Stateful.jsx

// Stateful mixin is default implementation for components with state() method.
// It is intended to be used as decorator.
export function Stateful(klass) {
  if (typeof klass === "function") {
    Stateful.prototype.getOwnPropertyNames().forEach(function (methodName) {
      Object.defineProperty(klass.prototype, methodName, {value: Stateful.prototype[methodName]});
    });
    
    return klass;
  }
  
  return Stateful;
}

export async function* state(state = {}, actions) {
  // fetch() method should (re)load data
  if (!state.ok && typeof this.fetch === "function") {
    yield state = {...state, ok: null}
    try {
      yield state = {...state, ...await this.fetch(state)};
    } catch (e) {
      return {...state, ok: false, error: e};
    }
  }
  
  // If component has no bound actions, just return state (possibly with fetch()'ed data).
  if (actions) {
    for (
      var done, action; 
      {done, value: action} = await actions.next() && !done;
    ) {
      if (typeof action === "function") {
        handler = action;
      } else if (typeof action.handler === "function") {
        handler = action.handler;
      } else if (typeof action.type === "string") {
        var methodName = "on" + action.type.split("_").map((part) => 
          part.substring(0, 1).toUpperCase() + part.substring(1).toLowerCase()
        ).join("");
        
        handler = this[methodName];
      }
      
      if (handler) {
        handler = handler.call(this, action, state);
        if (typeof handler.next === "function") {
          for (
            var handlerDone, handlerState; 
            {done: handlerDone, value: handlerState} = await handler.next() && !handlerDone;
          ) {
            yield state = {...state, ...handlerState};
          }
        } else if (typeof handler.then === "function") {
          yield state = {...state, ...await handler};
        } else {
          yield state = {...state, ...handler};
        }
        handler = undefined;
      }
    }
  }
  
  return state;
}

Object.defineProperty(Stateful.prototype, "state", {value: state});