Middleware

Middleware functions are functions that have access to the request object, req, the response object, res, and the next middleware function in the application’s request-response cycle. The next middleware function is commonly denoted by a variable named next.

Middleware functions can perform the following tasks:

• Execute any code
• Make changes to the request and the response objects
• End the request-response cycle
• Call the next middleware function in the stack

Exercise: We've already used one built-in middleware in our app - what was it?

Handle Page Not found

Try to navigate to a url that doesn't exist. What do you get? Open the Developer tools and check what status code do you get back.

app.use(function (req, res, next) {
  res.status(404).render('404');
});

Setup an error handler

You define error-handling middleware in the same way as other middleware, except with four arguments instead of three; specifically with the signature (err, req, res, next):

app.use(function (err, req, res, next) {
  console.error(err.stack)
  res.status(500).send('Something broke!')
})

Let's write a custom middleware

var myLogger = function (req, res, next) {
  console.log('LOGGED')
  next()
}

app.use(myLogger)

Exercise: Follow the tutorial for writing requestTime at https://expressjs.com/en/guide/writing-middleware.html and add it to your own application