A transform function to use with copy-webpack-plugin that attempts to gzip assets as they're copied.

gziptee.js

// A transform function to use with copy-webpack-plugin that
// gzips assets as they're copied.
//
var gziptee = function(src, dest) {
  const gzipSmallerThan = 0.8;

  function mkdirhier(dir) {
    if(path.resolve('.').length < dir.length) {
      mkdirhier(path.dirname(dir));
    }
    if (!fs.existsSync(dir)) {
      fs.appendFileSync('gzipper.log', 'Making ' +dir+ '\n');
      fs.mkdirSync(dir, 0777 & (~process.umask()));
    }
  }

  var gzipper = function(content, _path) {
    gzip(content, function(err, result) {
      if (err) {
        console.error(err);
      }
      else {
        if((result.length / content.length) < gzipSmallerThan) {
          var gzpath = path.resolve('.', path.join(dest, path.relative(src, _path)) + '.gz');
          fs.appendFileSync('gzipper.log', _path + ' to ' + gzpath + '\n');
          // The file hasn't been copied yet, so its parent might not exist yet
          mkdirhier(path.dirname(gzpath));
          fs.writeFile(gzpath, result);
        }
      }
    });
    return content;
  }
  return gzipper;
}

usage.js

new CopyWebpackPlugin([
	{
		from: 'src/assets',
		to: 'assets',
		transform: gziptee('src/assets', 'dist/assets')
	}
])