Error response formatting in Express - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ component_behavior

intermediate

What is the output of this Express error handler?

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

Express

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

A {"error":{"message":"Invalid input","status":400}}

B {"error":{"message":"Internal Server Error","status":500}}

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

D {"error":"Invalid input"}

💡 Hint

Look at how the error object properties are used in the JSON response.

The middleware sets the response status to err.status or 500 and sends a JSON object with an error key containing message and status from the error object.

📝 Syntax

intermediate

Which option causes a syntax error in Express error middleware?

Identify which error handler code snippet will cause a syntax error.

A

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

B

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

C

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D

app.use(function(err, req, res, next) {
  res.status(500).json({ error: err.message });
});

💡 Hint

Check the brackets and parentheses carefully.

Option B has a mismatched parenthesis in the JSON object causing a syntax error.

❓ state_output

advanced

What is the HTTP status code sent by this error handler?

Given this Express error middleware, what status code will be sent if err.status is undefined?

Express

app.use((err, req, res, next) => {
  res.status(err.status || 404).json({
    error: err.message || 'Not Found'
  });
});

A 200

B 500

C 404

D undefined

💡 Hint

Look at the fallback value used with the OR operator.

If err.status is undefined, the code uses 404 as the default status code.

🔧 Debug

advanced

Why does this error handler not send a JSON response?

This Express error handler does not send a JSON response as expected. What is the cause?

Express

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

A Calling next() after res.json() causes the response to be sent twice, breaking JSON output.

B Missing return statement after res.status(500) prevents response from sending.

C res.json() is called without setting status, so default 200 is sent.

D Error handler must have 3 parameters, not 4.

💡 Hint

Think about what happens when next() is called after sending a response.

Calling next() after res.json() passes control to next middleware, which can cause errors or multiple responses.

🧠 Conceptual

expert

Which option correctly formats an error response with a custom error code and message in Express?

You want to send a JSON error response with a custom code and message using Express error middleware. Which code snippet does this correctly?

A

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

B

app.use((err, req, res, next) => {
  res.status(400).json({
    code: err.code || 'ERR_CUSTOM',
    message: err.message || 'Custom error'
  });
});

C

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

D

app.use((err, req, res, next) => {
  res.status(err.statusCode || 400).json({
    error: {
      code: err.code || 'ERR_CUSTOM',
      message: err.message || 'Custom error'
    }
  });
});

💡 Hint

Check how status code and error details are included in the response.

Option D sets status code with fallback and includes error code and message inside an error object in JSON.