swr

Original：🇺🇸 English

Translated

SWR data-fetching expert guidance. Use when building React apps with client-side data fetching, caching, revalidation, mutations, optimistic UI, pagination, or infinite loading using the SWR library.

3installs

Sourcevercel-labs/vercel-plugin

Added on2026-03-07

NPX Install

npx skill4agent add vercel-labs/vercel-plugin swr

SKILL.md Content

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SWR — React Hooks for Data Fetching

You are an expert in SWR v2, the React Hooks library for data fetching by Vercel. SWR implements the stale-while-revalidate HTTP cache invalidation strategy — serve from cache first, then revalidate in the background.

Installation

bash

npm install swr

Core API

useSWR

tsx

import useSWR from 'swr'

const fetcher = (url: string) => fetch(url).then(res => res.json())

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

  if (isLoading) return <div>Loading...</div>
  if (error) return <div>Error loading data</div>
  return <div>Hello, {data.name}</div>
}

Key parameters:

```
key
```
— unique string, array, or function identifying the resource (often a URL)
```
fetcher
```
— async function that receives the key and returns data
```
options
```
— optional config object

Return values:

data

error

isLoading

isValidating

mutate

useSWRMutation

— Remote Mutations

tsx

import useSWRMutation from 'swr/mutation'

async function updateUser(url: string, { arg }: { arg: { name: string } }) {
  return fetch(url, { method: 'POST', body: JSON.stringify(arg) }).then(res => res.json())
}

function Profile() {
  const { trigger, isMutating } = useSWRMutation('/api/user', updateUser)

  return (
    <button disabled={isMutating} onClick={() => trigger({ name: 'New Name' })}>
      Update
    </button>
  )
}

useSWRInfinite

— Pagination & Infinite Loading

tsx

import useSWRInfinite from 'swr/infinite'

const getKey = (pageIndex: number, previousPageData: any[]) => {
  if (previousPageData && !previousPageData.length) return null
  return `/api/items?page=${pageIndex}`
}

function Items() {
  const { data, size, setSize, isLoading } = useSWRInfinite(getKey, fetcher)
  const items = data ? data.flat() : []

  return (
    <>
      {items.map(item => <div key={item.id}>{item.name}</div>)}
      <button onClick={() => setSize(size + 1)}>Load More</button>
    </>
  )
}

Global Configuration

Wrap your app (or a subtree) with

SWRConfig

to set defaults:

tsx

import { SWRConfig } from 'swr'

function App() {
  return (
    <SWRConfig value={{
      fetcher: (url: string) => fetch(url).then(res => res.json()),
      revalidateOnFocus: false,
      dedupingInterval: 5000,
    }}>
      <Dashboard />
    </SWRConfig>
  )
}

Revalidation Strategies

Strategy	Option	Default
On window focus	`revalidateOnFocus`	`true`
On network recovery	`revalidateOnReconnect`	`true`
On mount if stale	`revalidateIfStale`	`true`
Polling	`refreshInterval`	`0` (disabled)
Manual	Call `mutate()`	—

Optimistic Updates

tsx

const { trigger } = useSWRMutation('/api/user', updateUser, {
  optimisticData: (current) => ({ ...current, name: 'New Name' }),
  rollbackOnError: true,
  populateCache: true,
  revalidate: false,
})

Conditional Fetching

Pass

null

or a falsy key to skip fetching:

tsx

const { data } = useSWR(userId ? `/api/user/${userId}` : null, fetcher)

Error Retry

SWR retries on error by default with exponential backoff. Customize with:

tsx

useSWR(key, fetcher, {
  onErrorRetry: (error, key, config, revalidate, { retryCount }) => {
    if (error.status === 404) return // Don't retry on 404
    if (retryCount >= 3) return      // Max 3 retries
    setTimeout(() => revalidate({ retryCount }), 5000)
  },
})

Key Rules

Keys must be unique — two
```
useSWR
```
calls with the same key share cache and deduplicate requests
Fetcher is optional when set via
```
SWRConfig
```
mutate(key)
globally revalidates any hook matching that key
Array keys like
```
useSWR(['/api/user', id], fetcher)
```
— the fetcher receives the full array
Never call hooks conditionally — use conditional keys (
```
null
```
) instead

swr