b-o-a をもっと簡素化できないか

index.ts

// 1. Aciton の data を受けて次の Action type & data を返す関数群
// 2. 1 個の Action を N 個の ActionHandler で処理したい
// X 3. N 個の Action に共通の ActionHandler を追加したい
//   -> merge, chain がほしい。dummy の Action を追加するのは辛い

// framework
import { EventEmitter } from 'events';

type NextAction = (action: Action) => void;
type ActionHandler = (data: any, re: NextAction) => Action;
type Action = {
  type: string;
  data: any;
}
type App = { [actionType: string]: ActionHandler | ActionHandler[] };
const run = (app: App): void => {
  const emitter = new EventEmitter();
  const re: NextAction = (action: Action) => setTimeout(() => emitter.emit(action));
  emitter.on(action => {
    const { type, data } = action;
    const handler = app[type];
    const hoh = app[type];
    const handlers = Array.isArray(hoh) ? hoh : [hoh];
    handlers.forEach(handler => {
      re(handler(data, re));
    });
  });
};

// app
const action1: ActionHandler = (data, next) => {
  // 3.NG
  // oh...
  // let result = action4(data, next); //...
  return { type: 'action2', data: 123 };
};

const action2: ActionHandler = (data, next) => {
  // 3.NG
  // oh...
  // let result = action4(data, next); //...
  return null;
};

const action3: ActionHandler = (data, next) => {
  console.log(data);
  return null;
};

const action4: ActionHandler = (data, next) => {
  console.log('action 4');
  return null;
};

// 3. NG
// const app: App = {
//   'action1': [action1, action3, action4], // oh...
//   'action2': [action2, action4] // oh...
// };

const app: App = {
  'action1': [action1, action3],
  'action2': [action2]
};

run(app);