[node.js] Express: How to pass app-instance to routes from a different file?

1. To make your db object accessible to all controllers without passing it everywhere: make an application-level middleware which attachs the db object to every req object, then you can access it within in every controller.

// app.js
let db = ...;  // your db object initialized
const contextMiddleware = (req, res, next) => {
  req.db=db;
  next();
};
app.use(contextMiddleware);

2. to avoid passing app instance everywhere, instead, passing routes to where the app is

// routes.js  It's just a mapping.
exports.routes = [
  ['/', controllers.index],
  ['/posts', controllers.posts.index],
  ['/posts/:post', controllers.posts.show]
];

// app.js
var { routes }    = require('./routes');
routes.forEach(route => app.get(...route));
// You can customize this according to your own needs, like adding post request

The final app.js:

// app.js
var express   = require('express');
var app       = express.createServer();

let db = ...;  // your db object initialized
const contextMiddleware = (req, res, next) => {
  req.db=db;
  next();
};
app.use(contextMiddleware);

var { routes }    = require('./routes');
routes.forEach(route => app.get(...route));

app.listen(3000, function() {
  console.log('Application is listening on port 3000');
});

Another version: you can customize this according to your own needs, like adding post request

// routes.js  It's just a mapping.
let get = ({path, callback}) => ({app})=>{
  app.get(path, callback);
}
let post = ({path, callback}) => ({app})=>{
  app.post(path, callback);
}
let someFn = ({path, callback}) => ({app})=>{
  // ...custom logic
  app.get(path, callback);
}
exports.routes = [
  get({path: '/', callback: controllers.index}),
  post({path: '/posts', callback: controllers.posts.index}),
  someFn({path: '/posts/:post', callback: controllers.posts.show}),
];

// app.js
var { routes }    = require('./routes');
routes.forEach(route => route({app}));