Global exception filters in NestJS - Step-by-Step Execution

Concept Flow - Global exception filters

Request received

↓

Controller or Service executes

↓

Exception thrown?

No→Send normal response

Yes↓

Global Exception Filter catches

↓

Process exception (log, format)

↓

Send error response to client

When a request causes an error, the global exception filter catches it, processes it, and sends a formatted error response.

Execution Sample

NestJS

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

@Catch()
export class AllExceptionsFilter implements ExceptionFilter {
  catch(exception: any, host: ArgumentsHost) {
    const ctx = host.switchToHttp();
    const response = ctx.getResponse();
    response.status(500).json({ message: 'Error occurred' });
  }
}

This code defines a global exception filter that catches all exceptions and sends a 500 error response with a message.

Execution Table

Step	Action	Exception Thrown?	Filter Catches?	Response Sent
1	Request received and controller method called	No	No	Normal response sent
2	Controller throws an exception	Yes	Yes	Filter catches exception
3	Filter processes exception	Yes	Yes	Sends JSON error response with status 500
4	Request ends	No	No	Response sent to client

💡 Execution stops after sending error response or normal response to client

Variable Tracker

Variable	Start	After Exception	After Filter	Final
exception	undefined	Error object	Handled by filter	Handled
response.statusCode	200	200	500	500
response.body	Normal data	Not set	{"message":"Error occurred"}	{"message":"Error occurred"}

Key Moments - 3 Insights

Why does the filter catch exceptions thrown anywhere in the app?

Because it is registered globally, it listens for exceptions from all controllers and services, as shown in execution_table step 2 and 3.

What happens if no exception is thrown?

The filter does not run and the normal response is sent, as shown in execution_table step 1.

How does the filter change the HTTP response?

It sets the status code to 500 and sends a JSON error message, as tracked in variable_tracker for response.statusCode and response.body.

Visual Quiz - 3 Questions

Look at the execution table, at which step does the filter catch the exception?

A Step 2

B Step 1

C Step 4

D Step 3

💡 Hint

Check the 'Filter Catches?' column in execution_table rows

According to variable_tracker, what is the response status code after the filter processes the exception?

A 200

B 404

C 500

D 400

💡 Hint

Look at response.statusCode after filter in variable_tracker

If the controller does not throw an exception, what response is sent?

A Error response with status 500

B Normal response

C No response

D Exception filter response

💡 Hint

See execution_table step 1 where no exception is thrown

Concept Snapshot

Global Exception Filters in NestJS:
- Catch exceptions thrown anywhere in app
- Registered globally to handle all errors
- Modify HTTP response (status, body)
- Send formatted error messages
- Keeps error handling centralized and consistent

Full Transcript

Global exception filters in NestJS catch errors thrown during request handling anywhere in the app. When a controller or service throws an exception, the global filter catches it, processes it by setting the HTTP status code and response body, then sends a formatted error response to the client. If no exception occurs, the filter does not run and the normal response is sent. This centralizes error handling and keeps responses consistent.