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# typesafe-actions

Typesafe utilities designed to reduce types **verbosity**
and **complexity**  in Redux Architecture.

_This library is part of the [React & Redux TypeScript Guide](https://github.com/piotrwitek/react-redux-typescript-guide)_ ecosystem :book:  

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:tada: _Now updated to support **TypeScript v3.5**_ :tada:

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---

**Features**

- **minimalistic** - according to `size-snapshot` (Minified: 3.48 KB, Gzipped: 1.03 KB), check also on [bundlephobia](https://bundlephobia.com/result?p=typesafe-actions)
- **secure and optimized** - no external dependencies, bundled in 3 different formats (`cjs`, `esm` and `umd` for browser) with separate optimized bundles for dev & prod (same as `react`)
- **focus on quality** - complete test-suite for an entire API surface containing regular runtime tests and extra type-tests to guarantee **type soundness** and to prevent regressions in the future TypeScript versions
- **focus on performance** - integrated performance benchmarks to guarantee that the computational complexity of types are in check and there are no slow-downs when your application grow `npm run benchmark:[2,10,50]`


**Codesandbox links**

- Reference Todo-App implementation using `typesafe-actions`: [Link](https://codesandbox.io/s/github/piotrwitek/typesafe-actions/tree/master/codesandbox)
- Starter to help reproduce bug reports: [Link](https://codesandbox.io/s/github/piotrwitek/typesafe-actions/tree/master/codesandbox)

---

## Table of Contents

<!-- START doctoc generated TOC please keep comment here to allow auto update -->
<!-- DON'T EDIT THIS SECTION, INSTEAD RE-RUN doctoc TO UPDATE -->


- [Motivation](#motivation)
- [Installation](#installation)
- [Compatibility Notes](#compatibility-notes)
- [Contributing Guide](#contributing-guide)
- [Funding](#funding)
- [Tutorial](#tutorial)
  - [Constants](#constants)
  - [Actions](#actions)
    - [1. Basic actions](#1-basic-actions)
    - [2. FSA compliant actions](#2-fsa-compliant-actions)
    - [3. Custom actions (non-standard use-cases)](#3-custom-actions-non-standard-use-cases)
  - [Action Helpers](#action-helpers)
    - [Using action-creators instances instead of type-constants](#using-action-creators-instances-instead-of-type-constants)
    - [Using regular type-constants](#using-regular-type-constants)
  - [Reducers](#reducers)
    - [Extending internal types to enable type-free syntax with `createReducer`](#extending-internal-types-to-enable-type-free-syntax-with-createreducer)
    - [Using createReducer API with type-free syntax](#using-createreducer-api-with-type-free-syntax)
    - [Alternative usage with regular switch reducer](#alternative-usage-with-regular-switch-reducer)
  - [Async-Flows](#async-flows)
    - [With `redux-observable` epics](#with-redux-observable-epics)
    - [With `redux-saga` sagas](#with-redux-saga-sagas)
- [API Docs](#api-docs)
  - [Action-Creators API](#action-creators-api)
    - [`action`](#action)
    - [`createAction`](#createaction)
    - [`createStandardAction`](#createstandardaction)
    - [`createCustomAction`](#createcustomaction)
    - [`createAsyncAction`](#createasyncaction)
  - [Reducer-Creators API](#reducer-creators-api)
    - [`createReducer`](#createreducer)
  - [Action-Helpers API](#action-helpers-api)
    - [`getType`](#gettype)
    - [`isActionOf`](#isactionof)
    - [`isOfType`](#isoftype)
  - [Type-Helpers API](#type-helpers-api)
    - [`ActionType`](#actiontype)
    - [`StateType`](#statetype)
- [Migration Guides](#migration-guides)
  - [`v3.x.x` to `v4.x.x`](#v3xx-to-v4xx)
  - [`v2.x.x` to `v3.x.x`](#v2xx-to-v3xx)
  - [`v1.x.x` to `v2.x.x`](#v1xx-to-v2xx)
  - [Migrating from `redux-actions` to `typesafe-actions`](#migrating-from-redux-actions-to-typesafe-actions)
- [Recipes](#recipes)
  - [Restrict Meta type in `action` creator](#restrict-meta-type-in-action-creator)
- [Compare to others](#compare-to-others)
  - [`redux-actions`](#redux-actions)
- [MIT License](#mit-license)

<!-- END doctoc generated TOC please keep comment here to allow auto update -->

--- 

## Motivation

When I started to combine Redux with TypeScript, I was trying to use [redux-actions](https://redux-actions.js.org/) to reduce the maintainability cost and boilerplate of **action-creators**. Unfortunately, the results were intimidating: incorrect type signatures and broken type-inference cascading throughout the entire code-base [(click here for a detailed comparison)](#redux-actions).

Existing solutions in the wild have been either **too verbose because of redundant type annotations** (hard to maintain) or **used classes** (hinders readability and requires using the **new** keyword 😱)

**So I created `typesafe-actions` to address all of the above pain points.**

The core idea was to design an API that would mostly use the power of TypeScript **type-inference** 💪 to lift the "maintainability burden" of type annotations. In addition, I wanted to make it "look and feel" as close as possible to the idiomatic JavaScript ❤️ , so we don't have to write the redundant type annotations that which will create additional noise in your code.

[⇧ back to top](#table-of-contents)

---

## Installation

```bash
// NPM
npm install typesafe-actions

// YARN
yarn add typesafe-actions
```

[⇧ back to top](#table-of-contents)

---

## Compatibility Notes

**TypeScript support**

- `typesafe-actions@1.X.X` - minimal TS v2.7
- `typesafe-actions@2.X.X` - minimal TS v2.9
- `typesafe-actions@3.X.X` - minimal TS v3.2
- `typesafe-actions@4.X.X` - minimal TS v3.2

**Browser support**

It's compatible with all modern browsers.

For older browsers support (e.g. IE <= 11) and some mobile devices you need to provide the following polyfills:
- [Object.assign](https://developer.mozilla.org/pl/docs/Web/JavaScript/Referencje/Obiekty/Object/assign#Polyfill)
- [Array.prototype.includes](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Array/includes)

**Recommended polyfill for IE**

To provide the best compatibility please include a popular polyfill package in your application, such as `core-js` or `react-app-polyfill` for `create-react-app`.
Please check the `React` guidelines to learn how to do that: [LINK](https://reactjs.org/docs/javascript-environment-requirements.html)
A polyfill fo IE11 is included in our `/codesandbox` application.

[⇧ back to top](#table-of-contents)

---

## Contributing Guide

We are open for contributions. If you're planning to contribute please make sure to read the contributing guide: [CONTRIBUTING.md](/CONTRIBUTING.md)

[⇧ back to top](#table-of-contents)

---

## Funding
**Typesafe-Actions** is an independent open-source project created by people investing their free time for the benefit of our community.

If you are using **Typesafe-Actions** please consider donating as this will guarantee the project will be updated and maintained in the long run.

Issues can be funded by anyone interested in them being resolved. Reward will be transparently distributed to the contributor handling the task through the IssueHunt platform.

[![Let's fund issues in this repository](https://issuehunt.io/static/embed/issuehunt-button-v1.svg)](https://issuehunt.io/repos/110746954)

[⇧ back to top](#table-of-contents)

---

## Tutorial

To showcase the flexibility and the power of the **type-safety** provided by this library, let's build the most common parts of a typical todo-app using a Redux architecture:

> **WARNING**  
> Please make sure that you are familiar with the following concepts of programming languages to be able to follow along: [Type Inference](https://www.typescriptlang.org/docs/handbook/type-inference.html), [Control flow analysis](https://github.com/Microsoft/TypeScript/wiki/What%27s-new-in-TypeScript#control-flow-based-type-analysis), [Tagged union types](https://github.com/Microsoft/TypeScript/wiki/What%27s-new-in-TypeScript#tagged-union-types), [Generics](https://www.typescriptlang.org/docs/handbook/generics.html) and [Advanced Types](https://www.typescriptlang.org/docs/handbook/advanced-types.html).

[⇧ back to top](#table-of-contents)

### Constants

> **RECOMMENDATION:**  
> When using `typesafe-actions` in your project you won't need to export and reuse **string constants**. It's because **action-creators** created by this library have static property with **action type** that you can easily access using **actions-helpers** and then use it in reducers, epics, sagas, and basically any other place. This will simplify your codebase and remove some boilerplate code associated with the usage of **string constants**. Check our `/codesandbox` application to learn some best-practices to create such codebase.

**Limitations of TypeScript when working with string constants** - when using **string constants** as action `type` property, please make sure to use **simple string literal assignment with const**. This limitation is coming from the type-system, because all the **dynamic string operations** (e.g. string concatenation, template strings and also object used as a map) will widen the literal type to its super-type, `string`. As a result this will break contextual typing for **action** object in reducer cases.

```ts
// Example file: './constants.ts'

// WARNING: Incorrect usage
export const ADD = prefix + 'ADD'; // => string
export const ADD = `${prefix}/ADD`; // => string
export default {
   ADD: '@prefix/ADD', // => string
}

// Correct usage
export const ADD = '@prefix/ADD'; // => '@prefix/ADD'
export const TOGGLE = '@prefix/TOGGLE'; // => '@prefix/TOGGLE'
export default ({
  ADD: '@prefix/ADD', // => '@prefix/ADD'
} as const) // working in TS v3.4 and above => https://github.com/Microsoft/TypeScript/pull/29510
```

[⇧ back to top](#table-of-contents)

### Actions

Different projects have different needs, and conventions vary across teams, and this is why `typesafe-actions` was designed with flexibility in mind. It provides three different major styles so you can choose whichever would be the best fit for your team.

#### 1. Basic actions
`action` and `createAction` are creators that can create **actions** with predefined properties ({ type, payload, meta }). This make them concise but also opinionated.
 
Important property is that resulting **action-creator** will have a variadic number of arguments and preserve their semantic names `(id, title, amount, etc...)`.

This two creators are very similar and the only real difference is that `action` **WILL NOT WORK** with **action-helpers**.

```ts
import { action, createAction } from 'typesafe-actions';

export const add = (title: string) => action('todos/ADD', { id: cuid(), title, completed: false });
// add: (title: string) => { type: "todos/ADD"; payload: { id: string, title: string, completed: boolean; }; }

export const add = createAction('todos/ADD', action => {
  // Note: "action" callback does not need "type" parameter
  return (title: string) => action({ id: cuid(), title, completed: false });
});
// add: (title: string) => { type: "todos/ADD"; payload: { id: string, title: string, completed: boolean; }; }
```

#### 2. FSA compliant actions
This style is aligned with [Flux Standard Action](https://github.com/redux-utilities/flux-standard-action), so your **action** object shape is constrained to `({ type, payload, meta, error })`. It is using **generic type arguments** for `meta` and `payload` to simplify creation of type-safe action-creators.

It is important to notice that in the resulting **action-creator** arguments are also constrained to the predefined: `(payload, meta)`, making it the most opinionated creator.

> **TIP**: This creator is the most compatible with `redux-actions` in case you are migrating.

```ts
import { createStandardAction } from 'typesafe-actions';

export const toggle = createStandardAction('todos/TOGGLE')<string>();
// toggle: (payload: string) => { type: "todos/TOGGLE"; payload: string; }

export const add = createStandardAction('todos/ADD').map(
  (title: string) => ({
    payload: { id: cuid(), title, completed: false },
  })
);
// add: (payload: string) => { type: "todos/ADD"; payload: { id: string, title: string, completed: boolean; }; }
```

#### 3. Custom actions (non-standard use-cases)

This approach will give us the most flexibility of all creators, providing a variadic number of named parameters and custom properties on **action** object to fit all the custom use-cases.

```ts
import { createCustomAction } from 'typesafe-actions';

const add = createCustomAction('todos/ADD', type => {
  return (title: string) => ({ type, id: cuid(), title, completed: false });
});
// add: (title: string) => { type: "todos/ADD"; id: string; title: string; completed: boolean; }
```

> **TIP**: For more examples please check the [API Docs](#table-of-contents).

> **RECOMMENDATION**  
> Common approach is to create a `RootAction` in the central point of your redux store - it will represent all possible action types in your application. You can even merge it with third-party action types as shown below to make your model complete.

```ts
// types.d.ts
// example of including `react-router` actions in `RootAction`
import { RouterAction, LocationChangeAction } from 'react-router-redux';
import { TodosAction } from '../features/todos';

type ReactRouterAction = RouterAction | LocationChangeAction;

export type RootAction =
  | ReactRouterAction
  | TodosAction;
```

[⇧ back to top](#table-of-contents)

### Action Helpers

Now I want to show you **action-helpers** and explain their use-cases. We're going to implement a side-effect responsible for showing a success toast when user adds a new todo.

Important thing to notice is that all these helpers are acting as a **type-guard** so they'll narrow **tagged union type** (`RootAction`) to a specific action type that we want.

#### Using action-creators instances instead of type-constants

Instead of **type-constants** we can use **action-creators** instance to match specific actions in reducers and epics cases. It works by adding a static property on **action-creator** instance which contains the `type` string. 

The most common one is `getType`, which is useful for regular reducer switch cases:

```ts
  switch (action.type) {
    case getType(todos.add):
      // below action type is narrowed to: { type: "todos/ADD"; payload: Todo; }
      return [...state, action.payload];
    ...
```

Then we have the `isActionOf` helper which accept **action-creator** as first parameter matching actions with corresponding type passed as second parameter (it's a curried function).

```ts
// epics.ts
import { isActionOf } from 'typesafe-actions';

import { add } from './actions';

const addTodoToast: Epic<RootAction, RootAction, RootState, Services> = (action$, store, { toastService }) =>
  action$.pipe(
    filter(isActionOf(add)),
    tap(action => { // here action type is narrowed to: { type: "todos/ADD"; payload: Todo; }
      toastService.success(...);
    })
    ...
    
  // Works with multiple actions! (with type-safety up to 5)
  action$.pipe(
    filter(isActionOf([add, toggle])) // here action type is narrowed to a smaller union:
    // { type: "todos/ADD"; payload: Todo; } | { type: "todos/TOGGLE"; payload: string; }
```

#### Using regular type-constants
Alternatively if your team prefers to use regular **type-constants** you can still do that.

We have an equivalent helper (`isOfType`) which accept **type-constants** as parameter providing the same functionality.

```ts
// epics.ts
import { isOfType } from 'typesafe-actions';

import { ADD } from './constants';

const addTodoToast: Epic<RootAction, RootAction, RootState, Services> = (action$, store, { toastService }) =>
  action$.pipe(
    filter(isOfType(ADD)),
    tap(action => { // here action type is narrowed to: { type: "todos/ADD"; payload: Todo; }
    ...
    
  // Works with multiple actions! (with type-safety up to 5)
  action$.pipe(
    filter(isOfType([ADD, TOGGLE])) // here action type is narrowed to a smaller union:
    // { type: "todos/ADD"; payload: Todo; } | { type: "todos/TOGGLE"; payload: string; }
```

> **TIP:** you can use action-helpers with other types of conditional statements.

```ts
import { isActionOf, isOfType } from 'typesafe-actions';

if (isActionOf(actions.add, action)) {
  // here action is narrowed to: { type: "todos/ADD"; payload: Todo; }
}
// or with type constants
if (isOfType(types.ADD, action)) {
  // here action is narrowed to: { type: "todos/ADD"; payload: Todo; }
}
```

[⇧ back to top](#table-of-contents)

### Reducers

#### Extending internal types to enable type-free syntax with `createReducer`

We can extend internal types of `typesafe-actions` module with `RootAction` definition of our application so that you don't need to pass generic type arguments with `createReducer` API:

```ts
// types.d.ts
import { StateType, ActionType } from 'typesafe-actions';

export type RootAction = ActionType<typeof import('./actions').default>;

declare module 'typesafe-actions' {
  interface Types {
    RootAction: RootAction;
  }
}

// now you can use
createReducer(...)
// instead of
createReducer<State, Action>(...)
```

#### Using createReducer API with type-free syntax

We can prevent a lot of boilerplate code and type errors using this powerfull and completely typesafe API.

Using handleAction chain API:
```ts
// using action-creators
const counterReducer = createReducer(0)
  // state and action type is automatically inferred and return type is validated to be exact type
  .handleAction(add, (state, action) => state + action.payload)
  .handleAction(add, ... // <= error is shown on duplicated or invalid actions
  .handleAction(increment, (state, _) => state + 1)
  .handleAction(... // <= error is shown when all actions are handled
  
  // or handle multiple actions using array
  .handleAction([add, increment], (state, action) =>
    state + (action.type === 'ADD' ? action.payload : 1)
  );

// all the same scenarios are working when using type-constants
const counterReducer = createReducer(0)
  .handleAction('ADD', (state, action) => state + action.payload)
  .handleAction('INCREMENT', (state, _) => state + 1);
  
counterReducer(0, add(4)); // => 4
counterReducer(0, increment()); // => 1
```

#### Alternative usage with regular switch reducer

First we need to start by generating a **tagged union type** of actions (`TodosAction`). It's very easy to do by using `ActionType` **type-helper**.

```ts
import { ActionType } from 'typesafe-actions';

import * as todos from './actions';
export type TodosAction = ActionType<typeof todos>;
```

Now we define a regular reducer function by annotating `state` and `action` arguments with their respective types (`TodosAction` for action type).

```ts
export default (state: Todo[] = [], action: TodosAction) => {
```

Now in the switch cases we can use the `type` property of action to narrowing the union type of `TodosAction` to an action that is corresponding to that type.

```ts
  switch (action.type) {
    case getType(add):
      // below action type is narrowed to: { type: "todos/ADD"; payload: Todo; }
      return [...state, action.payload];
    ...
```

[⇧ back to top](#table-of-contents)

### Async-Flows

#### With `redux-observable` epics

To handle an async-flow of http request lets implement an `epic`. The `epic` will call a remote API using an injected `todosApi` client, which will return a Promise that we'll need to handle by using three different actions that correspond to triggering, success and failure.

To help us simplify the creation process of necessary action-creators, we'll use `createAsyncAction` function providing us with a nice common interface object `{ request: ... , success: ... , failure: ... }` that will nicely fit with the functional API of `RxJS`.
This will mitigate **redux verbosity** and greatly reduce the maintenance cost of type annotations for **actions** and **action-creators** that would otherwise be written explicitly.

```ts
// actions.ts
import { createAsyncAction } from 'typesafe-actions';

const fetchTodosAsync = createAsyncAction(
  'FETCH_TODOS_REQUEST',
  'FETCH_TODOS_SUCCESS',
  'FETCH_TODOS_FAILURE',
  'FETCH_TODOS_CANCEL'
)<string, Todo[], Error, string>();

// epics.ts
import { fetchTodosAsync } from './actions';

const fetchTodosFlow: Epic<RootAction, RootAction, RootState, Services> = (action$, store, { todosApi }) =>
  action$.pipe(
    filter(isActionOf(fetchTodosAsync.request)),
    switchMap(action =>
      from(todosApi.getAll(action.payload)).pipe(
        map(fetchTodosAsync.success),
        catchError(pipe(fetchTodosAsync.failure, of)),
        takeUntil(action$.pipe(filter(isActionOf(fetchTodosAsync.cancel)))),
      )
    );
```

#### With `redux-saga` sagas
With sagas it's not possible to achieve the same degree of type-safety as with epics because of limitations coming from `redux-saga` API design.

Typescript issues:
- [Typescript does not currently infer types resulting from a `yield` statement](https://github.com/Microsoft/TypeScript/issues/2983) so you have to manually assert the type  e.g. `const response: Todo[] = yield call(...`

*Here is the latest recommendation although it's not fully optimal. If you managed to cook something better, please open an issue to share your finding with us.*

```ts
import { createAsyncAction } from 'typesafe-actions';

const fetchTodosAsync = createAsyncAction(
  'FETCH_TODOS_REQUEST',
  'FETCH_TODOS_SUCCESS',
  'FETCH_TODOS_FAILURE'
)<string, Todo[], Error>();

function* addTodoSaga(action: ReturnType<typeof fetchTodosAsync.request>): Generator {
    const response: Todo[] = yield call(todosApi.getAll, action.payload);

    yield put(fetchTodosAsync.success(response));
  } catch (err) {
    yield put(fetchTodosAsync.failure(err));
  }
}
```

[⇧ back to top](#table-of-contents)

---

## API Docs

### Action-Creators API

#### `action`

_Simple **action factory function** to simplify creation of type-safe actions._

> **WARNING**:  
> This approach will **NOT WORK** with **action-helpers** (such as `getType` and `isActionOf`) because it is creating **action objects** while all the other creator functions are returning **enhanced action-creators**.

```ts
action(type, payload?, meta?, error?)
```

Examples:
[> Advanced Usage Examples](src/action.spec.ts)

```ts
const increment = () => action('INCREMENT');
// { type: 'INCREMENT'; }

const createUser = (id: number, name: string) =>
  action('CREATE_USER', { id, name });
// { type: 'CREATE_USER'; payload: { id: number; name: string }; }

const getUsers = (params?: string) =>
  action('GET_USERS', undefined, params);
// { type: 'GET_USERS'; meta: string | undefined; }
```

> **TIP**: Starting from TypeScript v3.4 you can achieve similar results using new `as const` operator.

```ts
const increment = () => ({ type: 'INCREMENT' } as const);
```

#### `createAction`

_Create an enhanced action-creator with unlimited number of arguments._
- Resulting action-creator will preserve semantic names of their arguments  `(id, title, amount, etc...)`.
- Returned action object have predefined properties `({ type, payload, meta })`

```ts
createAction(type)
createAction(type, actionCallback => {
  return (namedArg1, namedArg2, ...namedArgN) => actionCallback(payload?, meta?)
})
```
> **TIP**: Injected `actionCallback` argument is similar to `action` API but doesn't need the "type" parameter

Examples:
[> Advanced Usage Examples](src/create-action.spec.ts)

```ts
import { createAction } from 'typesafe-actions';

// - with type only
const increment = createAction('INCREMENT');
dispatch(increment());
// { type: 'INCREMENT' };

// - with type and payload
const add = createAction('ADD', action => {
  return (amount: number) => action(amount);
});
dispatch(add(10));
// { type: 'ADD', payload: number }

// - with type and meta
const getTodos = createAction('GET_TODOS', action => {
  return (params: Params) => action(undefined, params);
});
dispatch(getTodos('some_meta'));
// { type: 'GET_TODOS', meta: Params }

// - and finally with type, payload and meta
const getTodo = createAction('GET_TODO', action => {
  return (id: string, meta: string) => action(id, meta);
});
dispatch(getTodo('some_id', 'some_meta'));
// { type: 'GET_TODO', payload: string, meta: string }
```

[⇧ back to top](#table-of-contents)

#### `createStandardAction`

_Create an enhanced action-creator compatible with [Flux Standard Action](https://github.com/redux-utilities/flux-standard-action) to reduce boilerplate and enforce convention._
- Resulting action-creator have predefined arguments `(payload, meta)`
- Returned action object have predefined properties `({ type, payload, meta, error })`
- But it also contains a `.map()` method that allow to map `(payload, meta)` arguments to a custom action object `({ customProp1, customProp2, ...customPropN })`

```ts
createStandardAction(type)()
createStandardAction(type)<TPayload, TMeta?>()
createStandardAction(type).map((payload, meta) => ({ customProp1, customProp2, ...customPropN }))
```

> **TIP**: Using `undefined` as generic type parameter you can make the action-creator function require NO parameters.

Examples:
[> Advanced Usage Examples](src/create-standard-action.spec.ts)

```ts
import { createStandardAction } from 'typesafe-actions';

// Very concise with use of generic type arguments
// - with type only
const increment = createStandardAction('INCREMENT')();
const increment = createStandardAction('INCREMENT')<undefined>();
increment(); // { type: 'INCREMENT' } (no parameters are required)


// - with type and payload
const add = createStandardAction('ADD')<number>();
add(10); // { type: 'ADD', payload: number }

// - with type and meta
const getData = createStandardAction('GET_DATA')<undefined, string>();
getData(undefined, 'meta'); // { type: 'GET_DATA', meta: string }

// - and finally with type, payload and meta
const getData = createStandardAction('GET_DATA')<number, string>();
getData(1, 'meta'); // { type: 'GET_DATA', payload: number, meta: string }

// Can map payload and meta arguments to a custom action object
const notify = createStandardAction('NOTIFY').map(
  (payload: string, meta: Meta) => ({
    from: meta.username,
    message: `${username}: ${payload}`,
    messageType: meta.type,
    datetime: new Date(),
  })
);

dispatch(notify('Hello!', { username: 'Piotr', type: 'announcement' }));
// { type: 'NOTIFY', from: string, message: string, messageType: MessageType, datetime: Date }
```

[⇧ back to top](#table-of-contents)

#### `createCustomAction`

_Create an enhanced action-creator with unlimited number of arguments and custom properties on action object._
- Resulting action-creator will preserve semantic names of their arguments  `(id, title, amount, etc...)`.
- Returned action object have custom properties `({ type, customProp1, customProp2, ...customPropN })`

```ts
createCustomAction(type, type => {
  return (namedArg1, namedArg2, ...namedArgN) => ({ type, customProp1, customProp2, ...customPropN })
})
```

Examples:
[> Advanced Usage Examples](src/create-action-with-type.spec.ts)

```ts
import { createCustomAction } from 'typesafe-actions';

const add = createCustomAction('CUSTOM', type => {
  return (first: number, second: number) => ({ type, customProp1: first, customProp2: second });
});

dispatch(add(1));
// { type: "CUSTOM"; customProp1: number; customProp2: number; }
```

[⇧ back to top](#table-of-contents)

#### `createAsyncAction`

_Create an object containing three enhanced action-creators to simplify handling of async flows (e.g. network request - request/success/failure)._

```ts
createAsyncAction(
  requestType, successType, failureType, cancelType?
)<TRequestPayload, TSuccessPayload, TFailurePayload, TCancelPayload?>()
```

##### `AsyncActionCreator`

```ts
type AsyncActionCreator<
  [TRequestType, TRequestPayload],
  [TSuccessType, TSuccessPayload],
  [TFailureType, TFailurePayload],
  [TCancelType, TCancelPayload]?
> = {
  request: StandardActionCreator<TRequestType, TRequestPayload>,
  success: StandardActionCreator<TSuccessType, TSuccessPayload>,
  failure: StandardActionCreator<TFailureType, TFailurePayload>,
  cancel?: StandardActionCreator<TCancelType, TCancelPayload>,
}
```

> **TIP**: Using `undefined` as generic type parameter you can make the action-creator function require NO parameters.

Examples:
[> Advanced Usage Examples](src/create-async-action.spec.ts)

```ts
import { createAsyncAction, AsyncActionCreator } from 'typesafe-actions';

const fetchUsersAsync = createAsyncAction(
  'FETCH_USERS_REQUEST',
  'FETCH_USERS_SUCCESS',
  'FETCH_USERS_FAILURE'
)<string, User[], Error>();

dispatch(fetchUsersAsync.request(params));

dispatch(fetchUsersAsync.success(response));

dispatch(fetchUsersAsync.failure(err));

const fn = (
  a: AsyncActionCreator<
    ['FETCH_USERS_REQUEST', string],
    ['FETCH_USERS_SUCCESS', User[]],
    ['FETCH_USERS_FAILURE', Error]
  >
) => a;
fn(fetchUsersAsync);

// There is 4th optional argument to declare cancel action
const fetchUsersAsync = createAsyncAction(
  'FETCH_USERS_REQUEST',
  'FETCH_USERS_SUCCESS',
  'FETCH_USERS_FAILURE'
  'FETCH_USERS_CANCEL'
)<string, User[], Error, string>();

dispatch(fetchUsersAsync.cancel('reason'));

const fn = (
  a: AsyncActionCreator<
    ['FETCH_USERS_REQUEST', string],
    ['FETCH_USERS_SUCCESS', User[]],
    ['FETCH_USERS_FAILURE', Error],
    ['FETCH_USERS_CANCEL', string]
  >
) => a;
fn(fetchUsersAsync);
```

[⇧ back to top](#table-of-contents)

---

### Reducer-Creators API

#### `createReducer`

_Create a typesafe reducer_

```ts
createReducer<TState, TRootAction>(initialState, handlersMap?)
  .handleAction(type, reducer)
  .handleAction([type1, type2, ...typeN], reducer)
  .handleAction(actionCreator, reducer)
  .handleAction([actionCreator1, actionCreator2, ...actionCreatorN], reducer)
```

Examples:
[> Advanced Usage Examples](src/create-reducer.spec.ts)

> **TIP:** You can use reducer API with a **type-free** syntax by [Extending internal types](#extending-internal-types-to-enable-type-free-syntax-with-createreducer), otherwise you'll have to pass generic type arguments like in below examples
```ts
// type-free syntax doesn't require generic type arguments
const counterReducer = createReducer(0, { 
  ADD: (state, action) => state + action.payload,
  [getType(increment)]: (state, _) => state + 1,
})
```

Using type-constants as keys in the object map:
```ts
import { createReducer, getType } from 'typesafe-actions'

type State = number;
type Action = { type: 'ADD', payload: number } | { type: 'INCREMENT' };

const counterReducer = createReducer<State, Action>(0, { 
  ADD: (state, action) => state + action.payload,
  [getType(increment)]: (state, _) => state + 1,
})

counterReducer(0, add(4)); // => 4
counterReducer(0, increment()); // => 1
```

Using handleAction chain API:
```ts
// using action-creators
const counterReducer = createReducer<State, Action>(0)
  .handleAction(add, (state, action) => state + action.payload)
  .handleAction(increment, (state, _) => state + 1)

  // handle multiple actions by using array
  .handleAction([add, increment], (state, action) =>
    state + (action.type === 'ADD' ? action.payload : 1)
  );

// all the same scenarios are working when using type-constants
const counterReducer = createReducer<State, Action>(0)
  .handleAction('ADD', (state, action) => state + action.payload)
  .handleAction('INCREMENT', (state, _) => state + 1);
  
counterReducer(0, add(4)); // => 4
counterReducer(0, increment()); // => 1
```

Extend or compose various reducers together - every operation is completely typesafe:
```ts
const newCounterReducer = createReducer<State, Action>(0)
  .handleAction('SUBTRACT', (state, action) => state - action.payload)
  .handleAction('DECREMENT', (state, _) => state - 1);

const bigReducer = createReducer<State, Action>(0, {
  ...counterReducer.handlers, // typesafe
  ...newCounterReducer.handlers, // typesafe
  SUBTRACT: decrementReducer.handlers.DECREMENT, // <= error, wrong type
})
```

[⇧ back to top](#table-of-contents)

---

### Action-Helpers API

#### `getType`

_Get the **type** property value (narrowed to literal type) of given enhanced action-creator._

```ts
getType(actionCreator)
```

[> Advanced Usage Examples](src/get-type.spec.ts)

Examples:
```ts
import { getType, createStandardAction } from 'typesafe-actions';

const add = createStandardAction('ADD')<number>();

// In switch reducer
switch (action.type) {
  case getType(add):
    // action type is { type: "ADD"; payload: number; }
    return state + action.payload;

  default:
    return state;
}

// or with conditional statements
if (action.type === getType(add)) {
  // action type is { type: "ADD"; payload: number; }
}
```

[⇧ back to top](#table-of-contents)

#### `isActionOf`

_Check if action is an instance of given enhanced action-creator(s)
(it will narrow action type to a type of given action-creator(s))_


> **WARNING**: Regular action creators and [action](#action) will not work with this helper

```ts
// can be used as a binary function
isActionOf(actionCreator, action)
// or as a curried function
isActionOf(actionCreator)(action)
// also accepts an array
isActionOf([actionCreator1, actionCreator2, ...actionCreatorN], action)
// with its curried equivalent
isActionOf([actionCreator1, actionCreator2, ...actionCreatorN])(action)
```

Examples:
[> Advanced Usage Examples](src/is-action-of.spec.ts)

```ts
import { addTodo, removeTodo } from './todos-actions';

// Works with any filter type function (`Array.prototype.filter`, lodash, ramda, rxjs, etc.)
// - single action
[action1, action2, ...actionN]
  .filter(isActionOf(addTodo)) // only actions with type `ADD` will pass
  .map((action) => {
    // action type is { type: "todos/ADD"; payload: Todo; }
    ...
    
// - multiple actions
[action1, action2, ...actionN]
  .filter(isActionOf([addTodo, removeTodo])) // only actions with type `ADD` or 'REMOVE' will pass
  .do((action) => {
    // action type is { type: "todos/ADD"; payload: Todo; } | { type: "todos/REMOVE"; payload: Todo; }
    ...
      
// With conditional statements
// - single action
if(isActionOf(addTodo, action)) {
  return iAcceptOnlyTodoType(action.payload);
  // action type is { type: "todos/ADD"; payload: Todo; }
}
// - multiple actions
if(isActionOf([addTodo, removeTodo], action)) {
  return iAcceptOnlyTodoType(action.payload);
  // action type is { type: "todos/ADD"; payload: Todo; } | { type: "todos/REMOVE"; payload: Todo; }
}
```

[⇧ back to top](#table-of-contents)

#### `isOfType`

_Check if action type property is equal given type-constant(s)
(it will narrow action type to a type of given action-creator(s))_

```ts
// can be used as a binary function
isOfType(type, action)
// or as curried function
isOfType(type)(action)
// also accepts an array
isOfType([type1, type2, ...typeN], action)
// with its curried equivalent
isOfType([type1, type2, ...typeN])(action)
```

Examples:
[> Advanced Usage Examples](src/is-of-type.spec.ts)

```ts
import { ADD, REMOVE } from './todos-types';

// Works with any filter type function (`Array.prototype.filter`, lodash, ramda, rxjs, etc.)
// - single action
[action1, action2, ...actionN]
  .filter(isOfType(ADD)) // only actions with type `ADD` will pass
  .map((action) => {
    // action type is { type: "todos/ADD"; payload: Todo; }
    ...
    
// - multiple actions
[action1, action2, ...actionN]
  .filter(isOfType([ADD, REMOVE])) // only actions with type `ADD` or 'REMOVE' will pass
  .do((action) => {
    // action type is { type: "todos/ADD"; payload: Todo; } | { type: "todos/REMOVE"; payload: Todo; }
    ...
      
// With conditional statements
// - single action
if(isOfType(ADD, action)) {
  return iAcceptOnlyTodoType(action.payload);
  // action type is { type: "todos/ADD"; payload: Todo; }
}
// - multiple actions
if(isOfType([ADD, REMOVE], action)) {
  return iAcceptOnlyTodoType(action.payload);
  // action type is { type: "todos/ADD"; payload: Todo; } | { type: "todos/REMOVE"; payload: Todo; }
}
```

[⇧ back to top](#table-of-contents)

---

### Type-Helpers API
Below helper functions are very flexible generalizations, works great with nested structures and will cover numerous different use-cases.

#### `ActionType`

_Powerful type-helper that will infer union type from **import * as ...** or **action-creator map** object._

```ts
import { ActionType } from 'typesafe-actions';

// with "import * as ..."
import * as todos from './actions';
export type TodosAction = ActionType<typeof todos>;
// TodosAction: { type: 'action1' } | { type: 'action2' } | { type: 'action3' }

// with nested action-creator map case
const actions = {
  action1: createAction('action1'),
  nested: {
    action2: createAction('action2'),
    moreNested: {
      action3: createAction('action3'),
    },
  },
};
export type RootAction = ActionType<typeof actions>;
// RootAction: { type: 'action1' } | { type: 'action2' } | { type: 'action3' }
```

[⇧ back to top](#table-of-contents)

#### `StateType`

_Powerful type helper that will infer state object type from **reducer function** and **nested/combined reducers**._

> **WARNING**: working with redux@4+ types

```ts
import { combineReducers } from 'redux';
import { StateType } from 'typesafe-actions';

// with reducer function
const todosReducer = (state: Todo[] = [], action: TodosAction) => {
  switch (action.type) {
    case getType(todos.add):
      return [...state, action.payload];
    ...
export type TodosState = StateType<typeof todosReducer>;

// with nested/combined reducers
const rootReducer = combineReducers({
  router: routerReducer,
  counters: countersReducer,
});
export type RootState = StateType<typeof rootReducer>;
```

[⇧ back to top](#table-of-contents)

---

## Migration Guides

### `v3.x.x` to `v4.x.x`
From `v4.x.x` all action creators will use `undefined` instead of `void` as a generic type parameter to make the action-creator function require NO parameters.
```ts
const increment = createStandardAction('INCREMENT')<undefined>();
increment(); // <= no parameters required

const fetchUsersAsync = createAsyncAction(
  'FETCH_USERS_REQUEST',
  'FETCH_USERS_SUCCESS',
  'FETCH_USERS_FAILURE'
)<undefined, User[], Error>();
fetchUsersAsync.request(); // <= no parameters required
```

### `v2.x.x` to `v3.x.x`
`v3.x.x` API is backward compatible with `v2.x.x`. You'll only need to update typescript dependency to `> v3.1`.

### `v1.x.x` to `v2.x.x`
In v2 we provide a `createActionDeprecated` function compatible with v1 API to help with incremental migration.

```ts
// in v1 we created action-creator like this:
const getTodo = createAction('GET_TODO',
  (id: string, meta: string) => ({
    type: 'GET_TODO',
    payload: id,
    meta: meta,
  })
);

getTodo('some_id', 'some_meta'); // { type: 'GET_TODO', payload: 'some_id', meta: 'some_meta' }

// in v2 API we offer few different styles - please choose your preference
const getTodoNoHelpers = (id: string, meta: string) => action('GET_TODO', id, meta);

const getTodoWithHelpers = createAction('GET_TODO', action => {
  return (id: string, meta: string) => action(id, meta);
});

const getTodoFSA = createStandardAction('GET_TODO')<string, string>();

const getTodoCustom = createStandardAction('GET_TODO').map(
  ({ id, meta }: { id: string; meta: string; }) => ({
    payload: id,
    meta,
  })
);
```

### Migrating from `redux-actions` to `typesafe-actions`

- createAction(s)

```ts
createAction(type, payloadCreator, metaCreator) => createStandardAction(type)() || createStandardAction(type).map(payloadMetaCreator)

createActions() => // COMING SOON!
```

- handleAction(s)

```ts
handleAction(type, reducer, initialState) => createReducer(initialState).handleAction(type, reducer)

handleActions(reducerMap, initialState) => createReducer(initialState, reducerMap)
```

> TIP: If migrating from JS -> TS, you can swap out action-creators from `redux-actions` with action-creators from `typesafe-actions` in your `handleActions` handlers. This works because the action-creators from `typesafe-actions` provide the same `toString` method implementation used by `redux-actions` to match actions to the correct reducer.

- combineActions

Not needed because each function in the API accept single value or array of values for action types or action creators.

[⇧ back to top](#table-of-contents)

---

## Recipes

### Restrict Meta type in `action` creator
Using this recipe you can create an action creator with restricted Meta type with exact object shape.

```tsx
export type MetaType = {
  analytics?: {
    eventName: string;
  };
};

export const actionWithRestrictedMeta = <T extends string, P>(
  type: T,
  payload: P,
  meta: MetaType
) => action(type, payload, meta);

export const validAction = (payload: string) =>
  actionWithRestrictedMeta('type', payload, { analytics: { eventName: 'success' } }); // OK!

export const invalidAction = (payload: string) =>
  actionWithRestrictedMeta('type', payload, { analytics: { excessProp: 'no way!' } }); // Error
// Object literal may only specify known properties, and 'excessProp' does not exist in type '{ eventName: string; }
```

[⇧ back to top](#table-of-contents)

---

## Compare to others

Here you can find out a detailed comparison of `typesafe-actions` to other solutions.

### `redux-actions`
Lets compare the 3 most common variants of action-creators (with type only, with payload and with payload + meta)

Note: tested with "@types/redux-actions": "2.2.3"

**- with type only (no payload)**

##### redux-actions
```ts
const notify1 = createAction('NOTIFY');
// resulting type:
// () => {
//   type: string;
//   payload: void | undefined;
//   error: boolean | undefined;
// }
```

> with `redux-actions` you can notice the redundant nullable `payload` property and literal type of `type` property is lost (discrimination of union type would not be possible)

##### typesafe-actions
```ts
const notify1 = () => action('NOTIFY');
// resulting type:
// () => {
//   type: "NOTIFY";
// }
```

> with `typesafe-actions` there is no excess nullable types and no excess properties and the action "type" property is containing a literal type

**- with payload**

##### redux-actions
```ts
const notify2 = createAction('NOTIFY',
  (username: string, message?: string) => ({
    message: `${username}: ${message || 'Empty!'}`,
  })
);
// resulting type:
// (t1: string) => {
//   type: string;
//   payload: { message: string; } | undefined;
//   error: boolean | undefined;
// }
```

> first the optional `message` parameter is lost, `username` semantic argument name is changed to some generic `t1`, `type` property is widened once again and `payload` is nullable because of broken inference

##### typesafe-actions
```ts
const notify2 = (username: string, message?: string) => action(
  'NOTIFY',
  { message: `${username}: ${message || 'Empty!'}` },
);
// resulting type:
// (username: string, message?: string | undefined) => {
//   type: "NOTIFY";
//   payload: { message: string; };
// }
```

> `typesafe-actions` infer very precise resulting type, notice working optional parameters and semantic argument names are preserved which is really important for great intellisense experience

**- with payload and meta**

##### redux-actions
```ts
const notify3 = createAction('NOTIFY',
  (username: string, message?: string) => (
    { message: `${username}: ${message || 'Empty!'}` }
  ),
  (username: string, message?: string) => (
    { username, message }
  )
);
// resulting type:
// (...args: any[]) => {
//   type: string;
//   payload: { message: string; } | undefined;
//   meta: { username: string; message: string | undefined; };
//   error: boolean | undefined;
// }
```

> this time we got a completely broken arguments arity with no type-safety because of `any` type with all the earlier issues

##### typesafe-actions
```ts
/**
 * typesafe-actions
 */
const notify3 = (username: string, message?: string) => action(
  'NOTIFY',
  { message: `${username}: ${message || 'Empty!'}` },
  { username, message },
);
// resulting type:
// (username: string, message?: string | undefined) => {
//   type: "NOTIFY";
//   payload: { message: string; };
//   meta: { username: string; message: string | undefined; };
// }
```

> `typesafe-actions` never fail to `any` type, even with this advanced scenario all types are correct and provide complete type-safety and excellent developer experience 

[⇧ back to top](#table-of-contents)

---

## MIT License

Copyright (c) 2017 Piotr Witek <piotrek.witek@gmail.com> (http://piotrwitek.github.io)
