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Learn to build an interactive quiz app in JavaScript with multiple-choice questions, timer, score tracking, progress bar, result screen, and localStorage high score.
What You'll Learn
- How to structure quiz data as an array of question objects
- How to navigate through questions and track the current index
- How to implement a countdown timer per question
- How to display immediate feedback (correct / wrong + explanation)
- How to show a results screen with score and retry option
Project Overview
A quiz app presents one question at a time. The user picks an answer, gets feedback, and moves to the next question. After all questions, a results screen shows the final score with a breakdown.
Component Structure
How It Works — Flow Diagram
Step-by-Step Build
Step 1 — Quiz Data
Example
const questions = [
{
text: "Which keyword declares a block-scoped variable?",
options: ["var", "let", "function", "const"],
correct: 1, // index of correct answer
explanation: "`let` is block-scoped. `var` is function-scoped. `const` is also block-scoped but immutable."
},
{
text: "What does `typeof null` return?",
options: ["null", "undefined", "object", "string"],
correct: 2,
explanation: "A known JavaScript quirk: `typeof null === 'object'`. It's a bug that was never fixed for backward compatibility."
},
{
text: "Which array method creates a new array?",
options: ["push()", "pop()", "map()", "sort()"],
correct: 2,
explanation: "`map()` returns a new array. `push()`, `pop()`, and `sort()` mutate the original."
},
{
text: "What is the output of `0.1 + 0.2 === 0.3`?",
options: ["true", "false", "NaN", "Error"],
correct: 1,
explanation: "Floating-point precision: `0.1 + 0.2 = 0.30000000000000004`, which is not exactly `0.3`."
},
{
text: "Which method removes the last element of an array?",
options: ["shift()", "pop()", "splice()", "unshift()"],
correct: 1,
explanation: "`pop()` removes and returns the last element. `shift()` removes the first."
}
];
Step 2 — State
js example
let currentIndex = 0;
let score = 0;
let answered = false;
let timerInterval;
let timeLeft = 15;Step 3 — Show Question
Example
function showQuestion(i) {
answered = false;
timeLeft = 15;
const q = questions[i];
document.getElementById("questionText").textContent = q.text;
document.getElementById("questionCount").textContent = `Question ${i + 1} / ${questions.length}`;
const pct = (i / questions.length) * 100;
document.getElementById("progressFill").style.width = `${pct}%`;
const optionsContainer = document.getElementById("options");
optionsContainer.innerHTML = q.options.map((opt, idx) => `
<button class="option-btn" data-idx="${idx}">${opt}</button>
`).join("");
document.getElementById("explanation").classList.add("hidden");
document.getElementById("nextBtn").classList.add("hidden");
startTimer();
}
Step 4 — Timer
Example
function startTimer() {
clearInterval(timerInterval);
document.getElementById("timer").textContent = `⏱ ${timeLeft}s`;
timerInterval = setInterval(() => {
timeLeft--;
document.getElementById("timer").textContent = `⏱ ${timeLeft}s`;
if (timeLeft <= 0) {
clearInterval(timerInterval);
if (!answered) autoFail();
}
}, 1000);
}
function autoFail() {
answered = true;
highlightAnswers(-1); // -1 = no selection
showExplanation();
document.getElementById("nextBtn").classList.remove("hidden");
}
Step 5 — Check Answer
Example
document.getElementById("options").addEventListener("click", (e) => {
const btn = e.target.closest(".option-btn");
if (!btn || answered) return;
answered = true;
clearInterval(timerInterval);
const selected = Number(btn.dataset.idx);
const correct = questions[currentIndex].correct;
if (selected === correct) score++;
highlightAnswers(selected);
showExplanation();
document.getElementById("nextBtn").classList.remove("hidden");
});
function highlightAnswers(selected) {
const correct = questions[currentIndex].correct;
document.querySelectorAll(".option-btn").forEach((btn, i) => {
btn.disabled = true;
if (i === correct) btn.classList.add("correct");
else if (i === selected) btn.classList.add("wrong");
});
}
function showExplanation() {
const el = document.getElementById("explanation");
el.textContent = questions[currentIndex].explanation;
el.classList.remove("hidden");
}
Step 6 — Next Button
Example
document.getElementById("nextBtn").addEventListener("click", () => {
currentIndex++;
if (currentIndex < questions.length) {
showQuestion(currentIndex);
} else {
showResults();
}
});
Step 7 — Results Screen
Example
function showResults() {
clearInterval(timerInterval);
const pct = Math.round((score / questions.length) * 100);
const best = localStorage.getItem("quizHighScore") || 0;
const isNew = score > best;
if (isNew) localStorage.setItem("quizHighScore", score);
document.getElementById("quizScreen").classList.add("hidden");
document.getElementById("resultScreen").classList.remove("hidden");
document.getElementById("finalScore").textContent = `${score} / ${questions.length} (${pct}%)`;
document.getElementById("highScore").textContent = `High Score: ${isNew ? score : best} / ${questions.length}`;
document.getElementById("resultMsg").textContent =
pct >= 80 ? "🏆 Excellent!" : pct >= 50 ? "👍 Good job!" : "📚 Keep practicing!";
}
document.getElementById("retryBtn").addEventListener("click", () => {
currentIndex = 0;
score = 0;
document.getElementById("resultScreen").classList.add("hidden");
document.getElementById("quizScreen").classList.remove("hidden");
showQuestion(0);
});
Console Output
Output
startQuiz() → showQuestion(0)
timer starts: 15 → 14 → 13...
User clicks "let" (correct, idx=1)
checkAnswer(1) → correct! score = 1
highlightAnswers(1) → btn[1].classList.add("correct")
explanation shown → nextBtn visible
User clicks Next → showQuestion(1)
timer resets to 15s
-- Timer runs out on question 3 --
timeLeft = 0 → autoFail()
highlightAnswers(-1) → only correct answer highlighted in green
-- After 5 questions --
showResults()
score = 3 / 5 = 60%
pct = 60 → "👍 Good job!"
localStorage.setItem("quizHighScore", "3")Edge Cases to Handle
| Situation | Expected Behavior |
|---|---|
| Timer reaches 0 | Auto-fail: highlight correct answer |
| User clicks twice | answered flag prevents double-counting |
| Last question | Next → show results (not crash) |
| Score = 0 | "📚 Keep practicing!" message |
| New high score | Update localStorage and indicate new record |
Challenge Yourself
- Shuffle questions — use Fisher-Yates shuffle to randomize question order
- Difficulty levels — tag questions as Easy/Medium/Hard and filter by level
- Category filter — organize questions by topic (Arrays, Closures, etc.)
- Question bank — store 50+ questions and randomly select 10 per quiz
- Leaderboard — use
localStorageto store top 5 scores with player names
Best Practices
- Use an
answeredboolean flag to block double-clicks - Always
clearInterval(timerInterval)before starting a new timer - Store the explanation inside the question object — keeps data and UI logic together
- Derive the progress bar from
currentIndex / total— don't maintain it separately - High scores should be stored as integers in
localStorage, not objects
Exam Focus
Revise definitions, diagrams, examples, and short-answer points for Build a JavaScript Quiz App - Step by Step Project Tutorial.
Interview Use
Prepare one clear explanation, one practical example, and one common mistake for this JavaScript Master Course topic.
Search Terms
javascript-complete-guide, javascript master course, javascript, complete, guide, mini, projects, quiz
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