npm install --save-dev @snowpack/plugin-babel

// snowpack.config.mjs
export default {
  plugins: [
    [
      '@snowpack/plugin-babel',
      {
        input: ['.js', '.mjs', '.jsx', '.ts', '.tsx'], // (optional) specify files for Babel to transform
        transformOptions: {
          // babel transform options
        },
      },
    ],
  ],
};