code: function umd() {
    let bundler = browserify(src, { debug: true, standalone: 'luhn' })
      .transform(babelify) // .configure({ optional: ['runtime'] })
      .bundle();
  
    return bundler
      .pipe(source(pack.main)) // gives streaming vinyl file object
      .pipe(buffer()) // convert from streaming to buffered vinyl file object
      .pipe(sourcemaps.init({ loadMaps: true })) // loads map from browserify file
      .pipe(sourcemaps.write('./'))
      .pipe(gulp.dest('./'));
  }
docs: umd standalone library with sourcemaps