Logging with Winston or Pino in NestJS - Step-by-Step Execution

Concept Flow - Logging with Winston or Pino

Start Application

↓

Initialize Logger (Winston or Pino)

↓

Application Runs

↓

Log Event Occurs

↓

Logger Formats Message

↓

Logger Outputs to Console/File

↓

Continue Application or Exit

The app starts, sets up the logger, then logs messages during runtime, formatting and outputting them as configured.

Execution Sample

NestJS

import { Logger } from '@nestjs/common';
const logger = new Logger('App');
logger.log('App started');
logger.error('Error happened');
logger.warn('Warning issued');

This code creates a NestJS logger and logs messages of different levels.

Execution Table

Step	Action	Logger Method	Message	Output Format	Output Destination
1	Initialize Logger	N/A	N/A	Default NestJS format	Console
2	Log info message	log	App started	[LOG] [App] App started	Console
3	Log error message	error	Error happened	[ERROR] [App] Error happened	Console
4	Log warning message	warn	Warning issued	[WARN] [App] Warning issued	Console
5	Application continues running	N/A	N/A	N/A	N/A

💡 Logging completes for these messages; application continues running.

Variable Tracker

Variable	Start	After Step 1	After Step 2	After Step 3	Final
logger	undefined	Logger instance created	Logger instance used	Logger instance used	Logger instance used

Key Moments - 2 Insights

Why do different logger methods like log, error, and warn produce different output formats?

Each method corresponds to a log level that prefixes the message with its level (e.g., [LOG], [ERROR]), as shown in execution_table rows 2, 3, and 4.

Does the logger stop the application after logging an error?

No, logging an error only outputs the message; the application continues running as shown in step 5 of the execution_table.

Visual Quiz - 3 Questions

Look at the execution table, what is the output format when logger.warn is called?

A [WARN] [App] Warning issued

B [ERROR] [App] Warning issued

C [INFO] [App] Warning issued

D Warning issued

💡 Hint

Check row 4 under Output Format in the execution_table.

At which step does the logger instance get created?

A Step 3

B Step 2

C Step 1

D Step 4

💡 Hint

Look at the Action column in the execution_table for initialization.

If you add a logger.debug('Debug info') call, what would change in the execution_table?

A The existing rows would change their output format

B A new row with Logger Method 'debug' and output format '[DEBUG] [App] Debug info' would appear

C The logger instance would be recreated

D No change would happen

💡 Hint

Adding a new log call adds a new row with that method and message.

Concept Snapshot

NestJS logging uses Logger class.
Create logger with new Logger('Context').
Use methods: log(), error(), warn(), debug() for levels.
Messages show level and context.
Outputs go to console or files if configured.
Logging does not stop app execution.

Full Transcript

This visual execution shows how NestJS logging works with Winston or Pino. First, the application starts and initializes a logger instance. Then, during runtime, the logger logs messages at different levels: info, error, and warning. Each log method formats the message with a level prefix and context name. The output goes to the console by default. Logging messages do not stop the application from running. This trace helps beginners see how logging calls translate into formatted outputs step-by-step.