Initial State and Reducer

In this lesson, we will define the initialState and reducer for a simple Todo app with useReducer.

We'll cover the following

Initial state #

We are developing a ToDo app. Our state has a list of ToDo items. Let’s put three items initially so that we see them when the app starts.

const initialState = {
  todos: [
    { id: 1, title: 'Wash dishes' },
    { id: 2, title: 'Study JS' },
    { id: 3, title: 'Buy ticket' },
  query: '',

The IDs are almost always recommended when we have a list in a state. We use them to specify the key prop in React.

Right now, an item has only title in addition to id. We will be using another property, completed, that is undefined initially.

There is also query which we will use to highlight ToDo titles.

Now, let’s define a reducer function to manipulate this state immutably.

Reducer #

A reducer is a function to update a state with an action. An action is an object, typically with a type property. A reducer returns a new state without mutating any objects in it.

As we need to keep track of nextId, we define it in advance as an easy solution.

let nextId = 4;

The solution is 4 because we have three items initially.

We then define our reducer.

const reducer = (state, action) => {
  switch (action.type) {
    case 'ADD_TODO':
      return {
        todos: [...state.todos, { id: nextId++, title: action.title }],
    case 'DELETE_TODO':
      return {
        todos: state.todos.filter(todo => !==,
    case 'TOGGLE_TODO':
      return {
        todos: =>
            ? { ...todo, completed: !todo.completed }
            : todo,
    case 'SET_QUERY':
      return {
        query: action.query,
      return state;

It is a bit long, but there is a pattern. For each action type, it creates a new state. Let’s summarize what each action type means.

  • ADD_TODO creates a new item and appends it to the end of the list. It requires the title.
  • DELETE_TODO removes an existing item from the list. It requires the id.
  • TOGGLE_TODO changes the completed flag of the item. It requires the id.
  • SET_QUERY sets query to highlight ToDo titles.

Store file #

We create a file named store.js and export some hooks and a component.

Using initialState and reducer, the file is as follows:

// store.js

import { useReducer } from 'react';
import { createContainer } from 'react-tracked';

const initialState = ...;

let nextId = 4;

const reducer = ...;

const useValue = () => useReducer(reducer, initialState);

export const {
  useUpdate: useDispatch,
} = createContainer(useValue);

That’s all we need to define our global state.

Next #

In the next lesson, we will create a TodoList component.

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