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.

Practice

1. What is the correct number of parameters for an error-handling middleware function in Node.js Express?

easy

A. One parameter: err

B. Three parameters: req, res, next

C. Two parameters: req, res

D. Four parameters: err, req, res, next

Solution

1. Step 1: Recall the error middleware signature

   Error-handling middleware in Express always has four parameters: err, req, res, and next.

2. Step 2: Understand the role of each parameter

   The first parameter is the error object, followed by the request, response, and next function to pass control.

3. Final Answer:

   Four parameters: err, req, res, next -> Option D

4. Quick Check:

   Error middleware = 4 parameters [OK]

Hint: Error middleware always has 4 parameters: err, req, res, next [OK]

Common Mistakes:

• Using 3 parameters like normal middleware
• Forgetting the err parameter
• Placing error middleware before routes

2. Which of the following is the correct syntax to define an error-handling middleware in Express?

easy

A. app.use((err, req, res, next) => { /* handle error */ })

B. app.use((req, res, next, err) => { /* handle error */ })

C. app.use((err, req, next, res) => { /* handle error */ })

D. app.use((req, err, res, next) => { /* handle error */ })

Solution

1. Step 1: Check parameter order in error middleware

   The correct order is err, req, res, next for error-handling middleware.

2. Step 2: Verify the syntax matches Express pattern

   app.use((err, req, res, next) => { /* handle error */ }) matches the correct order and syntax for error middleware.

3. Final Answer:

   app.use((err, req, res, next) => { /* handle error */ }) -> Option A

4. Quick Check:

   Correct parameter order = err, req, res, next [OK]

Hint: Error middleware params start with err, then req, res, next [OK]

Common Mistakes:

• Mixing parameter order
• Placing err parameter last or in the middle
• Using normal middleware syntax for error middleware

3. Consider this Express app code snippet:

app.get('/test', (req, res) => {
  throw new Error('Fail');
});

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

What will the client receive when requesting /test?

medium

A. A 404 Not Found error

B. A 500 status with 'Error caught' message

C. A 200 status with no response body

D. The server crashes with an uncaught exception

Solution

1. Step 1: Understand error thrown in route handler

   The route throws an error which Express catches and passes to error middleware.

2. Step 2: Error middleware sends 500 status and message

   The error middleware sets status 500 and sends 'Error caught' to the client.

3. Final Answer:

   A 500 status with 'Error caught' message -> Option B

4. Quick Check:

   Error thrown -> error middleware response [OK]

Hint: Thrown errors go to error middleware response [OK]

Common Mistakes:

• Expecting 200 OK despite error
• Thinking server crashes without middleware
• Confusing 404 with error middleware response

4. What is wrong with this error-handling middleware code?

app.use((err, req, res) => {
  res.status(500).send('Error!');
});

medium

A. It should have only two parameters

B. It should be placed before routes

C. It is missing the next parameter

D. It should not use res.status

Solution

1. Step 1: Check error middleware parameters

   Error middleware must have four parameters: err, req, res, next.

2. Step 2: Identify missing parameter

   The code is missing the next parameter, which is required even if unused.

3. Final Answer:

   It is missing the next parameter -> Option C

4. Quick Check:

   Error middleware needs 4 params including next [OK]

Hint: Error middleware always needs 4 parameters, including next [OK]

Common Mistakes:

• Omitting next parameter
• Placing error middleware before routes
• Using wrong parameter order

5. You want to log errors and then send a response in your Express app. Which is the best way to write the error-handling middleware?

app.use((err, req, res, next) => {
  console.error(err.stack);
  res.status(500).send('Something broke!');
});

What is a potential problem with this code if you add more error middleware after it?

hard

A. It does not call next(), so later error middleware won't run

B. It calls next() without an error, causing infinite loop

C. It sends response twice causing an error

D. It should not log errors in middleware

Solution

1. Step 1: Analyze middleware flow after sending response

   Once res.send is called, response is sent and middleware usually ends.

2. Step 2: Understand next() usage in error middleware

   If next() is not called, any following error middleware won't execute.

3. Step 3: Identify problem if more error middleware exists

   Without calling next(err), later error middleware is skipped, possibly missing logging or handling.

4. Final Answer:

   It does not call next(), so later error middleware won't run -> Option A

5. Quick Check:

   Missing next() call skips later error middleware [OK]

Hint: Call next(err) to continue error middleware chain [OK]

Common Mistakes:

• Not calling next() after sending response
• Calling next() without error causing loops
• Logging errors outside middleware