What is the next() function in Express middleware?

The next() function is the third parameter of every Express middleware function. Calling next() passes control to the next middleware in the stack. If you do not call next() and do not send a response, the request will hang. Calling next(err) with an argument skips all remaining regular middleware and jumps to the error-handling middleware (which has the signature (err, req, res, next)). This is the standard pattern for propagating errors. Calling next('route') skips remaining handlers for the current route and passes control to the next matching route.