Example plugin

api.js

import { join } from 'path'
import { parse } from 'url'
import requireModule from 'next/dist/server/require.js'

function getPath (url) {
  return parse(url || '/').pathname
}

export default (addRoute) => {
  addRoute('/api/:path+', async (url, { req, res, params }, {
    dir = process.cwd(),
    dev = false
  } = {}) => {
    let data = null

    try {
      const mod = await requireModule(join(dir, '.next', 'dist', getPath(url)))
      const fn = mod.default

      if (!fn) {
        throw new Error('Endpoint is malformed; `export default function ({req, res, params}, {dir, dev}) {  }`')
      }

      data = {
        success: true,
        body: fn({ req, res, params }, { dir, dev })
      }
    } catch (e) {
      data = {
        error: true,
        message: dev ? e.message : 'Internal server error'
      }
    }

    const json = JSON.stringify(data)
    res.setHeader('Content-Type', 'application/json')
    res.setHeader('Content-Length', Buffer.byteLength(json))
    res.end(json)
  }, 'GET')
}