# chainReducers

Creates a reducer that chains together a sequence of (sub-)reducers.

```js
// JavaScript

function chainReducers(firstChildReducer, ...otherChildReducers)
```

```ts
// TypeScript

function chainReducers<S = any, A extends Action = AnyAction>(
  firstChildReducer: Reducer<S, A>,
  ...otherChildReducers: SubReducer<S, A>[]
): Reducer<S, A>
```

## Details

Given a sequence of (sub-)reducers, `chainReducers` creates a reducer which forwards incoming actions to each of these "child" reducers. More specifically, the reducer:

1. calls the first child reducer with the received state
2. passes the resulting state (and the same action) to the next reducer
3. repeats step 2 until reaching the last reducer, whose returned state is finally returned by the `chainReducers` reducer.

Note that the first child reducer in the chain is the only one which needs to handle an `undefined` state by returning the initial state; all others can be *sub-reducers* - such as the ones returned by [`onAction`](/api/onaction.md) - which assume that they are only ever called with a defined, already-initialized state.

## Examples

Chaining `withInitialState` and `onAction` reducers:

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

const reducer = chainReducers(
  withInitialState(0),
  onAction('increment', state => state + 1),
  onAction('multiply', (state, { payload }) => state * payload)
)

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

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

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

## See Also

* [Reducers](/guides/reducers.md) guide
* [onAction](/api/onaction.md)
* [withInitialState](/api/withinitialstate.md)


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