Centralized error handler in Express - Step-by-Step Execution

Concept Flow - Centralized error handler

Request received

↓

Route handler runs

↓

Error occurs?

No→Send response

Yes↓

Pass error to next()

↓

Centralized error handler middleware

↓

Send error response

The request goes to a route handler. If an error happens, it is passed to the centralized error handler middleware, which sends the error response.

Execution Sample

Express

app.get('/user', (req, res, next) => {
  if (!req.query.id) {
    next(new Error('ID missing'));
  } else {
    res.send('User found');
  }
});

app.use((err, req, res, next) => {
  res.status(400).send(err.message);
});

This code checks if the query has an id. If not, it sends an error to the centralized error handler which returns a 400 status with the error message.

Execution Table

Step	Action	Condition	Result	Next
1	Request to /user without id	id missing?	Yes	Call next(error)
2	next(error) called	Error passed to middleware?	Yes	Centralized error handler runs
3	Error handler runs	Send response with error message	Status 400, message 'ID missing'	Response sent
4	Request to /user with id	id missing?	No	Send 'User found' response
5	Response sent	End of request	Success	Request complete

💡 Execution stops after sending response either from route or error handler.

Variable Tracker

Variable	Start	After Step 1	After Step 2	After Step 3	Final
req.query.id	undefined or value	undefined (missing) or value	undefined (error passed) or value	unchanged	unchanged
error	none	Error('ID missing') created or none	passed to error handler or none	handled or none	none
res.statusCode	200 default	200	400 if error	400 or 200	final status sent
res.body	empty	'User found' or empty	error message or 'User found'	sent response	sent response

Key Moments - 3 Insights

Why do we call next(error) instead of sending response directly in the route?

Calling next(error) passes the error to the centralized error handler middleware, keeping error handling in one place. See execution_table step 2 where next(error) triggers the error handler.

What happens if the route sends a response without calling next(error)?

The centralized error handler is not called. The response is sent immediately as in execution_table step 4 and 5.

Why does the error handler have four parameters (err, req, res, next)?

Express recognizes middleware with four parameters as error handlers. This lets Express route errors to this middleware, as shown in the concept_flow and execution_table step 3.

Visual Quiz - 3 Questions

Look at the execution table, what is the status code sent when an error occurs?

A 400

B 200

C 500

D 404

💡 Hint

Check execution_table row 3 where the error handler sends the response.

At which step does the centralized error handler middleware run?

A Step 1

B Step 2

C Step 3

D Step 4

💡 Hint

See execution_table where error handler sends the error response.

If the route always sends a response without calling next(error), what changes in the execution table?

A Error handler runs anyway

B next(error) is never called, error handler skipped

C Status code changes to 500

D Request never completes

💡 Hint

Look at execution_table step 4 and 5 where no error is passed.

Concept Snapshot

Centralized error handler in Express:
- Use next(error) in routes to pass errors
- Define error handler middleware with 4 params: (err, req, res, next)
- Error handler sends error response centrally
- Keeps error handling consistent and clean
- Middleware order matters: error handler last

Full Transcript

In Express, when a request comes in, the route handler runs. If an error happens, instead of sending a response immediately, the route calls next(error). This passes the error to a special middleware called the centralized error handler, which has four parameters: err, req, res, next. This middleware sends a response with the error message and status code. If no error occurs, the route sends the normal response. This keeps error handling in one place and makes code cleaner. The error handler middleware must be added after all routes. The execution table shows the steps: request received, error detected, next(error) called, error handler runs, response sent. Variables like req.query.id, error, res.statusCode, and res.body change accordingly during these steps.