redux-typescript-reducers

Redux TypeScript Reducers

Fluent syntax for defining typesafe reducers on top of typescript-fsa.

Installation

npm install --save redux-typescript-reducers typescript-fsa

Usage

This library allows you to define reducers by chaining a series of handlers for different action types and optionally providing an initial value. It builds on top of and assumes familiarity with the excellent typescript-fsa.

Suppose we have the setup:

import actionCreatorFactory from "typescript-fsa";
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 };
}

Using vanilla typescript-fsa, you would most likely define a reducer as follows:

import { Action } from "redux";
import { isType } from "typescript-fsa";

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 redux-typescript-reducers, this is exactly equivalent to the following code:

import { reducerWithInitialState } from "redux-typescript-reducer";

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.

API

reducerWithInitialState(initialState)

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.

reducerWithoutInitialState()

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 intial 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);

upcastingReducer()

Starts a reducer 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 programs will not have a use for this.

Example usage:

type State = StoppedState | RunningState;

interface StoppedState {
    type: "STOPPED";
}

interface StartedState {
    type: "STARTED";
    count: number;
}

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: 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");
    }
}

Copyright © 2017 David Philipson