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const optionsCount = 6;
const sitesURL = 'https://api.stackexchange.com/2.2/sites?pagesize=999&filter=!SmNU3t0u7kg0gEzR5B';
const questionsURL = 'https://api.stackexchange.com/2.2/questions?page=1&pagesize=100&order=desc&sort=votes&filter=!bA1d_Kvv2YPQWk';

let game = 
 questionReady: false,
 question: ,
 options: ,
 answer: 0,
 questionsAnswered: 0,
 questionsCorrect: 0,
 sites: 


let elements = 
 stats: null,
 question: null,
 options: ,
 next: null


document.addEventListener('DOMContentLoaded', init);

function init() 
 elements.stats = document.getElementById('stats');
 elements.question = document.getElementById('question');
 elements.next = document.getElementById('next');
 
 let answers = document.getElementById('answers');
 for(let i = 0; i < optionsCount; i++) 
 let button = document.createElement('button');
 button.addEventListener('click', makeGuess.bind(null, i));
 answers.appendChild(button);
 elements.options.push(button);
 
 
 loadSites().then(chooseQuestion);


function loadSites() 
 return fetch(sitesURL)
 .then(response => response.json())
 .then(data => 
 game.sites = data.items.filter(e => e.site_type === "main_site");
 )


function loadQuestions(index) 
 return fetch(questionsURL + '&site=' + game.sites[index].api_site_parameter)
 .then(response => response.json())
 .then(data => 
 game.sites[index].questions = data.items;
 )


async function chooseQuestion() 
 game.options = getOptions(optionsCount, game.sites.length);
 game.answer = randInt(optionsCount);
 game.question = await getQuestion(game.options[game.answer]);
 updateView();
 game.questionReady = true;


function getOptions(amount, max) 
 let options = ;
 while(options.length < amount) 
 let value = randInt(max);
 if(!options.includes(value)) 
 options.push(value);
 
 
 return options;


function randInt(limit) 
 return Math.floor(Math.random() * limit);


async function getQuestion(index) 
 if(!game.sites[index].questions) 
 await loadQuestions(index);
 
 let size = game.sites[index].questions.length;
 return game.sites[index].questions[randInt(size)];


function updateView() 
 elements.question.innerHTML = game.question.title;
 for(let i = 0; i < optionsCount; i++) 
 elements.options[i].innerHTML = game.sites[game.options[i]].name;
 


function makeGuess(guess) 
 if(game.questionReady) 
 game.questionsAnswered++;
 if(guess === game.answer) 
 game.questionsCorrect++;
 
 else 
 elements.options[guess].classList.add('wrong');
 
 elements.options[game.answer].classList.add('correct');
 elements.next.disabled = false;
 elements.question.setAttribute('href', game.question.link);
 let percent = Math.round(100 * game.questionsCorrect / game.questionsAnswered);
 elements.stats.innerText = `$game.questionsCorrect/$game.questionsAnswered - $percent%`;
 game.questionReady = false;
 


function onNextClick() 
 elements.question.removeAttribute('href');
 for(option of elements.options) 
 option.classList.remove('correct', 'wrong');
 
 elements.next.disabled = true;
 chooseQuestion();

.correct 
 background-color: #54da54;

.wrong 
 background-color: #ff6565;

#answers button 
 margin: 2px;

<p id="stats">0/0 - 0%</p>
<h2><a id="question" target="_blank"></a></h2>
<div id="answers"></div>
<br><br>
<button id="next" onclick="onNextClick()" disabled>Next question</button>