The first step when creating a full Redux application is to define so-called action types. They will be used to create actions in action creators and to handle actions in reducers. The idea here is to avoid making typos when defining, or comparing, the type property of actions.

Let's define the action types now:

1. Create a new src/actionTypes.js file.
2. Define and export the following constants in the newly created file:

export const FETCH_TODOS = 'FETCH_TODOS'
export const ADD_TODO = 'ADD_TODO'
export const TOGGLE_TODO = 'TOGGLE_TODO'
export const REMOVE_TODO = 'REMOVE_TODO'
export const FILTER_TODOS = 'FILTER_TODOS'

Now that we have defined our action types, we can start using them in action creators and reducers.