QR Scanner

Javascript QR Code Scanner based on Cosmo Wolfe’s javascript port of Google’s ZXing library.

In this library, several improvements have been applied over the original port:

According to our benchmarking this project’s scanner engine’s detection rate is about 2-3 times (and up to 8 times) as high as the one of the most popular javascript QR scanner library LazarSoft/jsqrcode. Also the other library oftentimes misreads the content of QR codes, while for this project no misreads occurred in the benchmarking.

The library supports scanning a continuous video stream from a web cam as well as scanning of single images.

The development of this library is sponsored by nimiq, world’s first browser based blockchain.



See https://nimiq.github.io/qr-scanner/demo/


To install via npm:

npm install --save qr-scanner

To install via yarn:

yarn add qr-scanner

Or simply copy qr-scanner.min.js and qr-scanner-worker.min.js to your project.


The QR Scanner consists of two files.

Setting up the API

qr-scanner.min.js is the main API as an es6 module and can be imported as follows:

import QrScanner from 'path/to/qr-scanner.min.js'; // if using plain es6 import
import QrScanner from 'qr-scanner'; // if installed via package and bundling with a module bundler like webpack or rollup

This requires the importing script to also be an es6 module or a module script tag, e.g.:

<script type="module">
    import QrScanner from 'path/to/qr-scanner.min.js';
    // do something with QrScanner

If your project is not based on es6 modules you can

Setting up the worker

qr-scanner-worker.min.js is a plain Javascript file for the separate worker thread and needs to be copied over to your project. You should then point QrScanner.WORKER_PATH to the location where it will be hosted:

QrScanner.WORKER_PATH = 'path/to/qr-scanner-worker.min.js';

Webpack specific worker setup

If you’re using Webpack to bundle your project, the file-loader or raw-loader might be interesting for you for handling the qr-scanner-worker.min.js dependency. Which one to choose depends on your use case.

Using file-loader

The file-loader automatically copies the worker script into your build and provides the path where it will be located in the build. At runtime, the worker will then be lazy-loaded from there when needed. Due to its ability to lazy-load the worker, using the file-loader is the preferred approach if you do not expect the QR scanner to be used every time a user uses your app or if the QR scanner is not launched right after loading the app.

You can add the file-loader to your project via:

npm install --save-dev file-loader

You can then use it to copy the worker file and obtain the WORKER_PATH:

import QrScannerWorkerPath from '!!file-loader!./node_modules/qr-scanner/qr-scanner-worker.min.js';
QrScannerLib.WORKER_PATH = QrScannerWorkerPath;

Note that the path to the worker file has to be set relatively to the source file where you use it. For example, if your source file using the QrScanner sits in /src/components, the correct import would be import QrScannerWorkerPath from '!!file-loader!../../node_modules/qr-scanner/qr-scanner-worker.min.js';.

Using raw-loader

The raw-loader bundles the worker as string into your build, thus no separate file gets generated in your build output. While this simplifies the build output and avoids an additional network request, it increases your bundle size and removes the ability to lazy-load the worker file only when needed.

You can add the raw-loader to your project via:

npm install --save-dev raw-loader

You can then use it to load the worker file and set the WORKER_PATH:

import qrScannerWorkerSource from '!!raw-loader!./node_modules/qr-scanner/qr-scanner-worker.min.js';
QrScanner.WORKER_PATH = URL.createObjectURL(new Blob([qrScannerWorkerSource]));

Note that the path to the worker file has to be set relatively to the source file where you use it. For example, if your source file using the QrScanner sits in /src/components, the correct import would be import qrScannerWorkerSource from '!!raw-loader!../../node_modules/qr-scanner/qr-scanner-worker.min.js';.


Web Cam Scanning

1. Create HTML

Create a <video> element where the web cam video stream should get rendered:


2. Create a QrScanner Instance

const qrScanner = new QrScanner(videoElem, result => console.log('decoded qr code:', result));

As an optional third parameter an error handler to be invoked on decoding errors can be specified. The default is QrScanner._onDecodeError.

As an optional fourth parameter a method can be provided that determines a region to which scanning should be restricted as a performance improvement. This region can optionally also be scaled down before performing the scan as an additional performance improvement. The region is specified as x, y, width and height; the dimensions for the downscaled region as downScaledWidth and downScaledHeight. Note that the aspect ratio between width and height and downScaledWidth and downScaledHeight should remain the same. By default, the scan region is restricted to a centered square of two thirds of the video width or height, whichever is smaller, and scaled down to a 400x400 square.

As an optional fifth parameter a preference for the camera to use can be specified. Allowed values are 'environment' and 'user'. The default is 'environment'.

To use the default value for an optional parameter, omit it or use undefined.

3. Start scanning


Call it when you’re ready to scan, for example on a button click or directly on page load. It will prompt the user for permission to use a camera. Note: to read from a Web Cam stream, your page must be served via HTTPS.

4. Stop scanning


If you want, you can stop scanning anytime and resume it by calling start() again.

Single Image Scanning

    .then(result => console.log(result))
    .catch(error => console.log(error || 'No QR code found.'));

Supported image sources are: HTMLImageElement, SVGImageElement, HTMLVideoElement, HTMLCanvasElement, ImageBitmap, OffscreenCanvas, File / Blob, Data URIs, URLs pointing to an image (if they are on the same origin or CORS enabled)

As an optional second parameter a region defined by x, y, width and height can be specified to which the search for a QR code should be restricted. As a performance improvement this region can be scaled down before performing the scan by providing a downScaledWidth and downScaledHeight. By default, the region spans the whole image and is not scaled down.

As an optional third parameter a manually created QR scanner engine instance to be reused can be specified. This improves performance if you’re scanning a lot of images. An engine can be manually created via QrScanner.createQrEngine(QrScanner.WORKER_PATH) (async). By default, no engine is reused for single image scanning.

As an optional fourth parameter a manually created canvas to be reused can be specified. This improves performance if you’re scanning a lot of images. A canvas can be manually created via a <canvas> tag in your markup or document.createElement('canvas'). By default, no canvas is reused for single image scanning.

As an optional fifth parameter you can request a provided canvas for reuse to not be resized, irrespective of the source image or source region dimensions. Note that the canvas and source region should have the same aspect ratio to avoid that the image to scan gets distorted which could make detecting QR codes impossible. By default, the canvas size is adapted to the source region dimensions for single image scanning.

To use the default value for an optional parameter, omit it or use undefined.

Checking for Camera availability

This library provides a utility method for checking whether the device has a camera. This can be useful for determining whether to offer the QR web cam scanning functionality to a user.

QrScanner.hasCamera(); // async

Color Inverted Mode

The scanner by default scans for dark QR codes on a bright background. You can change this behavior to scan for bright QR codes on dark background or for both at the same time:


Where inversionMode can be original, invert or both. The default for web cam scanning is original and for single image scanning both.

Color Correction

Change the weights for red, green and blue in the grayscale computation to improve contrast for QR codes of a specific color:

qrScanner.setGrayscaleWeights(red, green, blue, useIntegerApproximation = true);

Where red, green and blue should sum up to 256 if useIntegerApproximation === true and 1 otherwise. By default, these values are used.

Flashlight support

On supported browsers, you can check whether the used camera has a flash and turn it on or off. Note that qrScanner.start() has to be called and awaited first.

qrScanner.hasFlash(); // check whether the browser and used camera support turning the flash on; async.
qrScanner.isFlashOn(); // check whether the flash is on
qrScanner.turnFlashOn(); // turn the flash on if supported; async
qrScanner.turnFlashOff(); // turn the flash off if supported; async
qrScanner.toggleFlash(); // toggle the flash if supported; async.

Clean Up

You can destroy the QR scanner if you don’t need it anymore:

qrScanner = null;

This will stop the camera stream and web worker and cleans up event listeners.

Build the project

The project is prebuild in qr-scanner.min.js in combination with qr-scanner-worker.min.js. Building yourself is only necessary if you want to change the code in the /src folder. NodeJs is required for building.

Install required build packages:

npm install


npm run build