next.js/client/webpack-hot-middleware-client.js

import 'event-source-polyfill'
import webpackHotMiddlewareClient from 'webpack-hot-middleware/client?autoConnect=false'
import Router from '../lib/router'

const {
  __NEXT_DATA__: {
    assetPrefix
  }
} = window

export default () => {
  webpackHotMiddlewareClient.setOptionsAndConnect({
    overlay: 'false',
    reload: 'true',
    path: `${assetPrefix}/_next/webpack-hmr`
  })

  const handlers = {
    reload (route) {
      if (route === '/_error') {
        for (const r of Object.keys(Router.components)) {
          const { err } = Router.components[r]
          if (err) {
            // reload all error routes
            // which are expected to be errors of '/_error' routes
            Router.reload(r)
          }
        }
        return
      }

      if (route === '/_document') {
        window.location.reload()
        return
      }

      Router.reload(route)
    },

    change (route) {
      if (route === '/_document') {
        window.location.reload()
        return
      }

      const { err, Component } = Router.components[route] || {}

      if (err) {
        // reload to recover from runtime errors
        Router.reload(route)
      }

      if (Router.route !== route) {
        // If this is a not a change for a currently viewing page.
        // We don't need to worry about it.
        return
      }

      if (!Component) {
        // This only happens when we create a new page without a default export.
        // If you removed a default export from a exising viewing page, this has no effect.
        console.log(`Hard reloading due to no default component in page: ${route}`)
        window.location.reload()
      }
    }
  }

  webpackHotMiddlewareClient.subscribe((obj) => {
    const fn = handlers[obj.action]
    if (fn) {
      const data = obj.data || []
      fn(...data)
    } else {
      throw new Error('Unexpected action ' + obj.action)
    }
  })
}