express - When to use next() and return next() in Node.js

Question

Scenario: Consider the following is the part of code from a node web app.

app.get('/users/:id?', function(req, res, next){
    var id = req.params.id;
    if (id) {
        // do something
    } else {
        next(); //or return next();
    }
});

Issue: I am checking which one to go with just next() or return next(). Above sample code works exactly the same for both & did not show any difference in execution.

Question: Can some one put light on this, when to use next() and when to use return next() and some important difference?

Answer 1

As @Laurent Perrin's answer:

If you don't do it, you risk triggering the callback a second time later, which usually has devastating results

I give an example here if you write middleware like this:

app.use((req, res, next) => {
  console.log('This is a middleware')
  next()
  console.log('This is first-half middleware')
})

app.use((req, res, next) => {
  console.log('This is second middleware')
  next()
})

app.use((req, res, next) => {
  console.log('This is third middleware')
  next()
})

You will find out that the output in console is:

This is a middleware
This is second middleware
This is third middleware
This is first-half middleware

That is, it runs the code below next() after all middleware function finished.

However, if you use return next(), it will jump out the callback immediately and the code below return next() in the callback will be unreachable.