Logging exceptions in NestJS - Step-by-Step Execution

Concept Flow - Logging exceptions

Exception occurs in code

↓

Catch exception in catch block

↓

Log exception details

↓

Optional: Re-throw or handle gracefully

↓

Continue execution or exit

When an error happens, the program catches it, logs the details for review, then decides to continue or stop.

Execution Sample

NestJS

try {
  // code that may fail
} catch (error) {
  this.logger.error('Error caught:', error.stack);
}

This code tries to run something risky, and if it fails, it logs the error stack trace.

Execution Table

Step	Action	Code Executed	Result	Log Output
1	Start try block	try { riskyCode(); }	riskyCode runs	No log yet
2	Exception thrown	riskyCode() throws Error	Exception caught	No log yet
3	Catch block entered	catch (error) { ... }	Error object available	No log yet
4	Log error	this.logger.error('Error caught:', error.stack);	Error logged	Error caught: <stack trace>
5	End catch block	}	Catch block ends	Log recorded
6	Continue or exit	After catch block	Program continues or stops	Log remains for review

💡 Exception caught and logged, program flow continues or stops based on handling

Variable Tracker

Variable	Start	After Step 2	After Step 3	Final
error	undefined	Error instance created	Error instance available in catch	Error instance remains

Key Moments - 3 Insights

Why do we log error.stack instead of just error?

Logging error.stack shows the full trace of where the error happened, which helps debugging better than just the error message. See execution_table step 4.

What happens if we don't catch the exception?

If not caught, the program may crash or stop unexpectedly. Catching lets us log and handle errors gracefully, as shown in steps 2 and 3.

Can the program continue after logging the exception?

Yes, depending on handling after catch block (step 6), the program can continue or exit safely.

Visual Quiz - 3 Questions

Look at the execution table, at which step is the error object first available?

A Step 3

B Step 2

C Step 1

D Step 4

💡 Hint

Check the 'Result' column for when the error object is accessible in the catch block.

At which step does the actual logging happen?

A Step 2

B Step 4

C Step 3

D Step 5

💡 Hint

Look for the 'Log error' action and the 'Log Output' showing the error message.

If the catch block did not log the error, what would be missing in the execution table?

A Error object creation

B Exception thrown

C Log Output with error details

D Try block execution

💡 Hint

Refer to the 'Log Output' column in steps 4 and 5 to see what logging adds.

Concept Snapshot

try {
  // risky code
} catch (error) {
  this.logger.error('Error caught:', error.stack);
}

- Catch exceptions to prevent crashes
- Log error.stack for full trace
- Decide to continue or exit after logging

Full Transcript

In NestJS, when code might fail, we wrap it in a try block. If an error happens, the catch block runs. Inside catch, we log the error details using this.logger.error and include error.stack to see where the error happened. This helps us understand and fix issues. After logging, we can choose to continue running or stop safely. This process prevents unexpected crashes and keeps logs for debugging.