Catch decorator in NestJS - Step-by-Step Execution

Concept Flow - Catch decorator

Define Catch Decorator

↓

Apply Decorator to Method

↓

Method Executes

↓

Error Occurs?

No→Return Result

Yes↓

Catch Decorator Handles Error

↓

Return Custom Error Response

The Catch decorator wraps a method to catch errors during execution and handle them gracefully.

Execution Sample

NestJS

import { Catch, ExceptionFilter, ArgumentsHost } from '@nestjs/common';

@Catch()
export class MyExceptionFilter implements ExceptionFilter {
  catch(exception: any, host: ArgumentsHost) {
    return 'Error handled';
  }
}

Defines a Catch decorator that intercepts errors thrown in a method and returns a custom message.

Execution Table

Step	Action	Method Execution	Error Thrown?	Catch Decorator Response	Final Output
1	Method called	Runs normally	No	No action	Method result
2	Method called	Throws error	Yes	Catch decorator intercepts	Returns 'Error handled'
3	Execution ends	N/A	N/A	N/A	Output returned to caller

💡 Execution stops after method returns or catch decorator handles error

Variable Tracker

Variable	Start	After Step 1	After Step 2	Final
exception	undefined	undefined	Error object	Error object
host	undefined	Request context	Request context	Request context
method result	undefined	Normal result	undefined	Error handled

Key Moments - 2 Insights

Why does the catch decorator run only when an error occurs?

Because as shown in execution_table row 2, the catch decorator intercepts only when the method throws an error; otherwise, it lets the normal result pass (row 1).

What happens if the method does not throw an error?

The catch decorator does nothing and the method returns its normal result, as seen in execution_table row 1.

Visual Quiz - 3 Questions

Look at the execution_table, what is the final output when the method throws an error?

A Undefined

B Method result

C 'Error handled'

D Exception object

💡 Hint

Check execution_table row 2 under 'Final Output'

At which step does the catch decorator intercept the error?

A Step 2

B Step 1

C Step 3

D Never

💡 Hint

See execution_table row 2 under 'Catch Decorator Response'

If the method never throws an error, what will the catch decorator do?

A Return 'Error handled'

B Do nothing

C Modify the method result

D Throw a new error

💡 Hint

Refer to execution_table row 1 where 'Error Thrown?' is No

Concept Snapshot

Catch decorator in NestJS:
- Wraps methods to catch thrown errors
- Executes only if an error occurs
- Returns custom error response
- Allows normal method result if no error
- Used for centralized error handling

Full Transcript

The Catch decorator in NestJS is used to handle errors thrown by methods. When a method runs, if it completes without error, the catch decorator does nothing and the method's result is returned. If the method throws an error, the catch decorator intercepts it and returns a custom response, such as an error message. This helps keep error handling clean and centralized. The execution table shows the steps: method call, error thrown or not, catch decorator response, and final output. Variables like the exception object and method result change accordingly. Understanding when the catch decorator activates is key to using it effectively.