Custom exception classes in NestJS - Step-by-Step Execution

Concept Flow - Custom exception classes

Define Custom Exception Class

↓

Throw Custom Exception

↓

NestJS Exception Filter Catches

↓

Send HTTP Response with Custom Status & Message

↓

End

This flow shows how you create a custom exception class, throw it in your code, and how NestJS catches it to send a proper HTTP response.

Execution Sample

NestJS

import { HttpException, HttpStatus } from '@nestjs/common';

export class MyCustomException extends HttpException {
  constructor() {
    super('Custom error occurred', HttpStatus.BAD_REQUEST);
  }
}

Defines a custom exception class that sends a 400 Bad Request with a custom message.

Execution Table

Step	Action	Evaluation	Result
1	Define MyCustomException class extending HttpException	Class created	Ready to throw custom error
2	Throw new MyCustomException() in controller/service	Exception instance created	Exception thrown
3	NestJS catches exception	Checks exception type	Matches MyCustomException
4	NestJS sends HTTP response	Uses status 400 and message 'Custom error occurred'	Client receives 400 Bad Request with message
5	Request handling ends	No further processing	Response sent to client

💡 Exception thrown and handled, HTTP response sent, request cycle ends

Variable Tracker

Variable	Start	After Throw	After Catch	Final
exceptionInstance	undefined	MyCustomException instance	Caught by NestJS	Handled and response sent

Key Moments - 3 Insights

Why do we extend HttpException instead of Error?

Extending HttpException lets NestJS recognize and handle the exception properly with HTTP status codes, as shown in execution_table step 3.

What happens if we throw the custom exception without a message or status?

The constructor requires a message and status; missing these causes errors or default behavior, so step 2 in execution_table shows the importance of passing them.

How does NestJS know to send a 400 status code?

Because the custom exception passes HttpStatus.BAD_REQUEST to the super constructor, NestJS reads this in step 4 and sends the correct HTTP response.

Visual Quiz - 3 Questions

Look at the execution_table, what is the result of step 3?

A NestJS ignores the exception

B NestJS matches the exception type

C Exception is re-thrown

D HTTP response is sent immediately

💡 Hint

Check the 'Result' column in step 3 of execution_table

At which step does the client receive the HTTP response?

A Step 4

B Step 3

C Step 2

D Step 5

💡 Hint

Look at the 'Action' and 'Result' columns in execution_table step 4

If we change the status in the custom exception to 404, what changes in the execution?

A Exception is not caught

B Client receives 500 error

C NestJS sends 404 instead of 400 in step 4

D No HTTP response is sent

💡 Hint

Refer to how status is used in execution_table step 4

Concept Snapshot

Custom exception classes in NestJS extend HttpException.
Use constructor to set message and HTTP status.
Throw the custom exception in your code.
NestJS catches it and sends the HTTP response.
This helps send clear error messages with proper status codes.

Full Transcript

In NestJS, you create custom exception classes by extending HttpException. This allows you to define your own error messages and HTTP status codes. When you throw this custom exception in your controller or service, NestJS catches it automatically. It then sends an HTTP response to the client with the status and message you defined. This process helps you handle errors clearly and consistently in your app.