Node.js多个Express对象

Question

I have been trying to understand how Expressjs framework for Node.js handles routes in multiple files with no luck. My main index.js looks similar this:

var express = require('express');
var app = express();
app.use('/route', require('./api/route'));
app.set('important_var', global_var);

In the last line I set one global variable for my application, which is needed by all other routes.

In the second JS file './api/route.js', I do kinda the same thing:

var express = require('express');
var router = app.Router(); 
router.get('/path1', () => {...});

In the second file I need an important_var which was set on application level in index.js. How can I access it? Am I allowed to create new (if it is considered new) object with express()? Is variable app from index.js considered as singleton and won't be recreated? How should I handle these kind of situations?

Answer 1

If I understand your question correctly then these are the requirements you are trying to meet:

• A main file that configures the server.
• The main file sets "global" values that can be accessed by various routes within the application.
• A sub router file that can access the global values.

If those requirements are correct then the following is a way of meeting them the Express way.

Using app.set for storing app globals can lead to issues since it is for assigning an app setting value. Instead, app globals can safely be stored to a app.locals which is specifically intended for this use case.

So in your main file ( /index.js ):

const express = require('express');
const app = express();
const subRouter = require('./api/route');

app.locals.important_var = 'My App Global';

app.use('/route', subRouter);

Then to access the app global in your sub router file ( /api/route.js ) you would use the req.app.locals object which is a reference back to app.locals :

const express = require('express');
const router = express.Router();

module.exports = router;

router.get('/path1', (req, res) => {
  console.log(req.app.locals.important_var);
  // ...
});

Answer 2

To access values set with app.set() , you need to be able to call app.get() and it needs to be the same app object that you used when you called app.set() . Each app is an Express object and it keeps its own state.

When create route handlers using a router, you can access the app object that a given route is associated with via req.app . So, if you have code in a router you can access things on the app object like this:

router.get('/path1', () => {
    let data = req.app.get('important_var');
    // rest of route handler here
});

---

It may be more common to use middleware and set a property of your own on the req object that all route handlers could then access:

const express = require('express');
const app = express();
// custom middleware
app.use(function(req, res, next) {
    req.myImportantVar = someCalculation(important_var);
    next();
});

app.use('/route', require('./api/route'));

Then, in a router, you could do this:

const router = require('express').Router();
router.get('/path1', () => {
     console.log(req.myImportantVar);
     // rest of route handler here
});