Node + Express: Does Express clone the req and res objects for each request handler?

Question

If Javascript copies objects by reference, then does Express clone the req and res objects before passing them down to each request handler? If not, then how does Express handle possible conflicts between routes running simultaneously and using the same reference to req and res?

Answer 1

Express doesn't clone req and res. You can see that in this example app:

var http = require('http');
var express = require('express');

var app = express();
var testReq, testRes;

app.use(function(req, res, next) {
  console.log('middleware');
  testReq = req;
  testRes = res;
  next();
});

app.get("*", function(req,res) {
  console.log('route')
  console.log('req the same? ' + (req === testReq)); // logs true
  console.log('res the same? ' + (res === testRes)); // logs true

  res.send(200);
});


http.createServer(app).listen(8080);

Test with curl:

$ curl localhost:8080

This is a useful feature - it means that middleware functions can use req and res to pass data to downstream functions. For example an authorisation middleware might add a req.user property.

Concurrency isn't a concern here because Node.js is single threaded - it is not possible for two routes to run at any given time.

It also doesn't run a single request through multiple routes - you can add another get("*") route and you'll see that it won't get called.