plugin.js

const workerpool = require('workerpool');
let worker, pool;

module.exports = function plugin(snowpackConfig, options = {}) {
  // options validation
  if (options) {
    if (typeof options !== 'object') throw new Error(`options isn’t an object. Please see README.`);
    if (options.input && !Array.isArray(options.input))
      throw new Error(
        `options.input must be an array (e.g. ['.js', '.mjs', '.jsx', '.ts', '.tsx'])`,
      );
    if (options.input && !options.input.length)
      throw new Error(`options.input must specify at least one filetype`);
  }

  return {
    name: '@snowpack/plugin-babel',
    resolve: {
      input: options.input || ['.js', '.mjs', '.jsx', '.ts', '.tsx'],
      output: ['.js'], // always export JS
    },
    async load({filePath}) {
      if (!filePath) {
        return;
      }
      pool = pool || workerpool.pool(require.resolve('./worker.js'));
      worker = worker || (await pool.proxy());
      let encodedResult = await worker.transformFileAsync(filePath, {
        cwd: process.cwd(),
        ast: false,
        compact: false,
        sourceMaps: snowpackConfig.buildOptions.sourceMaps,
        ...(options.transformOptions || {}),
      });
      let {code, map} = JSON.parse(encodedResult);

      if (code) {
        // Some Babel plugins assume process.env exists, but Snowpack
        // uses import.meta.env instead. Handle this here since it
        // seems to be pretty common.
        // See: https://www.pika.dev/npm/snowpack/discuss/496
        code = code.replace(/process\.env/g, 'import.meta.env');
      }
      return {
        '.js': {
          code,
          map,
        },
      };
    },
    cleanup() {
      pool && pool.terminate();
    },
  };
};