next.js/client/next-dev.js

import evalScript from '../lib/eval-script'

const { __NEXT_DATA__: { errorComponent } } = window
const ErrorComponent = evalScript(errorComponent).default

require('react-hot-loader/patch')

const next = window.next = require('./')

const emitter = next.default(onError)

function onError (err) {
  // just show the debug screen but don't render ErrorComponent
  // so that the current component doesn't lose props
  next.render({ err, emitter })
}

let lastScroll

emitter.on('before-reactdom-render', ({ Component }) => {
  // Remember scroll when ErrorComponent is being rendered to later restore it
  if (!lastScroll && Component === ErrorComponent) {
    const { pageXOffset, pageYOffset } = window
    lastScroll = {
      x: pageXOffset,
      y: pageYOffset
    }
  }
})

emitter.on('after-reactdom-render', ({ Component }) => {
  if (lastScroll && Component !== ErrorComponent) {
    // Restore scroll after ErrorComponent was replaced with a page component by HMR
    const { x, y } = lastScroll
    window.scroll(x, y)
    lastScroll = null
  }
})