Catch decorator in NestJS - Mini Project: Build & Apply

Create a Custom Catch Decorator in NestJS

📖 Scenario: You are building a NestJS backend service. You want to create a custom decorator that catches errors from a method and handles them gracefully.

🎯 Goal: Build a custom @CatchError decorator that wraps a method and catches any errors thrown, returning a friendly error message instead.

📋 What You'll Learn

Create a custom decorator function called CatchError

Use the decorator to wrap a method in a class

Catch any errors thrown inside the method

Return a string 'Error caught' when an error occurs

💡 Why This Matters

🌍 Real World

Custom decorators help add reusable behavior like error handling to methods in NestJS services, making code cleaner and more maintainable.

💼 Career

Understanding decorators is essential for backend developers working with NestJS, as they are widely used for middleware, validation, and error handling.

1

Create a simple service class with a method that throws an error

Create a class called TestService with a method dangerousMethod that throws an error with the message 'Something went wrong'.

NestJS

class TestService {
  // Your code here
}

Need a hint?

Define a class and inside it a method that throws an error using throw new Error('Something went wrong').

2

Create the CatchError decorator function

Create a function called CatchError that returns a method decorator function. The decorator should accept target, propertyKey, and descriptor parameters.

NestJS

class TestService {
  dangerousMethod() {
    throw new Error('Something went wrong');
  }
}

// Your code here

Need a hint?

Create a function that returns another function with the parameters target, propertyKey, and descriptor.

3

Implement error catching inside the decorator

Inside the CatchError decorator, save the original method from descriptor.value. Replace descriptor.value with a new function that calls the original method inside a try-catch block. If an error occurs, return the string 'Error caught'.

NestJS

class TestService {
  dangerousMethod() {
    throw new Error('Something went wrong');
  }
}

function CatchError() {
  return function(target: any, propertyKey: string, descriptor: PropertyDescriptor) {
    // Your code here
  };
}

Need a hint?

Save the original method, then replace it with a function that uses try-catch to handle errors and return 'Error caught'.

4

Apply the @CatchError decorator to the method

Add the @CatchError() decorator above the dangerousMethod in the TestService class.

NestJS

class TestService {
  // Add the decorator here
  dangerousMethod() {
    throw new Error('Something went wrong');
  }
}

function CatchError() {
  return function(target: any, propertyKey: string, descriptor: PropertyDescriptor) {
    const originalMethod = descriptor.value;
    descriptor.value = function(...args: any[]) {
      try {
        return originalMethod.apply(this, args);
      } catch (error) {
        return 'Error caught';
      }
    };
  };
}

Need a hint?

Place @CatchError() directly above the method you want to decorate.