Changing state is optional

For both sync and async reducers, returning a new state is optional. If you don't plan on changing the state, simply return null. This is the same as returning the state unchanged:

class GetAmount extends Action {
  
  reduce() async {    
    let amount = await getAmount();
    
    if (amount == 0) 
      return null;
    else 
      return (state) => state.copy({counter: state.counter + amount}));
  }
}

This also works:

class GetAmount extends Action {
  
  reduce() async {    
    let amount = await getAmount();
    
    return (state) => 
      (amount == 0) 
        ? null
        : state.copy({counter: state.counter + amount}));        
  }
}

Returning null is also useful when your action should not change the state, but simply start other async processes, or dispatch other actions. For example:

class Initialize extends Action {
  
  reduce() {
    this.dispatch(new ReadDatabaseAction());        
    this.dispatch(new StartTimersAction());          
    this.dispatch(new TurnOnListenersAction());
              
    return null;          
  }
}

Note the reduce() method has direct access to dispatch.

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Next, let's see how to dispatch actions.