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.

Practice

1. What is the main benefit of using structured logging in Node.js applications?

easy

A. Logs are stored as plain text without any format.

B. Logs are only saved in memory and not written to files.

C. Logs are encrypted automatically for security.

D. Logs are saved in a clear, organized JSON format for easy reading and processing.

Solution

1. Step 1: Understand structured logging

   Structured logging means saving logs in a format like JSON that is easy to read and parse.

2. Step 2: Identify the benefit

   This format helps both humans and tools quickly understand log data.

3. Final Answer:

   Logs are saved in a clear, organized JSON format for easy reading and processing. -> Option D

4. Quick Check:

   Structured logging = JSON format logs [OK]

Hint: Structured logs use JSON for clarity and easy parsing [OK]

Common Mistakes:

• Thinking logs are encrypted by default
• Confusing plain text logs with structured logs
• Assuming logs are only stored in memory

2. Which of the following is the correct way to create a logger using the pino library in Node.js?

easy

A. const logger = require('pino')();

B. const logger = new pino.Logger();

C. const logger = pino.create();

D. const logger = require('pino').createLogger();

Solution

1. Step 1: Recall pino import syntax

   The pino library is imported using require('pino'), which returns a function.

2. Step 2: Create logger instance

   Calling this function with () creates the logger instance: require('pino')()

3. Final Answer:

   const logger = require('pino')(); -> Option A

4. Quick Check:

   pino logger created by calling require('pino')() [OK]

Hint: Use require('pino')() to create logger instance [OK]

Common Mistakes:

• Using createLogger() which does not exist
• Trying to use new keyword with pino
• Calling pino.create() which is invalid

3. What will be the output of the following Node.js code using pino?

const pino = require('pino');
const logger = pino();
logger.info({ user: 'alice', action: 'login' }, 'User action');

medium

A. User action: user=alice, action=login

B. {"level":30,"time":<timestamp>,"msg":"User action","user":"alice","action":"login"}

C. {"message":"User action","user":"alice","action":"login"}

D. Error: Missing log level

Solution

1. Step 1: Understand pino.info() output format

   pino logs messages as JSON with fields like level, time, msg, and any extra data.

2. Step 2: Analyze given code

   Calling logger.info with an object and message logs JSON with level 30 (info), timestamp, msg, and object keys.

3. Final Answer:

   {"level":30,"time":<timestamp>,"msg":"User action","user":"alice","action":"login"} -> Option B

4. Quick Check:

   pino logs JSON with level and message [OK]

Hint: pino logs JSON with level, time, msg, and extra data [OK]

Common Mistakes:

• Expecting plain text output instead of JSON
• Confusing message key name
• Thinking log level is missing

4. Identify the error in this code snippet using pino for structured logging:

const pino = require('pino');
const logger = pino();
logger.info('User logged in', { user: 'bob' });

medium

A. The order of arguments is incorrect; the object should come first.

B. Missing semicolon after require statement.

C. pino() must be called with configuration object.

D. logger.info cannot accept two arguments.

Solution

1. Step 1: Check pino.info argument order

   pino expects the first argument as an object with extra data, second as message string.

2. Step 2: Analyze given code

   The code passes message string first, then object, which is reversed and causes wrong output.

3. Final Answer:

   The order of arguments is incorrect; the object should come first. -> Option A

4. Quick Check:

   pino.info(object, message) argument order [OK]

Hint: Pass object first, then message string to logger.info [OK]

Common Mistakes:

• Swapping message and object arguments
• Thinking semicolon is mandatory for correctness
• Assuming pino() needs config always

5. You want to log user events with structured logs including user ID, event type, and timestamp using pino. Which code snippet correctly logs this data ensuring timestamp is included in the JSON output?

hard

A. logger.info({ userId: 123, event: 'click' }, 'User event at ' + new Date());

B. logger.info('User event', { userId: 123, event: 'click', timestamp: Date.now() });

C. logger.info({ userId: 123, event: 'click', time: new Date().toISOString() }, 'User event');

D. logger.info({ userId: 123, event: 'click', timestamp: Date.now() });

Solution

1. Step 1: Ensure correct argument order and timestamp format

   pino expects object first, then message string. Timestamp should be ISO string for readability.

2. Step 2: Check each option

   logger.info({ userId: 123, event: 'click', time: new Date().toISOString() }, 'User event'); passes object with userId, event, and ISO timestamp, then message string. This is correct.

3. Final Answer:

   logger.info({ userId: 123, event: 'click', time: new Date().toISOString() }, 'User event'); -> Option C

4. Quick Check:

   Object first with ISO timestamp, then message [OK]

Hint: Pass object with ISO timestamp first, then message string [OK]

Common Mistakes:

• Swapping message and object arguments
• Using Date.now() without formatting
• Concatenating message and timestamp in string