On page 52, example code chapter_03/04/app/javascript/controllers/favorite_toggle_controller.ts, which is:

import { Controller } from "@hotwired/stimulus";

export default class FavoriteToggleController extends Controller {
  static targets = ["elementToHide"];
  elementToHideTarget: HTMLElement;

  toggle(): void {
    this.elementToHideTarget.classList.toggle("hidden");
  }
}

The way it is, will not work, since elementToHideTarget will always be undefined. To work I needed to make the following change:

import { Controller } from "@hotwired/stimulus";

export default class FavoriteToggleController extends Controller {
  static targets = ["elementToHide"];
  declare readonly elementToHideTarget: HTMLElement;

  toggle(): void {
    this.elementToHideTarget.classList.toggle("hidden");
  }
}

I did the same change on page 55:

static values = { visible: Boolean };
declare visibleValue: boolean;

Note that this time I did not declare it as readonly, since I am changing its value on flipState() function.

Versions:

Ruby 3.1.2
Rails: 7.0.4
Gem stimulus-rails: 1.1.0
@hotwired/stimulus: 3.1.0