partially working solution

eslint.config.js

@@ -3,12 +3,12 @@ import eslintPluginPrettierRecommended from "eslint-plugin-prettier/recommended"
 import globals from "globals";
 
 export default [
-  {    
-    ignores: ["dist"]  
-  },
   {
-    ...js.configs.recommended,
-    ...eslintPluginPrettierRecommended,
+    ignores: ["dist"],
+  },
+  eslintPluginPrettierRecommended,
+  js.configs.recommended,
+  {
     rules: {
       "no-unused-vars": "warn",
       "no-undef": "warn",

lib/bookPriceFor.js

@@ -0,0 +1,50 @@
+import currency from "currency.js";
+import { bookUrl } from "./bookUrl";
+import { extractPriceFrom } from "./extractPriceFrom";
+import { getRate } from "./rates";
+import { cacheBookPrice, getBookPrice } from "./cache";
+import { format, isToday } from "date-fns";
+
+export const bookPriceFor = async (country) => {
+  l("looking price for", country.countryCode);
+
+  const url = bookUrl(country.countryCode);
+
+  const fromCache = getBookPrice(url);
+
+  if (fromCache && isToday(fromCache.cachedAt)) {
+    l("found book price in cache", fromCache);
+    return fromCache;
+  }
+
+  const countryPrice = await extractPriceFrom(url);
+
+  let convertedPrice;
+
+  if (countryPrice) {
+    const rate = await getRate(country.currencyCode);
+
+    l("rate for", country.currencyCode, "is", rate);
+
+    convertedPrice = currency(countryPrice).divide(rate);
+
+    convertedPrice = {
+      intValue: convertedPrice.intValue,
+      format: convertedPrice.format(),
+    };
+
+    l("converted price for", country.currencyCode, convertedPrice);
+  }
+
+  const newPrice = {
+    ...country,
+    countryPrice,
+    convertedPrice,
+    url,
+    cachedAt: format(new Date(), "yyyy-MM-dd"),
+  };
+
+  cacheBookPrice(newPrice, url);
+
+  return newPrice;
+};

lib/bookUrl.js

@@ -0,0 +1,6 @@
+export const bookUrl = (country) => {
+  const urlPattern = /^https:\/\/www\.kobo\.com\/../;
+  const newPath = `https://www.kobo.com/${country}`;
+
+  return window.location.href.replace(urlPattern, newPath);
+};

lib/cache.js

@@ -0,0 +1,18 @@
+const CACHE_PREFIX = "KOBOPRICE";
+
+export const initCache = () => {
+  if (!localStorage.getItem(CACHE_PREFIX)) {
+    setState({ books: {}, rates: null });
+  }
+};
+export const getState = () => JSON.parse(localStorage.getItem(CACHE_PREFIX));
+export const setState = (s) =>
+  localStorage.setItem(CACHE_PREFIX, JSON.stringify(s));
+export const getRates = () => getState().rates;
+export const cacheRates = (rates) => setState({ ...getState(), rates });
+export const getBookPrice = (url) => getState().books[url];
+export const cacheBookPrice = (price, url) => {
+  const state = getState();
+  state.books[url] = price;
+  setState(state);
+};

lib/countries.js

@@ -1,12 +1,12 @@
 export const COUNTRIES = [
   { countryCode: "ww", currencyCode: "usd" },
   { countryCode: "ca", currencyCode: "cad" },
-  // { countryCode: "us", currencyCode: "usd" },
+  { countryCode: "us", currencyCode: "usd" },
   { countryCode: "in", currencyCode: "inr" },
-  // { countryCode: "za", currencyCode: "zar" },
-  // { countryCode: "au", currencyCode: "aud" },
-  // { countryCode: "hk", currencyCode: "hkd" },
-  // { countryCode: "jp", currencyCode: "EUR" },
+  { countryCode: "za", currencyCode: "zar" },
+  { countryCode: "au", currencyCode: "aud" },
+  { countryCode: "hk", currencyCode: "hkd" },
+  { countryCode: "jp", currencyCode: "JPY" },
   // { countryCode: "my", currencyCode: "EUR" },
   // { countryCode: "nz", currencyCode: "EUR" },
   // { countryCode: "ph", currencyCode: "EUR" },

lib/currency.js

@@ -1,20 +0,0 @@
-import currency from "currency.js";
-
-export const baseCurrency = "usd";
-
-export const convertCurrency = async (price, outCurrency, rates) => {
-  return currency(price).divide(rates[baseCurrency][outCurrency]);
-};
-
-export const loadCurrencyRates = async () => {
-  l("loading currency rates");
-  try {
-    const res = await fetch(
-      `https://cdn.jsdelivr.net/npm/@fawazahmed0/currency-api@latest/v1/currencies/${baseCurrency}.json`,
-    );
-    return await res.json();
-  } catch (e) {
-    l("error", e);
-    return null;
-  }
-};

lib/extractPriceFrom.js

@@ -0,0 +1,48 @@
+const timeout = (duration) => new Promise((r) => setTimeout(r, duration));
+
+const observePriceOnPage = (page) =>
+  new Promise((res, rej) => {
+    timeout(10000).then(() => rej("price not found"));
+
+    var observer = new MutationObserver(() => {
+      const price = page.querySelector(
+        ".primary-right-container .pricing-details .active-price span",
+      ).textContent;
+
+      if (price) {
+        l("found price", price);
+        observer.disconnect();
+        res(price);
+      }
+    });
+
+    observer.observe(page, {
+      attributes: true,
+      childList: true,
+      characterData: true,
+      subtree: true,
+    });
+  });
+
+export const extractPriceFrom = async (url) => {
+  try {
+    l("going to", url);
+
+    const iframe = document.createElement("iframe");
+
+    iframe.src = url;
+
+    iframe.hidden = true;
+
+    document.body.append(iframe);
+
+    await new Promise((res) => (iframe.contentWindow.onload = res));
+
+    l("starting observing price on", url);
+
+    return await observePriceOnPage(iframe.contentDocument.body, url);
+  } catch (e) {
+    l("getPriceForCountry", e, url);
+    return "";
+  }
+};

lib/getPriceForCountry.js

@@ -1,56 +0,0 @@
-const timeout = (duration) => new Promise((r) => setTimeout(r, duration));
-
-const observePriceOnPage = (page) =>
-  new Promise((res, rej) => {
-    timeout(10000).then(() => rej(""));
-
-    var observer = new MutationObserver(() => {
-      const price = page
-        .querySelector(".active-price")
-        .querySelector("span").textContent;
-      if (price) {
-        observer.disconnect();
-        res(price);
-      }
-    });
-
-    observer.observe(page, {
-      attributes: true,
-      childList: true,
-      characterData: true,
-      subtree: true,
-    });
-  });
-
-const bookUrlForCountry = (country) => {
-  const urlPattern = /^https:\/\/www\.kobo\.com\/../;
-  const newPath = `https://www.kobo.com/${country}`;
-
-  return window.location.href.replace(urlPattern, newPath);
-};
-
-export const getPriceForCountry = (country) =>
-  new Promise((res) => {
-    const url = bookUrlForCountry(country);
-
-    l("going to", url);
-
-    const iframe = document.createElement("iframe");
-
-    iframe.src = url;
-
-    iframe.hidden = true;
-
-    document.body.append(iframe);
-
-    iframe.contentWindow.onload = () => {
-      l("starting observing price on", url);
-      observePriceOnPage(iframe.contentDocument.body, url)
-        .then(res)
-        .catch(() => {
-          l(`failed to find price for ${url}`);
-          res("");
-        })
-        .finally(() => document.body.removeChild(iframe));
-    };
-  });

lib/index.js

@@ -1,29 +1,94 @@
-import './logger'
+import "./logger";
 import { COUNTRIES } from "./countries";
-import { getPriceForCountry } from "./getPriceForCountry";
-import { loadCurrencyRates, convertCurrency } from "./currency";
+import { initCache } from "./cache";
+import { bookPriceFor } from "./bookPriceFor";
 
+/*
+TODO:
+- not all currencies parsed as expected: 
+  https://www.kobo.com/za/en/ebook/nine-minds
+  ZA: R300,60 -> $1,674.58
+  JP: 2,246 円 -> $Infinity
+- publish to stores
+- check in chrome
+- more durable source for rates
+- React for UI
+- More informative, show loading progress
+- clear stale cache
+*/
+
+const createPricesContainer = () => {
+  const pricingActionContainer = document
+    .querySelector(".primary-right-container")
+    .querySelector(".pricing-details")
+    .querySelector(".action-container");
+
+  const container = document.createElement("div");
+
+  container.style.display = "flex";
+  container.style.flexDirection = "column";
+
+  return pricingActionContainer.parentNode.insertBefore(
+    container,
+    pricingActionContainer,
+  );
+};
+
+const getCountriesPrice = async () => {
+  const prices = [];
+  for (const country of COUNTRIES) {
+    // intentionally blocking execution
+    // to resolve sequentially.
+    // It should prevent DOS and triggering captcha
+    prices.push(await bookPriceFor(country));
+  }
+
+  return prices;
+};
+
+const formatPrice = (price) => {
+  const convertedPrice = price?.convertedPrice?.format ?? "NO PRICE";
+
+  const countryPrice = price?.countryPrice ?? "NO PRICE";
+
+  return `${price.countryCode.toUpperCase()}: ${countryPrice} -> ${convertedPrice}`;
+};
+
+const sortPrices = (prices) =>
+  prices.sort(
+    (a, b) =>
+      (a?.convertedPrice?.intValue || Infinity) -
+      (b?.convertedPrice?.intValue || Infinity),
+  );
+
+const showPrices = (container, prices) => {
+  container.innerText = null;
+  prices.forEach((price) => {
+    const link = document.createElement("a");
+    link.href = price.url;
+    link.target = "_blank";
+    link.style.marginBottom = "5px";
+    link.innerText = formatPrice(price);
+    container.appendChild(link);
+  });
+};
 async function main() {
   try {
-    const rates = await loadCurrencyRates();
+    initCache();
 
-    l('currency rates', rates)
+    const container = createPricesContainer();
 
-    const prices = await Promise.all(
-      COUNTRIES.map(async (c) => {
-        l("looking price for", c.countryCode);
-        const originalPrice = await getPriceForCountry(c.countryCode);
-        const convertedPrice = await convertCurrency(
-          originalPrice,
-          c.currencyCode,
-          rates,
-        );
+    container.innerText = "LOADING PRICES...";
 
-        return { ...c, originalPrice, convertedPrice };
-      }),
-    );
+    const countriesPrice = await getCountriesPrice();
 
-    l(prices);
+    l("country prices", countriesPrice);
+
+    sortPrices(countriesPrice);
+
+    l("sorted prices", countriesPrice);
+
+    showPrices(container, countriesPrice);
   } catch (e) {
     l("error", e);
   } finally {

lib/rates.js

@@ -0,0 +1,41 @@
+import { isToday } from "date-fns";
+import { getRates, cacheRates } from "./cache";
+
+export const baseCurrency = "usd";
+
+export const loadCurrencyRates = async () => {
+  l("loading currency rates");
+
+  try {
+    const res = await fetch(
+      `https://cdn.jsdelivr.net/npm/@fawazahmed0/currency-api@latest/v1/currencies/${baseCurrency}.json`,
+    );
+    return await res.json();
+  } catch (e) {
+    l("loadCurrencyRates", e);
+    return null;
+  }
+};
+
+export const getRate = async (currency) => {
+  const cached = getRates();
+
+  if (cached && isToday(cached.date)) {
+    l("found rates in cache", cached);
+
+    return cached[baseCurrency][currency];
+  }
+
+  const newRates = await loadCurrencyRates();
+
+  if (!newRates) {
+    l("failed to download rates");
+    return 0;
+  }
+
+  l("new rates", newRates);
+
+  cacheRates(newRates);
+
+  return newRates[baseCurrency][currency];
+};

package-lock.json

@@ -9,7 +9,8 @@
       "version": "1.0.0",
       "license": "ISC",
       "dependencies": {
-        "currency.js": "^2.0.4"
+        "currency.js": "^2.0.4",
+        "date-fns": "^3.6.0"
       },
       "devDependencies": {
         "@eslint/js": "^9.6.0",
@@ -2145,6 +2146,22 @@
         "url": "https://github.com/open-cli-tools/concurrently?sponsor=1"
       }
     },
+    "node_modules/concurrently/node_modules/date-fns": {
+      "version": "2.30.0",
+      "resolved": "https://registry.npmjs.org/date-fns/-/date-fns-2.30.0.tgz",
+      "integrity": "sha512-fnULvOpxnC5/Vg3NCiWelDsLiUc9bRwAPs/+LfTLNvetFCtCTN+yQz15C/fs4AwX1R9K5GLtLfn8QW+dWisaAw==",
+      "dev": true,
+      "dependencies": {
+        "@babel/runtime": "^7.21.0"
+      },
+      "engines": {
+        "node": ">=0.11"
+      },
+      "funding": {
+        "type": "opencollective",
+        "url": "https://opencollective.com/date-fns"
+      }
+    },
     "node_modules/concurrently/node_modules/supports-color": {
       "version": "8.1.1",
       "resolved": "https://registry.npmjs.org/supports-color/-/supports-color-8.1.1.tgz",
@@ -2288,19 +2305,12 @@
       }
     },
     "node_modules/date-fns": {
-      "version": "2.30.0",
-      "resolved": "https://registry.npmjs.org/date-fns/-/date-fns-2.30.0.tgz",
-      "integrity": "sha512-fnULvOpxnC5/Vg3NCiWelDsLiUc9bRwAPs/+LfTLNvetFCtCTN+yQz15C/fs4AwX1R9K5GLtLfn8QW+dWisaAw==",
-      "dev": true,
-      "dependencies": {
-        "@babel/runtime": "^7.21.0"
-      },
-      "engines": {
-        "node": ">=0.11"
-      },
+      "version": "3.6.0",
+      "resolved": "https://registry.npmjs.org/date-fns/-/date-fns-3.6.0.tgz",
+      "integrity": "sha512-fRHTG8g/Gif+kSh50gaGEdToemgfj74aRX3swtiouboip5JDLAyDE9F11nHMIcvOaXeOC6D7SpNhi7uFyB7Uww==",
       "funding": {
-        "type": "opencollective",
-        "url": "https://opencollective.com/date-fns"
+        "type": "github",
+        "url": "https://github.com/sponsors/kossnocorp"
       }
     },
     "node_modules/debounce": {

package.json

@@ -7,9 +7,9 @@
   "scripts": {
     "test": "echo \"Error: no test specified\" && exit 1",
     "lint": "eslint --fix",
-    "build": "esbuild lib/index.js --bundle --minify --sourcemap --target=chrome58,firefox57,safari11,edge16 --outfile=dist/index.js",
+    "build": "esbuild lib/index.js --bundle --minify --sourcemap --target=chrome58,firefox57,safari11 --outfile=dist/index.js",
     "watch-build": "npm run build -- --watch",
-    "watch-ext": "web-ext run",
+    "watch-ext": "web-ext run --start-url kobo.com",
     "watch": "concurrently npm:watch-build npm:watch-ext"
   },
   "author": "",
@@ -26,6 +26,7 @@
     "web-ext": "^8.2.0"
   },
   "dependencies": {
-    "currency.js": "^2.0.4"
+    "currency.js": "^2.0.4",
+    "date-fns": "^3.6.0"
   }
 }