Export reducer instead of switch

createReducer.js

export default function(initialState, handlers) {
  return function(state = initialState, action) {
    const handler = handlers[action.type];
    
    return typeof hander === 'function' ? handler(state, action) : state;
  }
}

store.js

import * as todoHandlers from 'todos';
import createReducer from 'createReducer';
import { createStore } from 'redux';

export default createStore(createReducer({}, todoHandlers));

todo.js

export function MAKE_TODO(text, completed) {
  return {
    text: text,
    completed: completed
  };
}

export function COMPLETE_TODO(todo) {
  Object.assign({}, todo, {
    completed: true
  });
}

export function RENAME_TODO(todo, text) {
  Object.assign({}, todo, {
    text: text
  });
}

export function MAKE_TODO() {
  return [];
}

export function ADD_TODO(todos, text, completed) {
  return [...todos, MAKE_TODO(text, completed)];
}

export function MODIFY_TODO(todos, index, modify) {
  return [
    ...todos.slice(0, index),
    modify(todos[index]),
    ...todos.slice(index + 1)
  ];
}