Kiss State is a powerful and modern state management library for React, launched in May 2025. While new to React, Kiss has been available for Flutter for years, battle-tested in hundreds of real-world applications.

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Store and state

The store holds all application state. Here are a few examples:

// If your state is a number
const store = createStore<number>({initialState: 1});

// If your state is a plain JavaScript object
const store = createStore({initialState: {name: 'Mary', age: 25}});

// If your state is an instance of an ES6 class
class State { constructor(public name: string, public age: number){} }
const store = createStore<State>({initialState: new State('Mary', 25)});

Use a StoreProvider to add the store to your component tree:

function App() {
  return (
    <StoreProvider store={store}>
      <AppContent />
    </StoreProvider>
  );
};

Components use the state

The useAllState hook lets you access the state from any component. The component re-renders when the state changes.

function MyComponent() { 
  const state = useAllState();   
  
  return <div>{state.name} is {state.age} years old</div>;    
};

The useSelect hook selects only the part of the state that your component needs. The component re-renders only when that part changes.

function MyComponent() { 
  const name = useSelect((state) => state.name);   
  const age = useSelect((state) => state.age);
     
  return <div>{name} is {age} years old</div>;    
};

The useObject hook also causes the component to re-render only when needed:

function MyComponent() {
 
  const state = useObject((state) => {
    name: state.name, 
    age: state.age
  });
       
  return <div>{state.name} is {state.age} years old</div>;    
};

Actions and reducers

An action is a class with its own reducer function. This reducer has access to the current state and returns a new state.

class Increment extends Action {

  reduce() { 
    // The reducer has access to the current state
    return this.state + 1; // It returns a new state 
  };
}

Dispatch an action

The store's state is immutable.

The only way to change the store's state is by dispatching an action. This runs the action's reducer to get a new state. The new state replaces the current one, and affected components re-render.

// Dispatch an action
store.dispatch(new Increment());

// Dispatch multiple actions
store.dispatchAll([new Increment(), new LoadText()]);

// Dispatch an action and wait for it to finish
await store.dispatchAndWait(new Increment());

// Dispatch multiple actions and wait for them to finish
await store.dispatchAndWaitAll([new Increment(), new LoadText()]);

Components can dispatch actions

Hooks for dispatching actions include useDispatch, useDispatchAll, etc.

function MyComponent() { 
  const dispatch = useDispatch();  

  return (
    <Button onClick={() => dispatch(new LoadText())}> 
      Click me! 
    </Button>
  );
};

You can also get the store with useStore and dispatch actions directly:

function MyComponent() { 
  const store = useStore();  

  return (
    <Button onClick={() => store.dispatch(new LoadText())}> 
      Click me! 
    </Button>
  );
};

Actions can do asynchronous work

Actions can fetch data from the internet or do any other async work.

const store = createStore<string>({initialState: ''});

class LoadText extends Action {

  // This reducer returns a Promise
  async reduce() {

    // Download something from the internet
    let response = await fetch('https://dummyjson.com/todos/1');
    let text = await response.text(); 

    // Change the state with the downloaded information
    return (state) => text;
  }
}

Actions can throw errors

If an error occurs, you can simply throw it. In this case, the action will not change the state. Errors are caught globally and can be handled later in one central place.

If you throw a UserException, a type provided by Kiss, a dialog or another UI element opens automatically and shows the error message to the user.

class LoadText extends Action {
  
  async reduce() {
    let response = await fetch("https://dummyjson.com/todos/random/1");
    if (!response.ok) throw new UserException("Failed to load.");    
    
    let text = await response.text();     
    return (state) => text;
  }
}

Components can react to actions

To show a spinner while an asynchronous action is running, use useIsWaiting(ActionType).

To show an error message inside the component, use useIsFailed(ActionType).

function MyComponent() {

  const isWaiting = useIsWaiting(LoadText); 
  const isFailed = useIsFailed(LoadText);  
  const state = useAllState();  
  
  if (isWaiting) return <CircularProgress />
  if (isFailed) return <p>Loading failed...</p>;
  return <p>{state}</p>;
}

Actions can dispatch other actions

You can use dispatchAndWait to dispatch an action and wait for it to finish.

class LoadTextAndIncrement extends Action {

  async reduce() {
  
    // Dispatch and wait for the action to finish   
    await this.dispatchAndWait(new LoadText());
    
    // Only then increment the state
    return (state) => state.copy({ count: state.count + 1 });  
  }
}

You can also dispatch actions in parallel and wait for them to finish:

class BuyAndSell extends Action {

  async reduce() {  
    
    // Dispatch and wait for both actions to finish
    await this.dispatchAndWaitAll([
      new BuyAction('IBM'), 
      new SellAction('TSLA')
    ]);        

    return (state) => state.copy({ 
      message: `New cash balance is ${this.state.cash}` 
    });
  }
}

You can also use waitCondition to wait until the state meets a condition:

class SellStockForPrice extends Action {
  constructor(public stock: string, public price: number) { super(); }

  async reduce() {
  
    // Wait until the stock price is higher than the limit price
    await this.waitCondition(
      (state) => state.stocks.getPrice(this.stock) >= this.price
    );
    
    // Only then post the sell order to the backend
    let amount = await postSellOrder(this.stock);    
    
    return (state) => 
      state.copy({
        stocks: state.stocks.setAmount(this.stock, amount),
      });
  }
}

Add features to your actions

It's easy to add your own reusable features to actions. Kiss also includes several useful features out of the box:

NonReentrant

To prevent an action from running again while it is already running, add the nonReentrant property to your action class and set it to true.

class LoadText extends Action { 
  nonReentrant = true;
   
  reduce() { ... }
}

Retry

To retry an action a few times with exponential backoff if it fails, add the retry property to your action class.

class LoadText extends Action {   
  retry = {on: true}
         
  reduce() { ... }
}

And you can specify the retry policy:

class LoadText extends Action {

  retry = {
    initialDelay: 350, // Delay in milliseconds before the first retry
    maxRetries: 3,     // Number of retries before stopping
    multiplier: 2,     // Factor used to increase the delay after each retry
    maxDelay: 5000,    // Maximum delay between retries, in milliseconds
  }
   
  reduce() { ... }
}

CheckInternet

To check for an internet connection before running the action, add the checkInternet property. If there is no internet connection, the action stops. You can also show a dialog that says, "There is no internet connection. Please check your connection."

class LoadPrices extends Action {    
  checkInternet = { dialog: true } 
   
  async reduce() { ... } 
}

Debounce (soon)

To limit how often an action runs in response to rapid input, add a debounce property to your action class. For example, when a user types into a search bar, debouncing ensures that not every keystroke triggers a server request. Instead, the action waits until the user stops typing for a short time before running.

class SearchText extends Action {
  constructor(public searchTerm: string) { super(); }
  
  debounce = 300 // Milliseconds
   
  async reduce()  {      
    let result = await loadJson('https://example.com/?q=', searchTerm);
    return (state) => state.copy({searchResult: result});
  }   
}

Throttle (soon)

To prevent an action from running too frequently, you can add a throttle property to your action class. This means that once the action runs it's considered fresh, and it won't run again for a set period of time, even if you dispatch it again during that period. After this period ends, the action is considered stale and is ready to run again.

class LoadPrices extends Action {    
  throttle = 5000 // Milliseconds
   
  async reduce()  {      
    let result = await loadJson('https://example.com/prices');
    return (state) => state.copy({prices: result});
  } 
}

OptimisticUpdate (soon)

To provide instant feedback when an action saves data to the server, you can use an optimistic update. This feature changes the state immediately, before the server confirms that the update succeeded. If the server update fails, the state is changed back. You can also show a notification to the user.

class SaveName extends Action {    
  optimisticUpdate = { ... } 
   
  async reduce() { ... } 
}

Persist the state

You can add a persistor to save the state to the local device disk. It supports serialization of JavaScript objects and ES6 class instances.

const store = createStore<State>({  
  persistor: new Persistor(),
});  

Testing your app is easy

Just dispatch actions and wait for them to finish. Then verify the new state or check whether an error occurred.

class State {
  constructor(
    public items: string[], 
    public selectedItem: number
  ) {}
}

test('Selecting an item', async () => {

  const store = createStore<State>({      
    initialState: new State(['A', 'B', 'C'], -1)
  });
  
  // Should select item 2
  await store.dispatchAndWait(new SelectItem(2));
  expect(store.state.selectedItem).toBe('B');
  
  // Fail to select item 42
  let status = await store.dispatchAndWait(new SelectItem(42));    
  expect(status.originalError).toBeInstanceOf(UserException);          
});

Advanced setup

If you are the team lead, you can set up the app's infrastructure in one central place, so developers can focus on business logic.

You can add a stateObserver to collect app metrics, an errorObserver to log errors, an actionObserver to log information to the console during development, and a globalWrapError to change errors before they are handled.

const store = createStore<string>({    
  stateObserver: (action, prevState, newState, error, count) => { ... },
  errorObserver: (error, action, store) => { ... },
  actionObserver: (action, count, ini) => { ... },
  globalWrapError: (error) => { ... }
});  

For example, here we handle FirestoreError errors thrown by Firebase. We convert them into UserException errors, which are built-in types that automatically show a message to the user in an error dialog:

globalWrapError: (error: any) => {
   return (error instanceof FirestoreError)
      ? new UserException('Error connecting to Firebase')
      : error;
   }  

Advanced action configuration

The team lead can create a base action class that all actions will extend, and add some common functionality to it. For example, the base class can provide getter shortcuts to important parts of the state and helper methods to find information.

class State {  
  items: Item[];    
  selectedItem: number;
}

export abstract class Action extends KissAction<State> {

  // Convenience getters   
  get items() { return this.state.items; }
  get selectedItem() { return this.state.selectedItem; }
  
  // Selectors 
  findById(id) { return this.items.find((item) => item.id === id); }
  get selectedIndex() { return this.items.indexOf(this.selectedItem); }
  searchByText(text) { return this.items.find((item) => item.text.includes(text)); }
}

Now, all actions can use them to access the state in their reducers:

class SelectItem extends Action {
  constructor(public id: number) { super(); }

  reduce() {
    let item = this.findById(this.id);
    if (item === undefined) throw new Error('Item not found');
    return this.state.copy({selectedItem: item});
  }
}

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