Начало наработок

src/script.js

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+const LOCAL_DATA_KEY = 'example-for-kids';
+const START_PAGE = 'StartPage';
+const GAME_PAGE = 'GamePage';
+const RESULT_PAGE = 'ResultPage';
+const NO_DISPLAY_CLASS = 'd-none';
+
+const MAX_NUMBER = 10;
+
+const store = {
+    page: START_PAGE,
+    examples: [],
+};
+
+const getRandomNumber = (maxNumber) => Math.round(Math.random() * maxNumber);
+const getFirstNumber = () => getRandomNumber(MAX_NUMBER);
+const getSecondNumber = (firstNumber) => getRandomNumber(MAX_NUMBER - firstNumber);
+const getSum = (first, second) => first + second;
+const getExample = () => {
+    const first = getFirstNumber();
+    const second = getSecondNumber(first);
+
+    return {
+        first,
+        second,
+        sum: getSum(first, second),
+    };
+};
+
+const getData = (defaultData) => {
+    try {
+        return JSON.parse(localStorage.getItem(LOCAL_DATA_KEY)) || defaultData;
+    } catch (e) {
+        return defaultData;
+    }
+};
+const setData = (data) => {
+    localStorage.setItem(LOCAL_DATA_KEY, JSON.stringify(data));
+};
+
+const startPage = document.querySelector(`.${START_PAGE}`);
+const gamePage = document.querySelector(`.${GAME_PAGE}`);
+const resultPage = document.querySelector(`.${RESULT_PAGE}`);
+
+const renderPage = () => {
+    [startPage, gamePage, resultPage].forEach(page => {
+        if (page.classList.contains(store.page)) {
+            page.classList.remove(NO_DISPLAY_CLASS);
+        } else {
+            page.classList.add(NO_DISPLAY_CLASS);
+        }
+    });
+};
+
+renderPage();
+