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    2.0.0 • Public • Published

    TypeScript FSA npm version Build Status

    A simple Flux Action Creator and Flux Reducers library for @ngrx/store. Its goal is to provide simple yet type-safe experience with Flux actions. Created actions are FSA-compliant:

    interface FSAAction<P> {
      type: string;
      payload?: P;
      error?: boolean;
      meta?: Object;

    It allows you to define reducers by chaining a series of handlers for different action types and optionally providing an initial value.

    This library is heavily inpired (with minor adaptations to make it @ngrx/store compatible) by two awesome librairies:

    Table of Contents


    npm install --save ngrx-store-fsa-helpers
    yarn add ngrx-store-fsa-helpers



    import {actionCreatorFactory} from 'ngrx-store-fsa-helpers';
    const actionCreator = actionCreatorFactory();
    // Specify payload shape as generic type argument. 
    const somethingHappened = actionCreator<{foo: string}>('SOMETHING_HAPPENED');
    // Get action creator type.
    console.log(somethingHappened.type);  // SOMETHING_HAPPENED
    // Create action.
    const action = somethingHappened({foo: 'bar'});
    console.log(action);  // {type: 'SOMETHING_HAPPENED', payload: {foo: 'bar'}}  

    Async Action Creators

    Async Action Creators are objects with properties started, done and failed whose values are action creators.

    started actions will have the following shape

    interface FSAAction<P> {
      type: string;
      payload?: P;
      error?: boolean;
      meta?: Object | null;

    done actions will have the following shape

    interface SuccessFSAAction<P, S> {
      type: string;
      payload: {
                   params?: P;
                   result?: S;
       error?: boolean;
      meta?: Object;

    failed actions will have the following shape

    interface FailureFSAAction<P, E> {
      type: string;
      payload: {
                  params?: P;
                  error?: E;
      error: true;
      meta?: Object;
    import {actionCreatorFactory} from 'ngrx-store-fsa-helpers';
    const actionCreator = actionCreatorFactory();
    // specify parameters and result shapes as generic type arguments
    const doSomething = 
      actionCreator.async<{foo: string},   // parameter type
                          {barnumber},   // success type for field "result" in payload
                          {codenumber}   // error type for field "error" in payload
    console.log(doSomething.started({foo: 'lol'}));
    // {type: 'DO_SOMETHING_STARTED', payload: {foo: 'lol'}}
      params: {foo: 'lol'},
      result: {bar: 42},
    // {type: 'DO_SOMETHING_DONE', payload: {
    //   params: {foo: 'lol'},
    //   result: {bar: 42},
    // }}
      params: {foo: 'lol'},
      error: {code: 42},    
    // {type: 'DO_SOMETHING_FAILED', payload: {
    //   params: {foo: 'lol'},
    //   error: {code: 42},
    // }, error: true}

    Actions With Type Prefix

    You can specify a prefix that will be prepended to all action types. This is useful to namespace library actions as well as for large projects where it's convenient to keep actions near the component that dispatches them.

    // MyComponent.actions.ts
    import {actionCreatorFactory} from 'ngrx-store-fsa-helpers';
    const actionCreator = actionCreatorFactory('MyComponent');
    const somethingHappened = actionCreator<{foo: string}>('SOMETHING_HAPPENED');
    const action = somethingHappened({foo: 'bar'});
    // {type: 'MyComponent/SOMETHING_HAPPENED', payload: {foo: 'bar'}}  


    Suppose we have the setup:

    import {actionCreatorFactory} from 'ngrx-store-fsa-helpers';
    const actionCreator = actionCreatorFactory();
    interface State {
        name: string;
        balance: number;
        isFrozen: boolean;
    const INITIAL_STATE: State = {
        name: "Untitled",
        balance: 0,
        isFrozen: false,
    const setName = actionCreator<string>("SET_NAME");
    function setNameHandler(state: State, name: string): State {
        return { ...state, name };
    const addBalance = actionCreator<number>("ADD_BALANCE");
    function addBalanceHandler(state: State, addedBalance: number): State {
        return { ...state, balance: state.balance + addedBalance };
    const setIsFrozen = actionCreator<boolean>("SET_IS_FROZEN");
    function setIsFrozenHandler(state: State, isFrozen: boolean): State {
        return { ...state, isFrozen };

    Without Reducers Chaining

    import { Action } from '@ngrx/store';
    import { isType } from 'ngrx-store-fsa-helpers';
    function reducer(state = INITIAL_STATE, action: Action): State {
        if (isType(action, setName)) {
            return setNameHandler(state, action.payload);
        } else if (isType(action, addBalance)) {
            return addBalanceHandler(state, action.payload);
        } else if (isType(action, setIsFrozen)) {
            return setIsFrozenHandler(state, action.payload);
        } else {
            return state;

    With Reducers Chaining

    import { reducerWithInitialState } from 'ngrx-store-fsa-helpers';
    const reducer = reducerWithInitialState(INITIAL_STATE)
        .case(setName, setNameHandler)
        .case(addBalance, addBalanceHandler)
        .case(setIsFrozen, setIsFrozenHandler);

    Everything is typesafe. If the types of the action payload and handler don't line up, then TypeScript will complain.

    The reducer builders are immutable. Each call to .case() returns a new reducer rather than modifying the callee.



    actionCreatorFactory(prefix?: string, defaultIsError?: Predicate): ActionCreatorFactory

    Creates Action Creator factory with optional prefix for action types.

    • prefix?: string: Prefix to be prepended to action types.
    • defaultIsError?: Predicate: Function that detects whether action is error given the payload. Default is payload => payload instanceof Error.

    isType(action: Action, actionCreator: ActionCreator): boolean

    Returns true if action has the same type as action creator. Defines Type Guard that lets TypeScript know payload type inside blocks where isType returned true:

    const somethingHappened = actionCreator<{foo: string}>('SOMETHING_HAPPENED');
    if (isType(action, somethingHappened)) {
      // action.payload has type {foo: string};

    Starting a reducer chain


    Starts a reducer builder-chain which uses the provided initial state if passed undefined as its state. For example usage, see the Usage section above.


    Starts a reducer builder-chain without special logic for an initial state. undefined will be treated like any other value for the state.

    Redux seems to really want you to provide an initial state for your reducers. Its createStore API encourages it and combineReducers function enforces it. For the Redux author's reasoning behind this, see this thread. For this reason, reducerWithInitialState will likely be the more common choice, but the option to not provide an initial state is there in case you have some means of composing reducers for which initial state is unnecessary.

    Note that since the type of the state cannot be inferred from the initial state, it must be provided as a type parameter:

    const reducer = reducerWithoutInitialState<State>()
        .case(setName, setNameHandler)
        .case(addBalance, addBalanceHandler)
        .case(setIsFrozen, setIsFrozenHandler);


    Starts a builder-chain which produces a "reducer" whose return type is a supertype of the input state. This is most useful for handling a state which may be in one of several "modes", each of which responds differently to actions and can transition to the other modes. Many applications will not have a use for this.

    Note that the function produced is technically not a reducer because the initial and updated states are different types.

    Example usage:

    type State = StoppedState | RunningState;
    interface StoppedState {
        type: "STOPPED";
    interface StartedState {
        type: "STARTED";
        count: number;
    const INITIAL_STATE: State = { type: "STOPPED" };
    const startWithCount = actionCreator<number>("START_WITH_COUNT");
    const addToCount = actionCreator<number>("ADD_TO_COUNT");
    const stop = actionCreator<void>("STOP");
    function startWithCountHandler(state: StoppedState, count: number): State {
        return { type: "STARTED", count };
    function addToCountHandler(state: StartedState, count: number): State {
        return { ...state, count: state.count + count };
    function stopHandler(state: StartedState): State {
        return { type: "STOPPED" };
    const stoppedReducer = upcastingReducer<StoppedState, State>()
        .case(startWithCount, startWithCountHandler);
    const startedReducer = upcastingReducer<StartedState, State>()
        .case(addToCount, addToCountHandler)
        .case(stop, stopHandler);
    function reducer(state = INITIAL_STATE, action: Redux.Action): State {
        if (state.type === "STOPPED") {
            return stoppedReducer(state, action);
        } else if (state.type === "STARTED") {
            return startedReducer(state, action);
        } else {
            throw new Error("Unknown state");

    Reducer chain methods

    .case(actionCreator, handler(state, payload) => newState)

    Mutates the reducer such that it applies handler when passed actions matching the type of actionCreator. For examples, see Usage.

    .caseWithAction(actionCreator, handler(state, action) => newState)

    Like .case(), except that handler receives the entire action as its second argument rather than just the payload. This is useful if you want to read other properties of the action, such as meta or error, or if you want to pass the entire action unmodified to some other function. For an example, see Usage.

    .cases(actionCreators, handler(state, payload) => newState)

    Like .case(), except that multiple action creators may be provided and the same handler is applied to all of them. That is,

        [setName, addBalance, setIsFrozen],

    is equivalent to

        .case(setName, handler)
        .case(addBalance, handler)
        .case(setIsFrozen, handler);

    Note that the payload passed to the handler may be of the type of any of the listed action types' payloads. In TypeScript terms, this means it has type P1 | P2 | ..., where P1, P2, ... are the payload types of the listed action creators.

    The payload type is inferred automatically for up to four action types. After that, it must be supplied as a type annotation, for example:

    reducerWithInitialState(initialState).cases <
        { documentId: number } >

    .casesWithAction(actionCreators, handler(state, action) => newState)

    Like .cases(), except that the handler receives the entire action as its second argument rather than just the payload.

    .withHandling(updateBuilder(builder) => builder)

    Convenience method which applies the provided function to the current builder and returns the result. Useful if you have a sequence of builder updates (calls to .case(), etc.) which you want to reuse across several reducers.

    .default(handler(state, action) => newState)

    Produces a reducer which applies handler when no previously added .case(), .caseWithAction(), etc. matched. The handler is similar to the one in .caseWithAction(). Note that .default() ends the chain and internally does the same as .build(), because it is not intended that the chain be mutated after calling .default().

    This is useful if you have a "delegate" reducer that should be called on any action after handling a few specific actions in the parent.

    const NESTED_STATE = {
        someProp: "hello",
    const nestedReducer = reducerWithInitialState(NESTED_STATE)
    const INITIAL_STATE = {
        someOtherProp: "world"
    const reducer = reducerWithInitialState(INITIAL_STATE)
        .default((state, action) => ({
            nested: nestedReducer(state.nested, action),


    Returns a plain reducer function whose behavior matches the current state of the reducer chain. Further updates to the chain (through calls to .case()) will have no effect on this function.

    There are two reasons you may want to do this:

    1. You want to ensure that the reducer is not modified further

      Calling .build() is an example of defensive coding. It prevents someone from causing confusing behavior by importing your reducer in an unrelated file and adding cases to it.

    2. You want your package to export a reducer, but not have its types depend on typescript-fsa-reducers

      If the code that defines a reducer and the code that uses it reside in separate NPM packages, you may run into type errors since the exported reducer has type ReducerBuilder, which the consuming package does not recognize unless it also depends on typescript-fsa-reducers. This is avoided by returning a plain function instead.

    Example usage:

    const reducer = reducerWithInitialState(INITIAL_STATE)
        .case(setName, setNameHandler)
        .case(addBalance, addBalanceHandler)
        .case(setIsFrozen, setIsFrozenHandler)


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