Recall that middleware is just a function with a specific signature, namely (req, res, next). We have, for the most part, been using anonymous function definitions for this because our middleware has only been relevant to the route invoking it. There is nothing stopping us from defining functions and using them as middleware though. That is to say:

const logging = (req, res, next) => {
  console.log(req);
  next();
};

app.use(logging);

is a valid and reasonable way to introduce logging throughout all paths. It is also modifiable so that you can remove the app.use() line and replace it with a specific route method, or sprinkle it throughout the application without it being universal.

Up until this point we’ve only been giving each middleware-accepting method a single callback. With modular pieces like this, it is useful to know that methods such as app.use(), app.get(), app.post(), and so on all can take multiple callbacks as additional parameters. This results in code that looks like the following:

const authenticate = (req, res, next) => {
  ...
};

const validateData = (req, res, next) => {
  ...
};

const getSpell = (req, res, next) => {
  res.status(200).send(getSpellById(req.params.id));
};

const createSpell = (req, res, next) => {
  createSpellFromRequest(req);
  res.status(201).send();
};

const updateSpell = (req, res, next) => {
  updateSpellFromRequest(req);
  res.status(204).send();
}

app.get('/spells/:id', authenticate, getSpell);

app.post('/spells', authenticate, validateData, createSpell);

app.put('/spells/:id', authenticate, validateData, updateSpell);

In the above code sample, we created reusable middleware for authentication and data validation. We use the authenticate() middleware to verify a user is logged in before proceeding with the request and we use the validateData() middleware before performing the appropriate create or update function. Additional middleware can be placed at any point in this chain.