Structured logging with JSON in Express - Practice Problems & Coding Challenges

Challenge - 5 Problems

💻 Command Output

intermediate

Output of JSON structured log in Express middleware

Consider this Express middleware that logs request info as JSON. What is the output when a GET request to '/test' with header 'User-Agent: curl/7.68.0' is received?

Express

app.use((req, res, next) => {
  const logEntry = {
    method: req.method,
    url: req.url,
    userAgent: req.headers['user-agent']
  };
  console.log(JSON.stringify(logEntry));
  next();
});

A {"method":"GET","url":"/test"}

B {method: GET, url: /test, userAgent: curl/7.68.0}

C {"method":"GET","url":"/test","userAgent":"curl/7.68.0"}

D SyntaxError: Unexpected token m in JSON at position 1

💡 Hint

Remember JSON.stringify converts an object to a JSON string with double quotes.

The middleware creates an object with method, url, and userAgent keys, then logs it as a JSON string. The output is a JSON string with double quotes around keys and values.

❓ Configuration

intermediate

Correct JSON logging configuration for Winston in Express

Which Winston logger configuration correctly outputs logs in JSON format with timestamp and level fields?

A const logger = winston.createLogger({ format: winston.format.printf(info => `${info.level}: ${info.message}`), transports: [new winston.transports.Console()] });

B const logger = winston.createLogger({ format: winston.format.json(), transports: [new winston.transports.Console()] });

C const logger = winston.createLogger({ format: winston.format.simple(), transports: [new winston.transports.Console()] });

D const logger = winston.createLogger({ format: winston.format.colorize(), transports: [new winston.transports.Console()] });

💡 Hint

JSON format outputs structured logs as JSON objects.

Using winston.format.json() ensures logs are output as JSON strings with all info fields. Other formats produce plain text or colored output.

❓ Troubleshoot

advanced

Why does JSON.parse fail on logged Express request object?

You try to log the entire Express request object as JSON with JSON.stringify(req) and then parse it back with JSON.parse. It throws an error. Why?

Express

const jsonString = JSON.stringify(req);
const obj = JSON.parse(jsonString);

A JSON.stringify automatically removes all functions, so parse fails due to missing methods.

B JSON.parse cannot parse strings longer than 1KB.

C Express request object is a string, so JSON.stringify returns the same string causing parse error.

D Express request object contains circular references causing JSON.stringify to fail silently or produce incomplete output.

💡 Hint

Think about what kind of data structures JSON.stringify can handle.

Express request objects have circular references (e.g., req.connection points back to req), so JSON.stringify fails or produces incomplete JSON, causing JSON.parse to throw errors.

🔀 Workflow

advanced

Best workflow to add structured JSON logging with request IDs in Express

You want to add a unique request ID to every log entry in Express using structured JSON logging. Which workflow correctly achieves this?

A Use middleware to generate a UUID, attach it to req.id, then use a custom logger that includes req.id in every log entry.

B Generate UUID in each route handler and pass it manually to logger calls only in those handlers.

C Use global variable to store current request ID and read it in logger calls.

D Add request ID only in error handling middleware, ignoring normal requests.

💡 Hint

Think about how to consistently add request ID to all logs for a request.

Middleware runs for every request and can attach a unique ID to req object. A custom logger can then include this ID in all logs, ensuring consistent tracking.

✅ Best Practice

expert

Which logging practice improves observability in distributed Express apps?

In a distributed Express app with multiple services, which logging practice best improves observability and troubleshooting?

A Use structured JSON logs with consistent fields including trace IDs propagated across services.

B Log only error messages as plain text to reduce log size.

C Use different log formats per service to optimize for local readability.

D Disable logging in production to improve performance.

💡 Hint

Think about how to correlate logs across multiple services.

Structured JSON logs with consistent fields and trace IDs allow logs from different services to be correlated, improving observability and troubleshooting in distributed systems.