In the adding real features section, the application.ts excerpt is from introduction/03 when it should be introduction/02 (introduction/03/app/javascript/packs/application.ts). The sections around this excerpt are talking about the state of the code from 02.
(03 has the DayToggle class refactored out into the GenericToggle class, but the text doesn’t discuss this action until the Going Generic section starting on page 21. The 03 version of application.ts appears correctly in the excerpt on pp 25-26.)