next.js/build/webpack/plugins/pages-plugin.js

// @flow
import { ConcatSource } from 'webpack-sources'
import {
  IS_BUNDLED_PAGE_REGEX,
  ROUTE_NAME_REGEX
} from '../../../lib/constants'

export default class PagesPlugin {
  apply (compiler: any) {
    compiler.hooks.compilation.tap('PagesPlugin', (compilation) => {
      compilation.chunkTemplate.hooks.render.tap('PagesPluginRenderPageRegister', (modules, chunk) => {
        if (!IS_BUNDLED_PAGE_REGEX.test(chunk.name)) {
          return modules
        }

        let routeName = ROUTE_NAME_REGEX.exec(chunk.name)[1]

        // We need to convert \ into / when we are in windows
        // to get the proper route name
        // Here we need to do windows check because it's possible
        // to have "\" in the filename in unix.
        // Anyway if someone did that, he'll be having issues here.
        // But that's something we cannot avoid.
        if (/^win/.test(process.platform)) {
          routeName = routeName.replace(/\\/g, '/')
        }

        routeName = `/${routeName.replace(/(^|\/)index$/, '')}`

        const source = new ConcatSource()

        source.add(`__NEXT_REGISTER_PAGE('${routeName}', function() {
            var comp =
        `)
        source.add(modules)
        source.add(`
            return { page: comp.default }
          })
        `)

        return source
      })
    })
  }
}