From https://once.com/writebook

app/javascript/helpers/cookie_helpers.js

@@ -0,0 +1,22 @@
+export function setCookie(name, value = "", days = 365) {
+  let expires = ""
+
+  const date = new Date()
+  date.setTime(date.getTime() + days * 24 * 60 * 60 * 1000)
+  expires = `; expires=${date.toUTCString()}`
+
+  document.cookie = `${name}=${value}${expires}; path=/`
+}
+
+export function readCookie(name) {
+  const nameEQ = `${name}=`
+  const ca = document.cookie.split(";")
+
+  for (let i = 0; i < ca.length; i++) {
+    let c = ca[i]
+    while (c.charAt(0) === " ") c = c.substring(1, c.length)
+    if (c.indexOf(nameEQ) === 0) return c.substring(nameEQ.length, c.length)
+  }
+
+  return null
+}

app/javascript/helpers/form_helpers.js

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+import { FetchRequest } from "@rails/request.js"
+
+export async function submitForm(form) {
+  const request = new FetchRequest(form.method, form.action, {
+    body: new FormData(form)
+  })
+
+  return await request.perform()
+}

app/javascript/helpers/reading_progress_helpers.js

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+import { setCookie, readCookie } from "helpers/cookie_helpers"
+
+export function getReadingProgress(bookId) {
+  const progress = readCookie(`reading_progress_${bookId}`)
+
+  if (progress) {
+    const [ leafId, lastReadParagraph ] = progress.split("/")
+    return [ parseInt(leafId), parseInt(lastReadParagraph) || 0 ]
+  } else {
+    return [ undefined, 0 ]
+  }
+}
+
+export function storeReadingProgress(bookId, leafId, lastReadParagraphIndex) {
+  setCookie(`reading_progress_${bookId}`, `${leafId}/${lastReadParagraphIndex}`)
+}

app/javascript/helpers/timing_helpers.js

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+export function throttle(fn, delay = 1000) {
+  let timeoutId = null
+
+  return (...args) => {
+    if (!timeoutId) {
+      fn(...args)
+      timeoutId = setTimeout(() => timeoutId = null, delay)
+    }
+  }
+}
+
+export function debounce(fn, delay = 1000) {
+  let timeoutId = null
+
+  return (...args) => {
+    clearTimeout(timeoutId)
+    timeoutId = setTimeout(() => fn.apply(this, args), delay)
+  }
+}
+
+export function nextEventLoopTick() {
+  return delay(0)
+}
+
+export function onNextEventLoopTick(callback) {
+  setTimeout(callback, 0)
+}
+
+export function nextFrame() {
+  return new Promise(requestAnimationFrame)
+}
+
+export function nextEventNamed(eventName, element = window) {
+  return new Promise(resolve => element.addEventListener(eventName, resolve, { once: true }))
+}
+
+export function delay(ms) {
+  return new Promise(resolve => setTimeout(resolve, ms))
+}