createAction

A helper function for defining a Redux action type and creator.

function createAction(type, prepareAction?)

The usual way to define an action in Redux is to separately declare an action type constant and an action creator function for constructing actions of that type.

const INCREMENT = 'counter/increment'

function increment(amount) {
  return {
    type: INCREMENT,
    payload: amount
  }
}

const action = increment(3)
// { type: 'counter/increment', payload: 3 }

The createAction helper combines these two declarations into one. It takes an action type and returns an action creator for that type. The action creator can be called either without arguments or with a payload to be attached to the action. Also, the action creator overrides toString() so that the action type becomes its string representation.

const increment = createAction('counter/increment')

let action = increment()
// { type: 'counter/increment' }

action = increment(3)
// returns { type: 'counter/increment', payload: 3 }

console.log(increment.toString())
// 'counter/increment'

console.log(`The action type is: ${increment}`)
// 'The action type is: counter/increment'

Using Prepare Callbacks to Customize Action Contents

By default, the generated action creators accept a single argument, which becomes action.payload. This requires the caller to construct the entire payload correctly and pass it in.

In many cases, you may want to write additional logic to customize the creation of the payload value, such as accepting multiple parameters for the action creator, generating a random ID, or getting the current timestamp. To do this, createAction accepts an optional second argument: a "prepare callback" that will be used to construct the payload value.

import v4 from 'uuid/v4'

const addTodo = createAction('todos/add', function prepare(text) {
  return {
    payload: {
      text,
      id: v4(),
      createdAt: new Date().toISOString()
    }
  }
})

console.log(addTodo('Write more docs'))
/**
 * {
 *   type: 'todos/add',
 *   payload: {
 *     text: 'Write more docs',
 *     id: '1b9d6bcd-bbfd-4b2d-9b5d-ab8dfbbd4bed',
 *     createdAt: '2019-10-03T07:53:36.581Z'
 *   }
 * }
 **/

If provided, all arguments from the action creator will be passed to the prepare callback, and it should return an object with the payload field (otherwise the payload of created actions will be undefined). Additionally, the object can have a meta and/or an error field that will also be added to created actions. meta may contain extra information about the action, error may contain details about the action failure. These three fields (payload, meta and error) adhere to the specification of Flux Standard Actions.

Note: The type field will be added automatically.

Usage with createReducer()

Because of their toString() override, action creators returned by createAction() can be used directly as keys for the case reducers passed to createReducer().

const increment = createAction('counter/increment')
const decrement = createAction('counter/decrement')

const counterReducer = createReducer(0, {
  [increment]: (state, action) => state + action.payload,
  [decrement]: (state, action) => state - action.payload
})

This works because object keys that are not natively supported by JavaScript (like, in this case, functions) are implicitly converted to strings, and the action creators’ string representations happen to be the action types they produce.

Non-String Action Types

In principle, Redux lets you use any kind of value as an action type. Instead of strings, you could theoretically use numbers, symbols, or anything else (although it's recommended that the value should at least be serializable).

However, Redux Toolkit rests on the assumption that you use string action types. Specifically, some of its features rely on the fact that with strings, the toString() method of an createAction() action creator returns the matching action type. This is not the case for non-string action types because toString() will return the string-converted type value rather than the type itself.

const INCREMENT = Symbol('increment')
const increment = createAction(INCREMENT)

increment.toString()
// returns the string 'Symbol(increment)',
// not the INCREMENT symbol itself

increment.toString() === INCREMENT
// false

This means that, for instance, you cannot use a non-string-type action creator as a case reducer key for createReducer().

const INCREMENT = Symbol('increment')
const increment = createAction(INCREMENT)

const counterReducer = createReducer(0, {
  // The following case reducer will NOT trigger for
  // increment() actions because `increment` will be
  // interpreted as a string, rather than being evaluated
  // to the INCREMENT symbol.
  [increment]: (state, action) => state + action.payload,

  // You would need to use the action type explicitly instead.
  [INCREMENT]: (state, action) => state + action.payload
})

For this reason, we strongly recommend you to only use string action types.

actionCreator.match

Every generated actionCreator has a .match(action) method that can be used to determine if the passed action is of the same type as an action that would be created by the action creator.

This has different uses:

As a TypeScript TypeGuard

This match method is a TypeScript type guard and can be used to discriminate the payload type of an action.

This behavior can be particularly useful when used in custom middlewares, where manual casts might be neccessary otherwise.

const increment = createAction<number>('INCREMENT')

function someFunction(action: Action) {
  // accessing action.payload would result in an error here
  if (increment.match(action)) {
    // action.payload can be used as `number` here
  }
}

With redux-observable

The match method can also be used as a filter method, which makes it powerful when used with redux-observable:

const increment = createAction<number>('INCREMENT')

export const epic = (actions$: Observable<Action>) =>
  actions$.pipe(
    filter(increment.match),
    map(action => {
      // action.payload can be safely used as number here (and will also be correctly inferred by TypeScript)
      // ...
    })
  )