babel hook to load modules from several directories

babel-hook.js

/**
 * Hook babel compilation into require.
 *
 * To disable cache, see https://babeljs.io/docs/usage/require/#environment-variables
 *
 * ``` javascript
 * const lookupDirs = ['app/server', 'app/shared/lib', 'app/shared'];
 * const resolveModuleSource = require('babel-hooks');
 *
 * require('babel/register')({
 *   stage: 1,
 *   resolveModuleSource: resolveModuleSource(lookupDirs)
 * });
 * ```
 */
const fs = require('fs');
const path = require('path');
const extensions = ['js', 'jsx', 'json'];

module.exports = function resolveModuleSource(_lookupDirs) {
  var lookupDirs = _lookupDirs.map(function makeAbsolute(x) {
                      return path.join(__dirname, x);
                    });

  function resolveExtension(absolutePath) {
    var currTest;

    for (var i = 0; i < extensions.length; i++) {
      currTest = [absolutePath, extensions[i]].join('.');
      if (fs.existsSync(currTest)) {
        return currTest;
      }
    }
  }

  function resolveFilename(source/*, filename*/) {
    var file;
    if (source[0] === '.') {
      return source;
    }

    // this is an absolute path
    for (var i = 0; i < lookupDirs.length; i++) {
      file = path.join(lookupDirs[i], source);
      file = resolveExtension(file);
      if (file) {
        return file;
      }
    }

    // none of the special path matched
    return source;
  }

  return resolveFilename;
};

index.js

const lookupDirs = ['app/server', 'app/shared/lib', 'app/shared'];
const resolveModuleSource = require('./babel-hooks');

require('babel/register')({
  stage: 1,
  resolveModuleSource: resolveModuleSource(lookupDirs)
});

require('./app');

All files in lookupDirs can be imported just as if they were at the repository root:

For instance to import app/server/foo.js

import foo from 'foo';