index.js

const path = require('path');

const PATH_DELIMITER = '[\\\\/]'; // match 2 antislashes or one slash

/**
 * Stolen from https://stackoverflow.com/questions/10776600/testing-for-equality-of-regular-expressions
 */
const regexEqual = (x, y) => {
  return (x instanceof RegExp) && (y instanceof RegExp) &&
    (x.source === y.source) && (x.global === y.global) &&
    (x.ignoreCase === y.ignoreCase) && (x.multiline === y.multiline);
};

const generateIncludes = (modules) => {
  return modules.map(module => (new RegExp(`${safePath(module)}(?!.*node_modules)`)));
};

const generateExcludes = (modules) => {
  return [new RegExp(`node_modules${PATH_DELIMITER}(?!(${modules.map(safePath).join('|')})(?!.*node_modules))`)];
};

/**
 * On Windows, the Regex won't match as Webpack tries to resolve the
 * paths of the modules. So we need to check for \\ and /
 */
const safePath = (module) => module.split('/').join(PATH_DELIMITER);

/**
 * Actual Next.js plugin
 */
const withTm = (nextConfig = {}) => {
  const { transpileModules = [] } = nextConfig;
  const includes = generateIncludes(transpileModules);
  const excludes = generateExcludes(transpileModules);

  return Object.assign({}, nextConfig, {
    webpack (config, options) {
      // Safecheck for Next < 5.0
      if (!options.defaultLoaders) {
        throw new Error(
          'This plugin is not compatible with Next.js versions below 5.0.0 https://err.sh/next-plugins/upgrade'
        );
      }

      // Avoid Webpack to resolve transpiled modules path to their real path
      config.resolve.symlinks = false;

      if(Array.isArray(config.externals)) {
        config.externals = config.externals.map(external => {
          if (typeof external !== 'function') return external;
          return (ctx, req, cb) => {
            return includes.find(include =>
              req.startsWith('.')
                ? include.test(path.resolve(ctx, req))
                : include.test(req)
            )
              ? cb()
              : external(ctx, req, cb);
          };
        }); 
      }

      // Add a rule to include and parse all modules
      config.module.rules.push({
        test: /\.+(js|jsx|ts|tsx)$/,
        loader: options.defaultLoaders.babel,
        include: includes
      });

      if (typeof nextConfig.webpack === 'function') {
        return nextConfig.webpack(config, options);
      }

      return config;
    },

    // webpackDevMiddleware needs to be told to watch the changes in the
    // transpiled modules directories
    webpackDevMiddleware (config) {
      // Replace /node_modules/ by the new exclude RegExp (including the modules
      // that are going to be transpiled)
      // https://github.com/zeit/next.js/blob/815f2e91386a0cd046c63cbec06e4666cff85971/packages/next/server/hot-reloader.js#L335
      const ignored = config.watchOptions.ignored.filter(
        regexp => !regexEqual(regexp, /[\\/]node_modules[\\/]/)
      ).concat(excludes);

      config.watchOptions.ignored = ignored;

      if (typeof nextConfig.webpackDevMiddleware === 'function') {
        return nextConfig.webpackDevMiddleware(config);
      }

      return config;
    }
  });
};

module.exports = withTm;