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Error-Handling Middleware

Express allows you to capture and handle errors using special middleware. An error-handling middleware function has four parameters: (err, req, res, next).

Synchronous errors (thrown in your route handlers) automatically skip normal middleware and go straight to this error handler.
Asynchronous errors (in async functions) are automatically passed to the error handler in Express 5.

When an error reaches your error handler, you can respond with a 500 status code or any other status you choose.

function errorHandler(
  err: Error,
  req: Request,
  res: Response,
  next: NextFunction,
) {
  console.error("Uh oh, spaghetti-o");
  res.status(500).json({
    error: "Boots has fallen",
  });
}

app.use(errorHandler);

Error handling middleware needs to be defined last, after all your other app.use() and route handlers (app.post, app.get, etc.), but before app.listen.

Catching Errors in Async Code

In Express 5, unhandled async errors automatically go to the error handler.

app.post("/api", async (req, res) => {
  await handler(req, res);
});

You can still use try/catch when you need to handle an error before re-throwing it.

app.post("/api", (req, res, next) => {
  Promise.resolve(handler(req, res)).catch(next);
});

Assignment

Create and mount an error-handling middleware that responds with 500 for unhandled errors.
It should log the error using console.log and return a JSON response:

{
  "error": "Something went wrong on our end"
}

Throw an error in your async route handler when a chirp is too long.
Test that your error-handling middleware catches it and returns the generic 500 error: Something went wrong on our end.

Run and submit the CLI tests.