.

Knowledge checkQuestion 1

What is the cleanup function for in an effect?

Select an answer to continue

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Lesson 8 of 300 · 0% of course