root.js

if (module.hot) {
  var hot = require('./index').hot
  var cache = require.cache

  // access parent
  var parent = cache[module.parents[0]]
  // remove itself from a cache
  delete cache[module.id]
  // setup hot for caller

  exports.hot = hot(Object.assign({ id: parent.i }, parent))
} else {
  // prod mode
  exports.hot = function(a) {
    return a
  }
}