leporello-js/docs/examples/todos/src/reducers/todos.js

const todos = (state = [], action) => {
  if(action.type == 'ADD_TODO') {
    return [
      ...state,
      {
        id: action.id,
        text: action.text,
        completed: false
      }
    ]
  } else if(action.type == 'TOGGLE_TODO') {
    return state.map(todo =>
      (todo.id === action.id)
        ? {...todo, completed: !todo.completed}
        : todo
    )
  } else {
    return state
  }
}

export default todos