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 the application state. 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 ES6 class object
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. It rebuilds 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. It rebuilds 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 only rebuilds 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 that contains its own reducer function. This reducer has access to the current state, and its goal is to return a new and modified 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 state is immutable.

The only way to change the store state is by dispatching an action. This will run the action's reducer to get a new state that will replace the current one, and rebuild all components that use it.

// 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

The hooks to dispatch actions are useDispatch, useDispatchAll etc.

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

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

Or getting the store with useStore also allows you to dispatch actions:

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

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

Actions can do asynchronous work

Actions can download information 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 something bad happens, you may simply throw an error. In this case, the state will not change. Errors are caught globally and can be handled in a central place, later.

In particular, if you throw a UserException, which is a type provided by Kiss, a dialog (or other UI) will open automatically, showing 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 isWaiting(action).

To show an error message inside the component, use isFailed(action).

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 changes in a certain way:

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 your actions, but they come out of the box with some interesting ones:

NonReentrant

To prevent an action from being dispatched while it's 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, // Milliseconds delay before the first attempt
    maxRetries: 3,     // Number of retries before giving up
    multiplier: 2,     // Delay increase factor for each retry
    maxDelay: 5000,    // Max milliseconds delay between retries
  }
   
  reduce() { ... }
}

CheckInternet

To check if there is internet before running the action, add the checkInternet property. If there's no internet, the action aborts. Optionally, show a dialog to the user saying: "There is no Internet, please verify your connection".

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

Debounce (soon)

To limit how often an action occurs in response to rapid inputs, you can add a debounce property to your action class. For example, when a user types in a search bar, debouncing ensures that not every keystroke triggers a server request. Instead, it waits until the user pauses typing before acting.

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 try to dispatch it. 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 on actions that save information to the server, this feature immediately applies state changes as if they were already successful, before confirming with the server. If the server update fails, the change is rolled back and, optionally, a notification can inform the user of the issue.

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 classes.

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 if some error was thrown.

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 set up the app's infrastructure in a central place, and allow your developers to concentrate solely on the business logic.

You can add a stateObserver to collect app metrics, an errorObserver to log errors, an actionObserver to print information to the console during development, and a globalWrapError to catch all errors.

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 may create a base action class that all actions will extend, and add some common functionality to it. For example, getter shortcuts to important parts of the state, and selectors to help find information.

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

export abstract class Action extends KissAction<State> {

  // Getter shortcuts   
  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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