What is the parameter “next” used for in Express?

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Suppose you have a simple block of code like this:

app.get("https://stackoverflow.com/", function(req, res){
    res.send('Hello World');
});

This function has two parameters, req and res, which represent the request and response objects respectively.

On the other hand, there are other functions with a third parameter called next. For example, lets have a look at the following code:

app.get('/users/:id?', function(req, res, next){ // Why do we need next?
    var id = req.params.id;
    if (id) {
        // do something
    } else {
        next(); // What is this doing?
    }
});

I can’t understand what the point of next() is or why its being used. In that example, if id doesn’t exist, what is next actually doing?

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