Tips for simulating a decorator function applied to a method used in the system under test using JEST

Question

Tips for simulating a decorator function applied to a method used in the system under test using JEST

I currently have a typescript class setup like this:

export class SystemUnderTest {

  @LogThisAction('something was done')
  public doSomething() {} 

}

It is clear that reflection is being used to execute a specific decoration function:

 export declare function LogThisAction(action: string): (target: any) => 
 void;

When running tests, I am not concerned with the actual implementation of the decorator function, so I attempt to mock it in this way:

 myModule = require(./DecoratorFunctions);
 myModule.LogThisAction = jest.fn();

However, this approach does not seem to be effective. Test results show:

● Test suite failed to run
TypeError: decorator is not a function
at DecorateProperty (node_modules/reflect-metadata/Reflect.js:553:33)

How can I accomplish my objective within the JEST framework?

javascript typescript jestjs

Answer 1

Answer №1

Your decorator functions by returning another function, essentially making it a higher-order function.

Therefore, your mock is incorrect and it needs to return a function. You can fix it by using the following code:

const myModule = require('./DecoratorFunctions');
myModule.LogThisAction = () => jest.fn();

Answer 2

Your decorator functions by returning another function, essentially making it a higher-order function.

Therefore, your mock is incorrect and it needs to return a function. You can fix it by using the following code:

const myModule = require('./DecoratorFunctions');
myModule.LogThisAction = () => jest.fn();

Answer 3

Answer №2

If you want to simulate the functionality of a module and its internal workings, you can utilize the

jest.mock

method.

jest.mock('./DecoratorFunctions', () => ({ LogThisAction: (item: any) => {
return (target, propertyKey, descriptor) => {
  // store the original method in a variable
  const originalMethod = descriptor.value as () => Promise<any>;
  descriptor.value = async function(...args) {
    originalMethod.apply(this, args);
    return response;
  };

  return descriptor;
}; }}));

By doing this, you can effectively imitate the behavior of the LogThisAction function.

Answer 4

If you want to simulate the functionality of a module and its internal workings, you can utilize the

jest.mock

method.

jest.mock('./DecoratorFunctions', () => ({ LogThisAction: (item: any) => {
return (target, propertyKey, descriptor) => {
  // store the original method in a variable
  const originalMethod = descriptor.value as () => Promise<any>;
  descriptor.value = async function(...args) {
    originalMethod.apply(this, args);
    return response;
  };

  return descriptor;
}; }}));

By doing this, you can effectively imitate the behavior of the LogThisAction function.

Tips for simulating a decorator function applied to a method used in the system under test using JEST

Answer №1

Answer №2

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