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Paul Kinlan for Google Web Dev

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Barcode detection in a Web Worker using Comlink

I’m a big fan of QRCodes, they are very simple and neat way to exchange data between the real world and the digital world. For a few years now I’ve had a little side project called QRSnapper — well it’s had a few names, but this is the one I’ve settled on — that uses the getUserMedia API to take live data from the user’s camera so that it can scan for QR Codes in near real time.

The goal of the app was to maintain 60fps in the UI and near instant detection of the QR Code, this meant that I had to put the detection code in to a Web Worker (pretty standard stuff). In this post I just wanted to quickly share how I used comlink to massively simplify the logic in the Worker.


import * as Comlink from './comlink.js';

const proxy = Comlink.proxy(new Worker('/scripts/qrworker.js')); 

export const decode = async function (context) {
  try {
    let canvas = context.canvas;
    let width = canvas.width;
    let height = canvas.height;
    let imageData = context.getImageData(0, 0, width, height);
    return await proxy.detectUrl(width, height, imageData);
  } catch (err) {

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qrworker.js (web worker)

import * as Comlink from './comlink.js';
import {qrcode} from './qrcode.js';

// Use the native API's
let nativeDetector = async (width, height, imageData) => {
  try {
    let barcodeDetector = new BarcodeDetector();
    let barcodes = await barcodeDetector.detect(imageData);
    // return the first barcode.
    if (barcodes.length > 0) {
      return barcodes[0].rawValue;
  } catch(err) {
    detector = workerDetector;

// Use the polyfil
let workerDetector = async (width, height, imageData) => {
  try {
    return qrcode.decode(width, height, imageData);
  } catch (err) {
    // the library throws an excpetion when there are no qrcodes.

let detectUrl = async (width, height, imageData) => {
  return detector(width, height, imageData);

let detector = ('BarcodeDetector' in self) ? nativeDetector : workerDetector;
// Expose the API to the client pages.
Comlink.expose({detectUrl}, self);

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I really love Comlink, I think it is a game changer of a library especially when it comes to creating idiomatic JavaScript that works across threads. Finally a neat thing here, is that the native Barcode detection API can be run inside a worker so all the logic is encapsulated away from the UI.

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Top comments (2)

plague69 profile image

Tried qrsnapper and it seems a bit faster than zxing-js which I'm using right now. However it does not read some QR codes (I think the text ones). Link QR's come up just great.

Your Git also includes a demo link which does not work because it's not https so can not getuermedia

eg what it won't read:

vikramchandra profile image
Vikram Sharma

Will it make development easier if one uses a barcode api like these.