From https://once.com/writebook

app/javascript/controllers/reading_progress_controller.js

@@ -0,0 +1,20 @@
+import { Controller } from "@hotwired/stimulus"
+import { getReadingProgress } from "helpers/reading_progress_helpers"
+
+export default class extends Controller {
+  static values = { bookId: Number }
+  static classes = [ "lastRead" ]
+
+  connect() {
+    this.#markLastReadLeaf()
+  }
+
+  #markLastReadLeaf() {
+    const [ leafId ] = getReadingProgress(this.bookIdValue)
+    const leafElement = leafId && this.element.querySelector(`#leaf_${leafId}`)
+
+    if (leafElement) {
+      leafElement.classList.add(this.lastReadClass)
+    }
+  }
+}