Access privateRuntimeConfig in express server

We have done precisely this by importing the Nuxt config into the file that configures the Express app, and using defu to combine public and private runtime configs, as Nuxt itself does:

    // nuxt.config.js

    export default {
      publicRuntimeConfig: {},
      privateRuntimeConfig: { redis: { url: process.env['REDIS_URL'] } }
    };

    // api/index.js

    import express from 'express';
    import defu  from 'defu';
    import { createClient as createRedisClient } from 'redis';
    
    const app = express();
    
    import nuxtConfig from '../nuxt.config.js';
    let runtimeConfig;
    
    app.use((req, res, next) => {
      if (!runtimeConfig) {
        // Load Nuxt config once, at runtime
        runtimeConfig = defu(nuxtConfig.privateRuntimeConfig, nuxtConfig.publicRuntimeConfig);
      }
      next();
    });
    
    // Subsequent middlewares will then be able to read from `runtimeConfig`
    app.use((req, res, next) => {
      const redisClient = createRedisClient({ url: runtimeConfig.redis.url });
      next();
    });
    
    export default app;