Logging with structured formats in Node.js - Practice Problems & Coding Challenges

Challenge - 5 Problems

💻 Command Output

intermediate

Output of JSON structured log with winston

What is the output of this Node.js code using winston logger configured for JSON format?

Node.js

import winston from 'winston';

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

logger.info('User login', { userId: 123, success: true });

A info: User login { userId: 123, success: true }

B {"level":"info","message":"User login","userId":123,"success":true}

C {"message":"User login","level":"info"}

D User login - userId: 123, success: true

💡 Hint

The winston JSON format outputs a single JSON object with all properties at top level.

Winston's json() format outputs logs as a single JSON string including level, message, and any extra metadata keys.

🧠 Conceptual

intermediate

Purpose of structured logging

Why is structured logging preferred over plain text logging in modern applications?

A It allows logs to be easily parsed and queried by log management tools.

B It automatically encrypts logs for security.

C It reduces the size of log files by compressing text.

D It prevents logs from being written to disk.

💡 Hint

Think about how logs are used in monitoring and debugging.

Structured logs use a consistent format like JSON, making it easy for tools to parse and analyze logs automatically.

❓ Configuration

advanced

Correct JSON logger configuration in Node.js

Which configuration correctly sets up a pino logger to output pretty-printed JSON logs to the console?

A

import pino from 'pino';
const logger = pino({ prettyPrint: true });
logger.info('Server started');

B

import pino from 'pino';
const logger = pino({ format: 'json' });
logger.info('Server started');

C

import pino from 'pino';
const logger = pino({ json: true });
logger.info('Server started');

D

import pino from 'pino';
const logger = pino({ prettyPrint: { colorize: true } });
logger.info('Server started');

💡 Hint

Check pino's documentation for prettyPrint options.

The prettyPrint option can be an object with colorize true to enable colored, pretty JSON output in pino.

❓ Troubleshoot

advanced

Why does this structured log miss fields?

Given this code snippet, why does the output JSON log miss the 'userId' field? const logger = winston.createLogger({ format: winston.format.json(), transports: [new winston.transports.Console()] }); logger.info('User login', { userId: 123 });

A Because winston's json format ignores metadata unless combined with splat format.

B Because the userId field is not a string and gets dropped.

C Because the Console transport does not support JSON format.

D Because the info method only accepts one argument.

💡 Hint

Check how winston handles extra metadata in JSON format.

Winston requires the splat format to include extra metadata fields in JSON output; otherwise, only message and level appear.

🔀 Workflow

expert

Order of middleware for structured logging in Express

In an Express.js app, you want to log each request with structured JSON format including method, url, and response time. Which middleware order is correct?

A 1,3,2,4

B 2,1,3,4

C 1,2,3,4

D 2,3,1,4

💡 Hint

Think about parsing JSON body before logging and measuring response time after request starts.

First parse JSON body, then log request start, then measure response time, finally handle routes.