Next JS 9: Proxy middleware with api and next-connect

[...slug].js

// [...slug].js
import nc from 'next-connect'

const dev = process.env.NODE_ENV !== 'production'

const handler = nc()

if (dev) {
  const { createProxyMiddleware } = require('http-proxy-middleware')
  handler.use(createProxyMiddleware({
    target: process.env.BACKEND_API,
    pathRewrite: {
      [`^/${process.env.API_PATH}`]: '',
    },
    changeOrigin: true,
  }))
}

export default handler

next.config

// next.config
module.exports = {
  optionalCatchAll: true
}