swr

What is swr?

The swr npm package is a React Hooks library for remote data fetching. The name 'SWR' is derived from stale-while-revalidate, a HTTP cache invalidation strategy popularized by HTTP RFC 5861. SWR is a strategy to first return the data from cache (stale), then send the fetch request (revalidate), and finally come with the up-to-date data.

What are swr's main functionalities?

Data Fetching

SWR provides a hook called useSWR for data fetching. You can pass a key and a fetcher function, and it will return the data, error, and other useful values for handling the UI state.

import useSWR from 'swr'

function Profile() {
  const { data, error } = useSWR('/api/user', fetcher)

  if (error) return <div>Failed to load</div>
  if (!data) return <div>Loading...</div>
  return <div>Hello, {data.name}!</div>
}

Automatic Revalidation

SWR automatically revalidates data when a user focuses on the window or when the network is reconnected. It can also be configured to revalidate data at a fixed interval.

import useSWR from 'swr'

function Profile() {
  const { data } = useSWR('/api/user', fetcher, {
    refreshInterval: 3000
  })

  // data will be revalidated every 3 seconds
  return <div>{data.name}</div>
}

Local Mutation

SWR allows you to mutate the local data immediately and revalidate it in the background. This provides an optimistic UI update experience.

import useSWR, { mutate } from 'swr'

function updateUsername(name) {
  mutate('/api/user', { ...data, name }, false)
  fetch('/api/user', {
    method: 'POST',
    body: JSON.stringify({ name })
  }).then(() => {
    mutate('/api/user')
  })
}

Other packages similar to swr

react-query

React Query is another library for fetching, caching, and updating data in React applications. It provides more advanced features like query cancellation, background fetching, and even pagination helpers. React Query is often compared to SWR for its similar use cases but offers a different API and additional features.

apollo-client

Apollo Client is a comprehensive state management library for JavaScript that enables you to manage both local and remote data with GraphQL. It is more complex and powerful than SWR, designed specifically for GraphQL, and provides features like caching, optimistic UI, and subscription support.

axios

Axios is a promise-based HTTP client for the browser and Node.js. While it is not a hook-based data fetching library like SWR, it is often used for making HTTP requests in React applications. Developers would use Axios for fetching data and then manage the caching and state themselves or with additional libraries.

README

swr.now.sh

Intro

SWR is a React Hooks library for remote data fetching.

The name “SWR” is derived from stale-while-revalidate, a HTTP cache invalidation strategy popularized by RFC 5861. SWR first returns the data from cache (stale), then sends the fetch request (revalidate), and finally comes with the up-to-date data again.

It features:

• Transport and protocol agnostic data fetching
• Fast page navigation
• Revalidation on focus
• Interval polling
• Local mutation
• Pagination
• TypeScript ready
• Suspense mode
• Minimal API

With SWR, components will get a stream of data updates constantly and automatically, Thus, the UI will be always fast and reactive.

Quick Start

import useSWR from 'swr'

function Profile () {
  const { data, error } = useSWR('/api/user', fetch)

  if (error) return <div>failed to load</div>
  if (!data) return <div>loading...</div>
  return <div>hello {data.name}!</div>
}

In this example, the React Hook useSWR accepts a key and a fetch function. key is a unique identifier of the data, normally a URL of the API. And the fetch accepts key as its parameter and returns the data asynchronously.

useSWR also returns 2 values: data and error. When the request (fetch) is not yet finished, data will be undefined. And when we get a response, it sets data and error based on the result of fetch and rerenders the component.

Note that fetch can be any asynchronous function, so you can use your favourite data-fetching library to handle that part.

API

useSWR

const {
  data,                                    // data for the given key (or undefined)
  error,                                   // error (or undefined)
  isValidating,                            // if the request is loading
  revalidate                               // function to trigger a validate manually
} = useSWR(
  key,                                     // a unique key for the data
  fn,                                      // Promise returning function to fetch your data
  swrOptions? = {
    suspense: false,                       // enabled React Suspense mode
    revalidateOnFocus: true,               // auto revalidate when window gets focused
    refreshWhenHidden: false,              // refresh while the window is invisible
    shouldRetryOnError: true,              // retry when fetch has an error
    refreshInterval: 0,                    // polling interval (disabled by default)
    errorRetryInterval: 5000,              // error retry interval (10s on slow network)
    focusThrottleInterval: 5000,           // keep focus revalidate requests in a time window
    dedupingInterval: 2000,                // deduping requests
    loadingTimeout: 3000,                  // timeout for triggering the onLoadingSlow event

    onLoadingSlow,                         // event handlers
    onSuccess,
    onError,
    onErrorRetry,

    fetcher                                // default fetcher function (same as `fn`)
  }
)

SWRConfig

A context to provide global configurations (swrOptions) for SWR.

import useSWR, { SWRConfig } from 'swr'

function App () {
  // all the SWRs inside will use `refreshInterval: 1000`
  return <SWRConfig value={{ refreshInterval: 1000 }}>
    <Profile/>
  </SWRConfig>
}

function Profile () {
  const { data, error } = useSWR('/api/user', fetch)
  // ...
}

mutate

With mutate, you can update your local data programmatically, while revalidating and finally replace it.

import useSWR, { mutate } from 'swr'

function Profile () {
  const { data } = useSWR('/api/user', fetch)

  return <div>
    <h1>My name is {data.name}.</h1>
    <button onClick={async () => {
      const newName = data.name.toUpperCase()
      // send a request to the API to update the data
      await requestUpdateUsername(newName)
      // update the local data immediately and revalidate (refetch)
      mutate('/api/user', { ...data, name: newName })
    }}>Uppercase my name!</button>
  </div>
}

trigger

You can broadcast a revalidation message to all SWR data inside any component by calling trigger(key).

import useSWR, { trigger } from 'swr'

function App () {
  return <div>
    <Profile />
    <button onClick={() => {
      // set the cookie as expired
      document.cookie = 'token=; expires=Thu, 01 Jan 1970 00:00:00 UTC; path=/;'
      // tell all SWRs with this key to revalidate
      trigger('/api/user')
    }}>
      Logout
    </button>
  </div>
}

Suspense Mode

You can enable the suspense option to use useSWR with React Suspense.

import { Suspense } from 'react'
import useSWR from 'swr'

function Profile () {
  const { data } = useSWR('/api/user', fetch, { suspense: true })
  return <div>hello, {data.name}</div>
}

function App () {
  return <Suspense fallback={<div>loading...</div>}>
    <Profile/>
  </Suspense>
}

Authors

• Shu Ding (@shuding_) – ZEIT
• Guillermo Rauch (@rauchg) – ZEIT
• Joe Haddad (@timer150) - ZEIT
• Paco Coursey (@pacocoursey) - ZEIT

Thanks to Ryan Chen for providing the awesome swr npm package name!

License

The MIT License.