diff --git a/infinite/index.ts b/infinite/index.ts
index b6600576e..c754985bf 100644
--- a/infinite/index.ts
+++ b/infinite/index.ts
@@ -41,7 +41,7 @@ export const infinite = ((<Data, Error, Args extends Arguments>(
 ): SWRInfiniteResponse<Data, Error> => {
   const rerender = useState({})[1]
   const didMountRef = useRef<boolean>(false)
-  const dataRef = useRef<Data[]>()
+  const dataRef = useRef<Readonly<Data[]>>()
 
   const {
     cache,
@@ -167,7 +167,12 @@ export const infinite = ((<Data, Error, Args extends Arguments>(
 
   const mutate = useCallback(
     (
-      data: Data[] | undefined | Promise<Data[]> | MutatorCallback<Data[]>,
+      data?:
+        | Data[]
+        | Readonly<Data[]>
+        | Promise<Data[]>
+        | Promise<Readonly<Data[]>>
+        | MutatorCallback<Data[]>,
       shouldRevalidate = true
     ) => {
       // It is possible that the key is still falsy.
@@ -190,7 +195,9 @@ export const infinite = ((<Data, Error, Args extends Arguments>(
   )
 
   // Function to load pages data from the cache based on the page size.
-  const resolvePagesFromCache = (pageSize: number): Data[] | undefined => {
+  const resolvePagesFromCache = (
+    pageSize: number
+  ): Readonly<Data[]> | undefined => {
     // return an array of page data
     const data: Data[] = []
 
diff --git a/src/types.ts b/src/types.ts
index dfe67fb69..d4222fa29 100644
--- a/src/types.ts
+++ b/src/types.ts
@@ -1,6 +1,7 @@
 import * as revalidateEvents from './constants/revalidate-events'
 
-export type FetcherResponse<Data = unknown> = Data | Promise<Data>
+type Async<Data> = Data | Promise<Data>
+export type FetcherResponse<Data = unknown> = Async<Data>
 
 export type Fetcher<Data = unknown, SWRKey extends Key = Key> =
   /**
@@ -142,8 +143,8 @@ export type Arguments = string | null | ArgumentsTuple | Record<any, any>
 export type Key = Arguments | (() => Arguments)
 
 export type MutatorCallback<Data = any> = (
-  currentValue?: Data
-) => Promise<undefined | Data> | undefined | Data
+  currentValue?: Readonly<Data>
+) => Async<undefined | Data | Readonly<Data>>
 
 export type Broadcaster<Data = any, Error = any> = (
   cache: Cache<Data>,
@@ -160,32 +161,35 @@ export type State<Data, Error> = {
   isValidating?: boolean
 }
 
+type MutatorData<Data> =
+  | Async<Data>
+  | Async<Readonly<Data>>
+  | MutatorCallback<Data>
+
 export type Mutator<Data = any> = (
   cache: Cache,
   key: Key,
-  data?: Data | Promise<Data> | MutatorCallback<Data>,
+  data?: MutatorData<Data>,
   shouldRevalidate?: boolean
-) => Promise<Data | undefined>
+) => Promise<Readonly<Data> | undefined>
 
 export interface ScopedMutator<Data = any> {
   /** This is used for bound mutator */
-  (
-    key: Key,
-    data?: Data | Promise<Data> | MutatorCallback<Data>,
-    shouldRevalidate?: boolean
-  ): Promise<Data | undefined>
+  (key: Key, data?: MutatorData<Data>, shouldRevalidate?: boolean): Promise<
+    Readonly<Data> | undefined
+  >
   /** This is used for global mutator */
   <T = any>(
     key: Key,
-    data?: T | Promise<T> | MutatorCallback<T>,
+    data?: MutatorData<T>,
     shouldRevalidate?: boolean
-  ): Promise<T | undefined>
+  ): Promise<Readonly<T> | undefined>
 }
 
 export type KeyedMutator<Data> = (
-  data?: Data | Promise<Data> | MutatorCallback<Data>,
+  data?: MutatorData<Data>,
   shouldRevalidate?: boolean
-) => Promise<Data | undefined>
+) => Promise<Readonly<Data> | undefined>
 
 // Public types
 
@@ -196,10 +200,10 @@ export type SWRConfiguration<
 > = Partial<PublicConfiguration<Data, Error, SWRKey>>
 
 export interface SWRResponse<Data, Error> {
-  data?: Data
-  error?: Error
-  mutate: KeyedMutator<Data>
+  data?: Readonly<Data>
+  error?: Readonly<Error>
   isValidating: boolean
+  mutate: KeyedMutator<Data>
 }
 
 export type KeyLoader<Args extends Arguments = Arguments> =
diff --git a/test/type/state.ts b/test/type/state.ts
new file mode 100644
index 000000000..56fb44a66
--- /dev/null
+++ b/test/type/state.ts
@@ -0,0 +1,35 @@
+import useSWR from 'swr'
+
+type ExpectType = <T>(value: T) => void
+const expectType: ExpectType = () => {}
+
+export function useInferDataType() {
+  // Should infer the data type
+  expectType<string | undefined>(useSWR('foo', i => i).data)
+  expectType<number | undefined>(useSWR('foo', () => 1).data)
+  expectType<{ foo: number } | undefined>(useSWR({ foo: 1 }, i => i).data)
+  expectType<{ foo: 1 } | undefined>(useSWR({ foo: 1 } as const, i => i).data)
+  expectType<string | undefined>(useSWR(['foo'], i => i[0]).data)
+
+  // Should mark data and error as readonly
+  expectType<Readonly<{}> | undefined>(useSWR({}, i => i).data)
+  expectType<Readonly<{}> | undefined>(useSWR<any, {}>('').error)
+}
+
+export function useInferMutateType() {
+  // Should infer the mutate argument type
+  useSWR('foo', i => i).mutate(v => {
+    expectType<string | undefined>(v)
+    return v
+  })
+
+  // Should mark it as readonly
+  useSWR({ foo: 1 }, i => i).mutate(v => {
+    expectType<Readonly<{ foo: number }> | undefined>(v)
+    return v
+  })
+  useSWR({ foo: 1 } as const, i => i).mutate(v => {
+    expectType<Readonly<{ foo: 1 }> | undefined>(v)
+    return v
+  })
+}
diff --git a/test/use-swr-local-mutation.test.tsx b/test/use-swr-local-mutation.test.tsx
index c6767e7eb..98e2a647c 100644
--- a/test/use-swr-local-mutation.test.tsx
+++ b/test/use-swr-local-mutation.test.tsx
@@ -44,7 +44,7 @@ describe('useSWR - local mutation', () => {
       const { data: state, mutate: setState } = useSWR(`${baseKey}--${key}`, {
         fallbackData
       })
-      return [state, setState]
+      return [state, setState] as const
     }
 
     function Page() {