leporello-js/docs/examples/todos-redux/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