Error response formatting in Node.js - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ Predict Output

intermediate

What is the output of this Express error handler?

Consider this Express.js error handling middleware. What JSON response will the client receive if an error with message 'Invalid input' and status 400 is passed?

Node.js

app.use((err, req, res, next) => {
  res.status(err.status || 500).json({
    error: {
      message: err.message || 'Internal Server Error',
      code: err.status || 500
    }
  });
});

A {"error":{"message":"Internal Server Error","code":500}}

B {"error":{"msg":"Invalid input","status":400}}

C {"message":"Invalid input","code":400}

D {"error":{"message":"Invalid input","code":400}}

💡 Hint

Look at how the error message and status are accessed and structured in the JSON response.

The middleware sends a JSON object with an 'error' key containing 'message' and 'code' properties. It uses the error's message and status if provided, otherwise defaults.

❓ Predict Output

intermediate

What error does this Express error handler cause?

What error will this Express error handler cause when an error is passed to it?

Node.js

app.use((err, req, res, next) => {
  res.status(err.status).json({
    error: err.message
  });
});

A No error, sends JSON with error message and undefined status code

B TypeError: Cannot read property 'status' of undefined

C ReferenceError: err is not defined

D SyntaxError: Unexpected token

💡 Hint

Consider what happens if err.status is undefined.

If err.status is undefined, res.status(undefined) defaults to 200 OK in Express, so no error is thrown. The JSON contains the error message but status code is 200.

❓ component_behavior

advanced

Which option correctly formats a JSON error response with stack trace only in development?

You want to send an error response JSON that includes the error message always, but includes the stack trace only if NODE_ENV is 'development'. Which code snippet does this correctly?

A res.status(err.status || 500).json({ message: err.message });

B res.status(err.status || 500).json({ message: err.message, stack: process.env.NODE_ENV === 'production' ? err.stack : undefined });

C res.status(err.status || 500).json({ message: err.message, stack: process.env.NODE_ENV === 'development' ? err.stack : undefined });

D res.status(err.status || 500).json({ message: err.message, stack: err.stack });

💡 Hint

Check the environment variable condition for including the stack trace.

Option C includes the stack trace only if NODE_ENV is 'development', which is the common practice to avoid exposing stack traces in production.

📝 Syntax

advanced

Which option causes a syntax error in this error response code?

Identify the option that will cause a syntax error when used as an Express error handler response.

Node.js

app.use((err, req, res, next) => {
  res.status(500).json({
    error: err.message,
    details: err.stack
  });
});

A res.status(500).json({ error: err.message, details: err.stack, })

B res.status(500).json({ error: err.message details: err.stack })

C res.status(500).json({ error: err.message, details: err.stack })

D res.status(500).json({ error: err.message + details: err.stack })

💡 Hint

Look for missing commas or invalid object syntax.

Option B misses a comma between properties, causing a syntax error.

🔧 Debug

expert

Why does this Express error handler send a 200 status instead of 500?

Given this error handler, why does the client receive a 200 OK status instead of 500 when an error occurs? app.use((err, req, res, next) => { res.status(err.status).json({ error: err.message }); });

A Because err.status is undefined, res.status(undefined) defaults to 200 OK in Express

B Because the error handler is missing next() call to propagate the error

C Because res.status() always sends 200 unless explicitly called with 500

D Because JSON responses always override status codes to 200

💡 Hint

Check what happens if err.status is not set.

If err.status is undefined, Express treats res.status(undefined) as res.status(200), so the response status is 200 OK instead of an error code.