• A string representing a path.
• A path pattern.
• A regular expression pattern to match paths.
• An array of combinations of any of the above.

• A middleware function.
• A series of middleware functions (separated by commas).
• An array of middleware functions.
• A combination of all of the above.

You can provide multiple callback functions that behave just like middleware, except that these callbacks can invoke next('route') to bypass the remaining route callback(s). You can use this mechanism to impose pre-conditions on a route, then pass control to subsequent routes if there is no reason to proceed with the current route.

When a callback function throws an error or returns a rejected promise, `next(err)` will be invoked automatically.

Since router and app implement the middleware interface, you can use them as you would any other middleware function.

For examples, see Middleware callback function examples.