Error-handling middleware in Node.js - Step-by-Step Execution

Concept Flow - Error-handling middleware

Request Received

↓

Middleware 1 Executes

↓

Middleware 2 Executes

↓

Error Occurs?

No→Next Middleware or Response

Yes↓

Error-handling Middleware Executes

↓

Send Error Response

↓

End

The request passes through normal middleware until an error occurs, then error-handling middleware runs to send an error response.

Execution Sample

Node.js

app.use((req, res, next) => {
  if (!req.user) {
    next(new Error('No user'));
  } else {
    next();
  }
});

app.use((err, req, res, next) => {
  res.status(500).send(err.message);
});

This code checks if a user exists; if not, it passes an error to the error handler which sends a 500 response.

Execution Table

Step	Middleware Type	Input	Action	Next Step
1	Normal Middleware	Request with no user	Checks req.user, finds none	Calls next(error) with Error('No user')
2	Error-handling Middleware	Error('No user')	Sends 500 status and error message	Response sent, ends request
3	Normal Middleware	Request with user	Calls next() without error	Passes to next middleware or response
4	No more middleware	No error	Sends normal response	Ends request

💡 Execution stops after error-handling middleware sends response or after normal middleware completes response.

Variable Tracker

Variable	Start	After Step 1	After Step 2	After Step 3	After Step 4
req.user	undefined or object	undefined (no user) or object (user)	unchanged	unchanged	unchanged
err	none	Error('No user') if no user, else none	passed to error handler or none	none	none
res.statusCode	200 (default)	200	500 if error sent	200	200

Key Moments - 3 Insights

Why does the error-handling middleware have 4 parameters (err, req, res, next)?

Because Express identifies error-handling middleware by the presence of 4 parameters. This middleware runs only when next() is called with an error, as shown in step 2 of the execution_table.

What happens if next() is called without an error?

The request continues to the next normal middleware or response, as shown in step 3 and 4 of the execution_table.

Can normal middleware send a response after calling next(error)?

No, once next(error) is called, Express skips normal middleware and goes directly to error-handling middleware, which sends the response (step 2).

Visual Quiz - 3 Questions

Look at the execution_table, what happens at step 2?

A Normal middleware processes the request

B Error-handling middleware sends a 500 response

C Request is passed to the next normal middleware

D The server crashes

💡 Hint

Check the 'Action' column at step 2 in the execution_table.

At which step does the middleware detect there is no user in the request?

A Step 1

B Step 2

C Step 3

D Step 4

💡 Hint

Look at the 'Input' and 'Action' columns in the execution_table for step 1.

If req.user exists, what will be the status code after all middleware run?

A 500

B 404

C 200

D 401

💡 Hint

Check the variable_tracker for res.statusCode after step 4.

Concept Snapshot

Error-handling middleware in Node.js Express has 4 parameters: (err, req, res, next).
It runs only when next() is called with an error.
Normal middleware calls next() to continue or next(error) to trigger error handling.
Error middleware sends error responses, stopping normal flow.
Always place error middleware after normal middleware.

Full Transcript

In Node.js Express, error-handling middleware is special middleware that runs only when an error occurs. It has four parameters: error, request, response, and next. When normal middleware detects a problem, it calls next with an error. This skips all normal middleware and runs the error handler. The error handler sends a response with an error message and status code, usually 500. If no error occurs, normal middleware calls next without arguments to continue processing. This flow ensures errors are caught and handled cleanly without crashing the server.