code: function customSourceProcessor(asset, callback) {
          const args =
            arguments.length > 2 && Array.prototype.slice.call(arguments, 2);
  
          const updatedSource = `// built with webpack and ${args.join("-")}
          // source from ${asset.filename} to ${asset.url}
          // default templating would have resulted in ${asset.content}
          ${asset.source}`;
          callback(updatedSource);
        }
docs: in supported version of Node.js use: template: (asset, callback, ...args) => {