Struggling with esbuild + Propshaft + CDN, and source mapping URL

[Alexander-Blair]

I'm hoping that I'm missing something really obvious, and I have a workaround to ensure the sourceMappingURL appears in my final JavaScript assets, however I've tried lots of things that haven't worked.

My esbuild configuration boiled down to its most important settings:

const buildOptions = {
  entryPoints: ['app/javascript/admin.js', 'app/javascript/employee.js'],
  outdir: 'app/assets/builds',
  assetNames: '[name]-[hash].digested',
  bundle: true,
  sourcemap: true,
  loader: {
    '.js': 'jsx',
    '.jpeg': 'file',
    '.png': 'file',
    '.svg': 'file',
    '.ts': 'ts',
  },
  publicPath: `${process.env.ASSET_HOST}/assets`,
};

Rails config: config.asset_host = ENV.fetch('ASSET_HOST', nil), where ASSET_HOST would be set to something like https://assets.blobby.com

So esbuild would create a file in app/assets/builds with an absolute path for the source map e.g.

//# sourceMappingURL=https://assets.blobby.com/assets/admin.js.map

And it seems that one of 2 things is true:

• Something is misconfigured on my end, causing the issue. I did notice that Rails mentions you don't actually need to prefix the http / https on the asset_host, although I did play with that and it made no difference
• Propshaft can't currently convert a linked source map living at an absolute URL

Here's the solution I used to get it working in the end:

ciass Propshaft::Compiler::SourceMappingUrls
  def url_prefix
    if (host = Rails.configuration.asset_host)
      "#{host}#{config.prefix}"
    else
      super
    end
  end
end

Thanks!