Mutation

Revalidate

You can get the mutate function from the useSWRConfig() hook, and broadcast a revalidation message globally to other SWR hooks^* using the same key by calling mutate(key).

This example shows how to automatically refetch the login info (e.g. inside <Profile/>) when the user clicks the “Logout” button.

import useSWR, { useSWRConfig } from 'swr'
 
function App() {
  const { mutate } = useSWRConfig()
 
  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
          mutate('/api/user')
        }}
      >
        Logout
      </button>
    </div>
  )
}

*: It broadcasts to SWR hooks under the same cache provider scope. If no cache provider exists, it will broadcast to all SWR hooks.

Mutation and POST Request

In many cases, applying local mutations to data is a good way to make changes feel faster — no need to wait for the remote source of data.

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

import useSWR, { useSWRConfig } from 'swr'
 
function Profile() {
  const { mutate } = useSWRConfig()
  const { data } = useSWR('/api/user', fetcher)
 
  return (
    <div>
      <h1>My name is {data.name}.</h1>
      <button
        onClick={async () => {
          const newName = data.name.toUpperCase()
 
          // update the local data immediately, but disable the revalidation
          mutate('/api/user', { ...data, name: newName }, false)
 
          // send a request to the API to update the source
          await requestUpdateUsername(newName)
 
          // trigger a revalidation (refetch) to make sure our local data is correct
          mutate('/api/user')
        }}
      >
        Uppercase my name!
      </button>
    </div>
  )
}

Clicking the button in the example above will locally update the client data, send a POST request to modify the remote data and try to fetch the latest one (revalidate).

But many POST APIs will just return the updated data directly, so we don’t need to revalidate again. Here’s an example showing the “local mutate - request - update” usage:

mutate('/api/user', newUser, false) // use `false` to mutate without revalidation
mutate('/api/user', updateUser(newUser), false) // `updateUser` is a Promise of the request,
// which returns the updated document

Mutate Based on Current Data

Sometimes, you want to update a part of your data based on the current data.

With mutate, you can pass an async function which will receive the current cached value, if any, and returns an updated document.

mutate('/api/todos', async todos => {
  // let's update the todo with ID `1` to be completed,
  // this API returns the updated data
  const updatedTodo = await fetch('/api/todos/1', {
    method: 'PATCH',
    body: JSON.stringify({ completed: true })
  })
 
  // filter the list, and return it with the updated item
  const filteredTodos = todos.filter(todo => todo.id !== '1')
  return [...filteredTodos, updatedTodo]
})

Returned Data from Mutate

Most probably, you need some data to update the cache. The data is resolved or returned from the promise or async function you passed to mutate.

The function passed to mutate will return an updated document which is used to update the corresponding cache value. If there is an error thown while executing the function, the error will be thrown so it can be handled appropriately.

try {
  const user = await mutate('/api/user', updateUser(newUser))
} catch (error) {
  // Handle an error while updating the user here
}

Bound Mutate

The SWR object returned by useSWR also contains a mutate() function that is pre-bound to the SWR’s key.

It is functionally equivalent to the global mutate function but does not require the key parameter.

import useSWR from 'swr'
 
function Profile() {
  const { data, mutate } = useSWR('/api/user', fetcher)
 
  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)
          // NOTE: key is not required when using useSWR's mutate as it's pre-bound
          mutate({ ...data, name: newName })
        }}
      >
        Uppercase my name!
      </button>
    </div>
  )
}

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