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Learn to build a QR code scanner in JavaScript using the device camera with jsQR library, file upload fallback, decoded result display, and link detection.
What You'll Learn
- How to access the device camera with
getUserMedia() - How to read video frames onto a Canvas using
drawImage() - How to decode QR codes from canvas pixel data using jsQR
- How to support file upload as a fallback (scan from image)
- How to detect URLs in the decoded result and make them clickable
Project Overview
The QR scanner streams the camera feed into a <video> element, repeatedly draws frames to an off-screen canvas, extracts pixel data, and passes it to the jsQR library which returns the decoded string if a QR code is found.
Component Structure
How It Works — Flow Diagram
Step-by-Step Build
Step 1 — Load jsQR
html example
<script src="https://cdn.jsdelivr.net/npm/jsqr@1.4.0/dist/jsQR.js"></script>Step 2 — HTML Structure
html example
<div class="scanner-app">
<h1>QR Code Scanner</h1>
<div class="scanner-viewport">
<video id="video" class="scanner-video" playsinline></video>
<canvas id="canvas" class="scanner-canvas hidden"></canvas>
<div class="scan-overlay">
<div class="scan-box"></div>
</div>
<div id="scanStatus" class="scan-status">Waiting to scan...</div>
</div>
<div class="scanner-controls">
<button id="startBtn" class="btn-start">📷 Start Camera</button>
<button id="stopBtn" class="btn-stop hidden">⏹ Stop</button>
<label class="btn-upload">
🖼 Upload Image
<input type="file" id="fileInput" accept="image/*" hidden>
</label>
</div>
<div id="resultSection" class="result-section hidden">
<h3>Decoded Result</h3>
<div id="resultText" class="result-text"></div>
<div class="result-actions">
<a id="openLink" class="btn-open hidden" target="_blank">🔗 Open Link</a>
<button id="copyResult" class="btn-copy">📋 Copy</button>
</div>
</div>
</div>Step 3 — Start Camera Stream
js example
const video = document.getElementById("video");
const canvas = document.getElementById("canvas");
const ctx = canvas.getContext("2d");
let stream = null;
let scanning = false;
let animFrameId = null;
async function startCamera() {
try {
stream = await navigator.mediaDevices.getUserMedia({
video: { facingMode: "environment", width: 640, height: 480 }
});
video.srcObject = stream;
await video.play();
scanning = true;
setStatus("🔍 Scanning...");
requestAnimationFrame(scanLoop);
document.getElementById("startBtn").classList.add("hidden");
document.getElementById("stopBtn").classList.remove("hidden");
} catch (err) {
setStatus(`❌ Camera error: ${err.message}`);
}
}Step 4 — Scan Loop
js example
function scanLoop() {
if (!scanning) return;
if (video.readyState === video.HAVE_ENOUGH_DATA) {
canvas.width = video.videoWidth;
canvas.height = video.videoHeight;
ctx.drawImage(video, 0, 0, canvas.width, canvas.height);
const imageData = ctx.getImageData(0, 0, canvas.width, canvas.height);
const code = jsQR(imageData.data, imageData.width, imageData.height, {
inversionAttempts: "dontInvert"
});
if (code) {
scanning = false;
stopCamera();
displayResult(code.data);
return;
}
}
animFrameId = requestAnimationFrame(scanLoop);
}Step 5 — Stop Camera
Example
function stopCamera() {
scanning = false;
if (animFrameId) cancelAnimationFrame(animFrameId);
if (stream) stream.getTracks().forEach(t => t.stop());
stream = null;
document.getElementById("startBtn").classList.remove("hidden");
document.getElementById("stopBtn").classList.add("hidden");
}
Step 6 — Display Result
js example
function displayResult(text) {
document.getElementById("resultSection").classList.remove("hidden");
document.getElementById("resultText").textContent = text;
setStatus("✅ QR code decoded!");
// Detect URL
const isUrl = /^https?:\/\//i.test(text);
const openLink = document.getElementById("openLink");
if (isUrl) {
openLink.href = text;
openLink.classList.remove("hidden");
} else {
openLink.classList.add("hidden");
}
}Step 7 — File Upload Fallback
Example
document.getElementById("fileInput").addEventListener("change", (e) => {
const file = e.target.files[0];
if (!file) return;
const img = new Image();
img.onload = () => {
canvas.width = img.width;
canvas.height = img.height;
ctx.drawImage(img, 0, 0);
const imageData = ctx.getImageData(0, 0, canvas.width, canvas.height);
const code = jsQR(imageData.data, imageData.width, imageData.height);
if (code) {
displayResult(code.data);
} else {
setStatus("❌ No QR code found in image");
}
};
img.src = URL.createObjectURL(file);
e.target.value = "";
});
Step 8 — Copy to Clipboard
Example
document.getElementById("copyResult").addEventListener("click", () => {
const text = document.getElementById("resultText").textContent;
navigator.clipboard.writeText(text).then(() => {
document.getElementById("copyResult").textContent = "✅ Copied!";
setTimeout(() => { document.getElementById("copyResult").textContent = "📋 Copy"; }, 2000);
});
});
Step 9 — Wire Buttons
js example
document.getElementById("startBtn").addEventListener("click", startCamera);
document.getElementById("stopBtn").addEventListener("click", stopCamera);Console Output
Output
startCamera()
→ navigator.mediaDevices.getUserMedia({ video: ... })
→ stream acquired → video.play()
→ requestAnimationFrame(scanLoop) started
scanLoop() cycle 1: video not ready yet → next frame
scanLoop() cycle 42: QR code found!
code.data = "https://wohtech.com"
→ stopCamera() called
→ displayResult("https://wohtech.com")
→ "Open Link" button visible
// File upload: user selects qr-image.png
img.onload → jsQR(imageData, 800, 600)
→ code.data = "WIFI:T:WPA;S:HomeNet;P:secret;;"
→ displayResult("WIFI:T:WPA;S:HomeNet;P:secret;;")
→ Not a URL → Open Link button hiddenEdge Cases to Handle
| Situation | Expected Behavior |
|---|---|
| Camera permission denied | Show error message, offer file upload |
| No QR in uploaded image | "No QR code found" status |
| Low light / blurry | inversionAttempts: "attemptBoth" helps |
| Non-URL decoded text | Open Link button hidden |
| Multiple QRs in frame | jsQR returns the first one found |
Challenge Yourself
- Continuous mode — don't stop after the first scan; keep scanning and show a history
- Torch/flashlight — toggle the camera's torch using
MediaTrackConstraints - Scan corner markers — draw corner dots on the canvas where jsQR found the QR
- Barcode support — add Barcode Detection API support for standard barcodes
- Sound feedback — play a beep when a code is successfully decoded
Best Practices
- Use
facingMode: "environment"for the back camera on mobile (better for scanning) - Always call
stream.getTracks().forEach(t => t.stop())when done — don't leave the camera on - Use
requestAnimationFrameinstead ofsetIntervalfor the scan loop — it syncs to screen refresh - The jsQR library works with raw pixel data (
Uint8ClampedArray) — passimageData.data - File upload fallback is essential — not all users want to grant camera access
Continue the lesson path
Study next in this module
01Mini ProjectsBuild a JavaScript QR Code Generator - Step by Step Project TutorialLearn to build a QR code generator in JavaScript using the QRCode.js library, with custom colors, size, logo overlay, and download as PNG functionality.02Mini ProjectsBuild a JavaScript Quiz App - Step by Step Project TutorialLearn to build an interactive quiz app in JavaScript with multiple-choice questions, timer, score tracking, progress bar, result screen, and localStorage high score.03Mini ProjectsBuild a JavaScript Prime Number Checker - Step by Step Project TutorialLearn to build a prime number checker in JavaScript with trial division, Sieve of Eratosthenes, prime factorization, and range prime finder visualization.04Mini ProjectsBuild a JavaScript Random Number Generator - Step by Step TutorialLearn to build a random number generator in JavaScript with min/max range, integer/decimal modes, history log, exclude list, and bulk generation.