[3065] Where is the source code of renderRoute located

I read the readme of nuxt.js, and there is an example of rendering an specific route.

nuxt.renderRoute('/about', context)
.then(function ({ html, error }) {
  // You can check error to know if your app displayed the error page for this route
  // Useful to set the correct status code if an error appended:
  if (error) {
    return res.status(error.statusCode || 500).send(html)
  }
  res.send(html)
})
.catch(function (error) {
  // And error appended while rendering the route
})

But I can not find the source code of the method of renderRoute in the source of nuxt.js. Where is it located?