If you still have the app running from a previous step, stop it with ctrl+c. Start the tests instead by running npm test from the root of the frontend-bootcamp folder.

First, take a look at the store interface in exercise/src/store/index.ts. Note that the Store interface has two keys: todos and filter. We'll concentrate on todos, which is an object where the keys are string IDs and the values are of a TodoItem type.
Open exercise/src/reducers/index.ts and fill in the missing case statements for the switch on action.type.
Open exercise/src/index.tsx and write separate dispatch calls.
Take a look what is written in the console (F12 on PC, cmd-option-I on Mac).
Install the Chrome or Firefox extensions
Observe the state changes, try doing "time travel"