Question

72

how to mock a constructor being initiated from inside a method with jest

rated 0 times [ 75] [ 3] / answers: 1 / hits: 8858 / 4 Years ago, tue, october 27, 2020, 12:00:00

i have a script that defines 2 classes, and 1 gets instantiated from within the constructor of the other. how can i mock the nested constructor, so i can test the parent constructor?

export default class Foo {

  // i want to test this constructor...

  constructor() {

    new Bar

  }

}



export class Bar {

  // but mock this constructor

  constructor() {}

}

additionally, i am trying to spy on the Bar constructor, to assert that it has been called

i have tried several different approaches, but haven't been able to get the results i am looking for. i am new to the jest mocking library

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analiseg

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4 Years ago

answered 4 Years ago martina · Accepted Answer

Need to make a little modification to the module export statement. Then, we can use jest.spyOn(object, methodName) method to mock implementation for Bar class. Take a look at the code after compiling. We create the mocked Bar in the module export object and we use it in the Foo class. It has the same reference as the mocked one.

Recommend way:

Dependency Injection

Each file contains ONLY ONE class. So that we can use jest.mock or jest.doMock method to mock the class without modifying the module export statement.

E.g.

index.ts:

export default class Foo {

  constructor() {

    new exports.Bar();

  }

}



class Bar {

  constructor() {

    console.log('real Bar constructor implmentation');

  }

}



exports.Bar = Bar;

index.test.ts:

import * as mod from './';



console.log(mod);



describe('64549093', () => {

  it('should pass', () => {

    const BarConstructorMock = jest.spyOn(mod as any, 'Bar').mockImplementationOnce(() => {

      console.log('fake Bar constructor implmentation');

    });

    new mod.default();

    expect(BarConstructorMock).toBeCalled();

  });

});

unit test result:

 PASS  src/stackoverflow/64549093/index.test.ts (9.905s)

  64549093

    ✓ should pass (5ms)



  console.log src/stackoverflow/64549093/index.test.ts:3

    { default: [Function: Foo], Bar: [Function: Bar] }



  console.log src/stackoverflow/64549093/index.test.ts:8

    fake Bar constructor implmentation



Test Suites: 1 passed, 1 total

Tests:       1 passed, 1 total

Snapshots:   0 total

Time:        11.751s, estimated 12s

About the configuration of jestjs, TypeScript, see example:
https://github.com/mrdulin/jest-codelab/tree/master/src/stackoverflow/64549093