lloydakresi / practice-for-week-10-express-error-handling-404

Practice: Handling "Resource Not Found" errors

A common feature for applications to implement is to present a friendly "Resource Not Found" message to end users when a request can't be matched to one of the API application's defined routes. In this practice, you will implement this feature using a combination of a middleware function and an error handler function.

Set up

cd into the server folder.

npm install the dependencies.

Run npm run dev to start the server with nodemon on localhost port 5000.

"Resource Not Found" middleware

Express has a default error that gets sent to the client when no response is sent after all middleware are executed. To see this generic response, browser to http://localhost:5000/some-unknown-resource (or any path that doesn't match one of your application's configured routes). You should see a message in the browser of Cannot GET /some-unknown-resource.

You want to create a "Resource Not Found" middleware that should create an error when none of the route handlers are matched to a request method/URL path combination. This middleware should override that default Express response by throwing an error if a request doesn't match any of the route handlers.

Add a middleware function after all the route handlers in the Express application in the server/app.js file.

In this function, create a new Error with the message of "Sorry, the requested resource couldn't be found".

The status code of a response should be 404 when something is not found in the server. Any error-handling middleware that . Add a statusCode property of 404 to the error object so that any error-handling middleware that catches this error knows that the error was thrown because something was not found.

Finally, pass the error into the next error-handling middleware.

Place this middleware function after all of your route handler. By doing so, this middleware function will only be invoked if a request fails to match any of your routes.

Why does the placement of this middleware matter?

In Express, the order of the middleware defined on the Express application matters. Express will attempt to match the request method/URL to the first middleware defined then the next until a response is sent or there are no more defined middleware to match.

At this point, you can test your "Resource Not Found" middleware function by browsing to http://localhost:5000/some-unknown-resource (or really any path that doesn't match one of your application's configured routes).

You should see the error message of "Error: Sorry, the requested resource couldn't be found" followed by a stack trace of the error message in the response instead of the default Express response of Cannot GET /some-unknown-resource.

Why does a response get sent back when you didn't explicitly send a response?

If no response is sent after an error gets thrown from a middleware, then Express will send a default response of the error message plus the error stack trace for the error that was thrown.

Catch-all errors middleware (error-handling middleware)

The error message that you see in the response sent to the client is Express' automatically generated error response. To override this automated response, you can create an error-handling middleware that will send a different response when there is an error caught by this middleware.

Create a catch-all errors middleware that sends a custom response for any request that throws an error in your application. Define an error handling middleware and connect it to the Express application after all route handlers and middleware.

Why should you place the "Catch-all" middleware AFTER the "Resource Not Found" middleware?

The "Resource Not Found" middleware throws an error that the "Catch-all" middleware should catch and send a custom response for.

In the middleware, log the error to the server console.

Then, extract the statusCode property on the error and set it as the status code for the response. If there is no statusCode property on the error caught by the error-handling middleware, then set the status code of the response to a default of 500.

Set the body of the response to a JSON containing a property of message with the error message as its value. Set another property of statusCode with the status code of the response as its value.

Make sure the middleware is connected as the very last middleware in your Express application so that it can catch any errors thrown in any of the route handlers or middleware before it.

Test your catch-all middleware function by browsing to http://localhost:5000/some-unknown-resource (or really any path that doesn't match one of your application's configured routes). Check the status code of the response and make sure that it is 404. Check the body of the response. It should be something like:

{
  "message": "Sorry, the requested resource couldn't be found.",
  "statusCode": 404
}

Testing all the other route handlers

Make sure the GET / endpoint still works properly. It should still display "GET / This is the root URL" as the response body.

Awesome! You just implemented a commonly implemented Express middleware for throwing an error when the request doesn't match any routes in your Express application.

You also implemented an error-handling middleware that catches all errors thrown in your Express application and sends a customized response for the error.

You can use these middleware or customize them further in your future Express applications.