There are different types of middleware; for our purposes, we'll group them into three different categories:
Built-in middleware is included with Express but not added to the application automatically. Like the other types, we need to opt-in to using it in our application.
We saw an example of built-in middleware when we added support for parsing JSON content out of the request body using server.use(express.json());.
Every type of middleware works in the same way. We tell Express about the middleware we want to turn on for our application by making a call to .use() on our server and passing .use() the piece of middleware we want to apply. This line must come after the server has been created by calling express().
Third-party middleware are npm modules that we can install and then import into our applications using require(). There are thousands of middleware modules we can use. There is no need to write our own in most cases.
Some popular middleware modules are:
Custom middleware is functions we write to perform specific tasks. We'll learn more about how to write and use them in the next section.
One thing that is not immediately obvious is that route handlers are middleware. Let's see this in action.
Suppose a client visits a non-existent endpoint in our current implementation. In that case, they will get a default message when a resource is not found on the server. In the case of a browser, it's "Cannot Get /urlWeTriedToAccess". This default message makes for a poor user experience. So please code along as we write a request handler that responds with a custom message for invalid URLs.
function(req, res) {
res.status(404).send("Ain't nobody got time for that!");
};This code is not complete yet, but you can see that it is, in fact, a request handler function. We know because the homies (req and res) are there.
Now let's just use it as if it was middleware:
server.use(function(req, res) {
res.status(404).send("Ain't nobody got time for that!");
});Almost there! The last step is adding this status after each route handler. That way, if the preceding middleware or route handlers do not respond to the request, this will become our catch-all and respond with the correct HTTP status code and a helpful message.
Now, requests to non-existent endpoints will result in a better experience for our clients.
Write a paragraph (or two) explaining Express middleware and how you would use it when writing Web APIs.
The versions of project dependencies used in the recording are slightly different from the ones used in the starter and solution repositories, but this should not affect the relevant code of the Guided Project.
The versions used in the repositories are more recent, and thus more similar to the versions you will install if you create a project from scratch.