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NestJSframework~10 mins

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

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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
StepActionEvaluationResult
1Define MyCustomException class extending HttpExceptionClass createdReady to throw custom error
2Throw new MyCustomException() in controller/serviceException instance createdException thrown
3NestJS catches exceptionChecks exception typeMatches MyCustomException
4NestJS sends HTTP responseUses status 400 and message 'Custom error occurred'Client receives 400 Bad Request with message
5Request handling endsNo further processingResponse sent to client
💡 Exception thrown and handled, HTTP response sent, request cycle ends
Variable Tracker
VariableStartAfter ThrowAfter CatchFinal
exceptionInstanceundefinedMyCustomException instanceCaught by NestJSHandled 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
Test your understanding
Look at the execution_table, what is the result of step 3?
ANestJS ignores the exception
BNestJS matches the exception type
CException is re-thrown
DHTTP 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?
AStep 4
BStep 3
CStep 2
DStep 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?
AException is not caught
BClient receives 500 error
CNestJS sends 404 instead of 400 in step 4
DNo 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.