code: function umd(cb) {
    const env = process.env.NODE_ENV || 'development';
    rollup({
      input: './src/ssr-window.js',
      plugins: [buble()],
      format: 'umd',
      name: 'ssrWindow',
      strict: true,
      sourcemap: env === 'development',
      banner,
    })
    .pipe(source('ssr-window.js', './src'))
    .pipe(buffer())
    .pipe(gulp.dest(`./${env === 'development' ? 'build' : 'dist'}/`))
    .on('end', () => {
      if (env === 'development') {
        if (cb) cb();
        return;
      }
      gulp.src('./dist/ssr-window.js')
      .pipe(sourcemaps.init())
      .pipe(uglify())
      .pipe(header(banner))
      .pipe(rename('ssr-window.min.js'))
      .pipe(sourcemaps.write('./'))
      .pipe(gulp.dest('./dist/'))
      .on('end', () => {
        if (cb) cb();
      });
    });
  }
docs: UMD DIST