Solving the Enigmatic “this” Conundrum: Issues with zxing-js/library and Rollup Build

If you’re reading this, chances are you’ve stumbled upon the frustrating error message “this has been rewritten to undefined” while trying to integrate zxing-js/library with Rollup build. Don’t worry, you’re not alone! This perplexing issue has plagued many developers, leaving them scratching their heads in confusion. But fear not, dear reader, for we’re about to embark on a journey to demystify this enigmatic problem and provide a clear, step-by-step guide to resolving it.

Understanding the Culprit: Rollup’s this-rewriting Magic

Before we dive into the solution, let’s take a moment to understand the root cause of this issue. Rollup, a popular JavaScript module bundler, is notorious for rewriting the this keyword in certain scenarios. This rewriting process, although intended to optimize code, can sometimes lead to unforeseen consequences.

In the case of zxing-js/library, this rewriting of this can result in the library’s internal functions losing their context, ultimately leading to the dreaded “this has been rewritten to undefined” error.

Step 1: Verify Your Rollup Configuration

Before we begin, ensure that your Rollup configuration is up-to-date and correctly set up. Double-check that you’ve included the necessary plugins and settings to handle ECMAScript modules (ESM) correctly.

// rollup.config.js
import { rollup } from 'rollup';
import { babel } from 'rollup-plugin-babel';
import { eslint } from 'rollup-plugin-eslint';
import { uglify } from 'rollup-plugin-uglify';

export default {
  input: 'src/index.js',
  output: {
    file: 'dist/bundle.js',
    format: 'esm'
  },
  plugins: [
    babel({
      babelHelpers: 'inline'
    }),
    eslint({
      exclude: 'node_modules/**'
    }),
    uglify()
  ]
};

Step 2: Configure zxing-js/library for Rollup Compatibility

Now that our Rollup configuration is in check, let’s focus on preparing zxing-js/library for the build process. Create a new file, zxing-adapter.js, in the same directory as your Rollup configuration file:

// zxing-adapter.js
import * as ZXing from 'zxing-js/library';

const zxingAdapter = {
  ...ZXing,
  BrowserBarcodeReader: class extends ZXing.BrowserBarcodeReader {
    async decode(): Promise<ZXing.Result> {
      // Ensure the correct context for the BrowserBarcodeReader instance
      return super.decode.apply(this);
    }
  }
};

export default zxingAdapter;

This adapter file will help us maintain the correct context for zxing-js/library’s internal functions, circumventing Rollup’s this rewriting.

Step 3: Update Your Rollup Configuration to Use the Adapter

Modify your Rollup configuration to include the zxing-adapter.js file and update the input file accordingly:

// rollup.config.js (updated)
import { rollup } from 'rollup';
import { babel } from 'rollup-plugin-babel';
import { eslint } from 'rollup-plugin-eslint';
import { uglify } from 'rollup-plugin-uglify';
import zxingAdapter from './zxing-adapter';

export default {
  input: 'zxing-adapter.js',
  output: {
    file: 'dist/bundle.js',
    format: 'esm'
  },
  plugins: [
    babel({
      babelHelpers: 'inline'
    }),
    eslint({
      exclude: 'node_modules/**'
    }),
    uglify()
  ]
};

Step 4: Use the Adapter in Your Application Code

Finally, update your application code to utilize the zxingAdapter:

// your-application-code.js
import zxingAdapter from './zxing-adapter';

const browserBarcodeReader = new zxingAdapter.BrowserBarcodeReader();

// Use the browserBarcodeReader instance as you would with the original ZXing.BrowserBarcodeReader

Bonus Step: Verify Your Barcode Scanning Functionality

To ensure everything is working as expected, add a simple test to verify barcode scanning functionality:

// test-barcode-scanning.js
import zxingAdapter from './zxing-adapter';

const browserBarcodeReader = new zxingAdapter.BrowserBarcodeReader();

// Create a sample barcode image
const barcodeImage = document.createElement('img');
barcodeImage.src = 'data:image/png;base64,iVBORw0KGg...'; // Replace with your barcode image

// Decode the barcode
browserBarcodeReader.decode(barcodeImage).then((result) => {
  console.log(`Decoded barcode: ${result.text}`);
}).catch((error) => {
  console.error(`Error decoding barcode: ${error}`);
});

Run this test script to verify that your zxing-js/library implementation, with the adapter, is correctly decoding barcodes.

Conclusion

Voilà! By following these steps, you should now have a fully functional zxing-js/library implementation that plays nicely with Rollup. Remember to keep your Rollup configuration up-to-date and to use the zxing-adapter.js file to ensure compatibility.

If you’re still encountering issues, don’t hesitate to reach out to the zxing-js/library community or seek help from fellow developers who have faced similar challenges.

Common Issues	Solutions
Rollup configuration not set up correctly	Verify Rollup configuration and update as necessary
zxing-adapter.js not properly configured	Review zxing-adapter.js implementation and ensure correct context for BrowserBarcodeReader instance
Barcode scanning functionality not working	Verify barcode image creation and decoding process; check for any errors in the console

Additional Resources

• zxing-js/library GitHub repository
• Rollup documentation
• Babel plugins documentation

By following this comprehensive guide, you should now have a solid understanding of how to overcome the “this has been rewritten to undefined” issue when using zxing-js/library with Rollup build. Happy coding!

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