Error-handling middleware in Express - Step-by-Step Execution

Concept Flow - Error-handling middleware

Request received

↓

Route handler runs

↓

Error occurs?

No→Send response

Yes↓

Pass error to next()

↓

Error-handling middleware runs

↓

Send error response

↓

End request

When a request causes an error, Express passes it to special middleware with 4 parameters to handle and respond with error info.

Execution Sample

Express

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

This middleware catches errors, logs the message, and sends a 500 response.

Execution Table

Step	Action	Error Present?	Middleware Called	Response Sent
1	Request received at /data	No	Route handler	No
2	Route handler runs	Yes (throws error)	Route handler	No
3	Error passed to next()	Yes	Error-handling middleware	No
4	Error-handling middleware logs error	Yes	Error-handling middleware	No
5	Error-handling middleware sends 500 response	Yes	Error-handling middleware	500 Server error
6	Request ends	No	None	Response sent

💡 Error handled and response sent, request cycle ends.

Variable Tracker

Variable	Start	After Step 2	After Step 3	After Step 5
err	undefined	Error object with message 'Something broke!'	Same error object	Same error object
res.statusCode	200 (default)	200 (default)	200 (default)	500
responseSent	false	false	false	true

Key Moments - 2 Insights

Why does error-handling middleware have 4 parameters instead of 3?

Express identifies error-handling middleware by the 4 parameters (err, req, res, next). This lets it run only when an error is passed, as shown in steps 3-5.

What happens if next() is called without an error in error-handling middleware?

If next() is called without an error, Express skips error middleware and looks for normal middleware, which can cause the error to be unhandled. See step 3 where next(err) is used to pass the error.

Visual Quiz - 3 Questions

Look at the execution table, at which step does the error-handling middleware first run?

A Step 2

B Step 3

C Step 4

D Step 5

💡 Hint

Check the 'Middleware Called' column to see when error-handling middleware starts.

According to the variable tracker, what is the response status code after step 5?

A 500

B 200

C 404

D undefined

💡 Hint

Look at the 'res.statusCode' row after step 5.

If the route handler did not throw an error, what would happen according to the flow?

A Error-handling middleware runs anyway

B next(err) is called with undefined

C Request ends after sending normal response

D Server crashes

💡 Hint

Refer to the concept flow where 'Error occurs?' is No.

Concept Snapshot

Error-handling middleware in Express:
- Has 4 parameters: (err, req, res, next)
- Runs only when next(err) is called
- Used to catch errors and send error responses
- Place after all other middleware
- Example: app.use((err, req, res, next) => { res.status(500).send('Error'); })

Full Transcript

In Express, error-handling middleware is special middleware with four parameters: err, req, res, and next. When a route handler or middleware calls next() with an error, Express skips normal middleware and runs error-handling middleware. This middleware can log the error and send a response like a 500 status code. The flow starts with a request, runs the route handler, and if an error occurs, passes it to error middleware which sends the error response. Variables like the error object and response status code change during this process. Understanding when error middleware runs and how it handles errors helps keep your app stable and user-friendly.