packages/next/client/dev-error-overlay/hot-dev-client.js
async function tryApplyUpdates (onHotUpdateSuccess) {
  if (!module.hot) {
    // HotModuleReplacementPlugin is not in Webpack configuration.
    console.error('HotModuleReplacementPlugin is not in Webpack configuration.')
    // window.location.reload();
    return
  }

  if (!isUpdateAvailable() || !canApplyUpdates()) {
    return
  }

  function handleApplyUpdates (err, updatedModules) {
    if (err || hadRuntimeError) {
      if (err) {
        console.warn('Error while applying updates, reloading page', err)
      }
      if (hadRuntimeError) {
        console.warn('Had runtime error previously, reloading page')
      }
      window.location.reload()
      return
    }

    if (typeof onHotUpdateSuccess === 'function') {
      // Maybe we want to do something.
      onHotUpdateSuccess()
    }

    if (isUpdateAvailable()) {
      // While we were updating, there was a new update! Do it again.
      tryApplyUpdates()
    }
  }

  // https://webpack.github.io/docs/hot-module-replacement.html#check
  try {
    const updatedModules = await module.hot.check(/* autoApply */ {
      ignoreUnaccepted: true
    })
    if (updatedModules) {
      handleApplyUpdates(null, updatedModules)
    }
  } catch (err) {
    handleApplyUpdates(err, null)
  }
}
async function tryApplyUpdates (onHotUpdateSuccess) {
  if (!module.hot) {
    // HotModuleReplacementPlugin is not in Webpack configuration.
    console.error('HotModuleReplacementPlugin is not in Webpack configuration.')
    // window.location.reload();
    return
  }

  if (!isUpdateAvailable() || !canApplyUpdates()) {
    return
  }

  function handleApplyUpdates (err, updatedModules) {
    if (err || hadRuntimeError) {
      if (err) {
        console.warn('Error while applying updates, reloading page', err)
      }
      if (hadRuntimeError) {
        console.warn('Had runtime error previously, reloading page')
      }
      window.location.reload()
      return
    }

    if (typeof onHotUpdateSuccess === 'function') {
      // Maybe we want to do something.
      onHotUpdateSuccess()
    }

    if (isUpdateAvailable()) {
      // While we were updating, there was a new update! Do it again.
      tryApplyUpdates()
    }
  }

  // https://webpack.github.io/docs/hot-module-replacement.html#check
  try {
    const updatedModules = await module.hot.check(/* autoApply */ {
      ignoreUnaccepted: true
    })
    if (updatedModules) {
      handleApplyUpdates(null, updatedModules)
    }
  } catch (err) {
    handleApplyUpdates(err, null)
  }
}
Attempt to update code on the fly, fall back to a hard reload.