谁在Express中间件中提供next（）函数？

Question

I'm confused about how next() works in Node.js and Express middleware.

There have been some other questions about middleware works, for example here , but I'm looking for a different answer.

The main question bugging me is, who is providing the next() function?

For example, in my standard generated express app, I'm given this code:

// catch 404 and forward to error handler
app.use(function(req, res, next) {
    var err = new Error('Not Found');
    err.status = 404;
    next(err);
});

But who calls this function and what is the nature of the next() that it provides? How do I know what the next() will look like? I have no idea what next() does.

Answer 1

An Express application is essentially a series of middleware calls.

Middleware is a function with access to the request object (req), the response object (res), and the next middleware in line in the request-response cycle of an Express application, commonly denoted by a variable named next.

As written above, Expressjs provides the next callback function. Next is just a way of calling the next middleware in the flow.

For many examples see Express's Using middleware