# withInitialState

Creates a reducer that provides an initial state value, optionally wrapping a sub-reducer.

```js
// JavaScript

function withInitialState(state, subReducer?)
```

```ts
// TypeScript

function withInitialState<S, A extends Action>(
  initialState: S,
  reducer?: InitializedReducer<S, A>
): Reducer<S, A>
```

## Details

Given a value, `withInitialState` creates a reducer which provides that value as the initial state - that is, the reducer returns the value if called with the state `undefined`.

If a `subReducer` is passed, the reducer will delegate to it once the state has been initialized. Used this way, `withInitialState` basically converts a sub-reducer that doesn't provide an initial state into a proper reducer that does.

Without `subReducer`, the reducer will simply return any non-`undefined` state it receives. This means it will not make any state changes after the initial state has been provided. This form of `withInitialState` is meant to be used as building block for reducer chains built with [`chainReducers`](/api/chainreducers.md).

## Examples

Using `withInitialState` without sub-reducer:

```js
import { withInitialState } from 'redux-preboiled'

const reducer = withInitialState(0)

reducer(undefined, { type: '' })
// => 0

reducer(123, { type: '' })
// => 123
```

Passing a sub-reducer:

```js
import { withInitialState } from 'redux-preboiled'

function counterReducer(state, action) {
  return action.type === 'increment' ? state + 1 : state
}

const reducer = withInitialState(0, counterReducer)

reducer(undefined, { type: '' })
// => 0

reducer(0, { type: 'increment' })
// => 1
```

Combining `withInitialState` with `chainReducers`:

```js
import { chainReducers, withInitialState } from 'redux-preboiled'

function incrementReducer(state, action) {
  return action.type === 'increment' ? state + 1 : state
}

function multiplyReducer(state, action) {
  return action.type === 'multiply' ? state * action.payload : state
}

const reducer = chainReducers(
  withInitialState(0),
  incrementReducer,
  multiplyReducer
)

reducer(undefined, { type: '' })
// => 0

reducer(0, { type: 'increment' })
// => 1

reducer(2, { type: 'multiply', payload: 4 })
// => 8
```

## See Also

* [`chainReducers`](/api/chainreducers.md)


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