# Optimistic Updates

 

## Optimistic Updates

When you're performing an update on some data that *already exists* in the cache via [`useMutation`](https://github.com/bgoonz/Learning-Redux/blob/master/repos/redux-toolkit/docs/rtk-query/usage/mutations/README.md), RTK Query gives you a few tools to implement an optimistic update. This can be a useful pattern for when you want to give the user the impression that their changes are immediate.

The core concepts are:

* when you start a query or mutation, `onQueryStarted` will be executed
* you manually update the cached data by dispatching `api.util.updateQueryData`
* then, in the case that `promiseResult` rejects, you roll it back via the `.undo` property of the object you got back from the earlier dispatch.

```ts
// file: types.ts noEmit
export interface Post {
  id: number
  name: string
}

// file: api.ts
import { createApi, fetchBaseQuery } from '@reduxjs/toolkit/query'
import { Post } from './types'

const api = createApi({
  baseQuery: fetchBaseQuery({
    baseUrl: '/',
  }),
  tagTypes: ['Post'],
  endpoints: (build) => ({
    getPost: build.query<Post, number>({
      query: (id) => `post/${id}`,
      providesTags: ['Post'],
    }),
    updatePost: build.mutation<void, Pick<Post, 'id'> & Partial<Post>>({
      query: ({ id, ...patch }) => ({
        url: `post/${id}`,
        method: 'PATCH',
        body: patch,
      }),
      invalidatesTags: ['Post'],
      // highlight-start
      async onQueryStarted({ id, ...patch }, { dispatch, queryFulfilled }) {
        const patchResult = dispatch(
          api.util.updateQueryData('getPost', id, (draft) => {
            Object.assign(draft, patch)
          })
        )
        try {
          await queryFulfilled
        } catch {
          patchResult.undo()
        }
      },
      // highlight-end
    }),
  }),
})
```

or, if you prefer the slightly shorter version with `.catch`

```diff
-      async onQueryStarted({ id, ...patch }, { dispatch, queryFulfilled }) {
+      onQueryStarted({ id, ...patch }, { dispatch, queryFulfilled }) {
        const patchResult = dispatch(
          api.util.updateQueryData('getPost', id, (draft) => {
            Object.assign(draft, patch)
          })
        )
-       try {
-         await queryFulfilled
-       } catch {
-         patchResult.undo()
-       }
+       queryFulfilled.catch(patchResult.undo)
      }
```

#### Example

[View Example](https://github.com/bgoonz/Learning-Redux/blob/master/repos/redux-toolkit/docs/rtk-query/usage/examples/README.md#react-optimistic-updates)


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