Looking to observe a decorated method using Jest even if the decorator poses a hurdle to method execution?

Question

Looking to observe a decorated method using Jest even if the decorator poses a hurdle to method execution?

In my TypeScript project, decorators are a crucial part of enhancing method behavior. However, I recently ran into a situation where a decorator could potentially block the execution of a method. To test this scenario using Jest, I aim to confirm whether the decorated method was called or not.

Consider the following example of a class with a theoretical CacheDecorator:

class TestClass {
  @CacheDecorator
  async methodToTest(arg1: string, arg2: number) {
    return { arg1, arg2 };
  }
}

The CacheDecorator assesses a cache and either returns a cached value without running methodToTest, or allows methodToTest to proceed if there is no valid cache entry.

I want to verify whether methodToTest is executed based on the caching conditions. The challenge I face is distinguishing between the decorator's actions and the actual method call when attempting to spy on methodToTest directly with jest.spyOn().

Is there an effective way to spy on methodToTest to determine if it has been executed, considering the behavior of this particular decorator?

typescript ts-jest

Answer 1

Answer №1

One of the challenges lies in distinguishing between the behavior of a decorator and the behavior of the method itself.

1. Delegate the primary logic to an internal method

Rather than embedding the main logic within the decorated method, you can delegate it to an inner method within the class:

class TestClass {
  async executeInternally(arg1: string, arg2: number) {
    return { arg1, arg2 };
  }

  @CacheDecorator
  async testMethod(arg1: string, arg2: number) {
    return await this.executeInternally(arg1, arg2);
  }
}

2. Monitor the internal method instead

In your Jest tests, you can spy on executeInternally rather than testMethod.

const instance = new TestClass();
const spy = jest.spyOn(instance, 'executeInternally');
await instance.testMethod('testArg1', 123);
expect(spy).toHaveBeenCalled();

By monitoring executeInternally, you are able to directly observe the actual method logic without interference from the decorator. This provides a clear indication of whether the decorator allowed testMethod to execute its logic or if it terminated early due to cached data.

Answer 2

One of the challenges lies in distinguishing between the behavior of a decorator and the behavior of the method itself.

1. Delegate the primary logic to an internal method

Rather than embedding the main logic within the decorated method, you can delegate it to an inner method within the class:

class TestClass {
  async executeInternally(arg1: string, arg2: number) {
    return { arg1, arg2 };
  }

  @CacheDecorator
  async testMethod(arg1: string, arg2: number) {
    return await this.executeInternally(arg1, arg2);
  }
}

2. Monitor the internal method instead

In your Jest tests, you can spy on executeInternally rather than testMethod.

const instance = new TestClass();
const spy = jest.spyOn(instance, 'executeInternally');
await instance.testMethod('testArg1', 123);
expect(spy).toHaveBeenCalled();

By monitoring executeInternally, you are able to directly observe the actual method logic without interference from the decorator. This provides a clear indication of whether the decorator allowed testMethod to execute its logic or if it terminated early due to cached data.

Looking to observe a decorated method using Jest even if the decorator poses a hurdle to method execution?

Answer №1

1. Delegate the primary logic to an internal method

2. Monitor the internal method instead

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